Preliminary support for Matlab objects

MatFileHandler.Tests/MatFileReaderTests.cs

@@ -292,7 +292,38 @@ namespace MatFileHandler.Tests
         [Test]
         public void TestObject()
         {
-            Assert.That(() => GetTests("bad")["object"], Throws.TypeOf<HandlerException>());
+            var matFile = GetTests("good")["object"];
+            var obj = matFile["object_"].Value as IMatObject;
+            Assert.IsNotNull(obj);
+            Assert.That(obj.ClassName, Is.EqualTo("Point"));
+            Assert.That(obj.FieldNames, Is.EquivalentTo(new[] { "x", "y" }));
+            Assert.That(obj["x", 0].ConvertToDoubleArray(), Is.EqualTo(new[] { 3.0 }));
+            Assert.That(obj["y", 0].ConvertToDoubleArray(), Is.EqualTo(new[] { 5.0 }));
+            Assert.That(obj["x", 1].ConvertToDoubleArray(), Is.EqualTo(new[] { -2.0 }));
+            Assert.That(obj["y", 1].ConvertToDoubleArray(), Is.EqualTo(new[] { 6.0 }));
+        }
+
+        /// <summary>
+        /// Test reading another object.
+        /// </summary>
+        [Test]
+        public void TestObject2()
+        {
+            var matFile = GetTests("good")["object2"];
+            var obj = matFile["object2"].Value as IMatObject;
+            Assert.IsNotNull(obj);
+            Assert.That(obj.ClassName, Is.EqualTo("Point"));
+            Assert.That(obj.FieldNames, Is.EquivalentTo(new[] { "x", "y" }));
+            Assert.That(obj["x", 0, 0].ConvertToDoubleArray(), Is.EqualTo(new[] { 3.0 }));
+            Assert.That(obj["y", 0, 0].ConvertToDoubleArray(), Is.EqualTo(new[] { 5.0 }));
+            Assert.That(obj["x", 1, 0].ConvertToDoubleArray(), Is.EqualTo(new[] { 1.0 }));
+            Assert.That(obj["y", 1, 0].ConvertToDoubleArray(), Is.EqualTo(new[] { 0.0 }));
+            Assert.That(obj["x", 0, 1].ConvertToDoubleArray(), Is.EqualTo(new[] { -2.0 }));
+            Assert.That(obj["y", 0, 1].ConvertToDoubleArray(), Is.EqualTo(new[] { 6.0 }));
+            Assert.That(obj["x", 1, 1].ConvertToDoubleArray(), Is.EqualTo(new[] { 0.0 }));
+            Assert.That(obj["y", 1, 1].ConvertToDoubleArray(), Is.EqualTo(new[] { 1.0 }));
+            Assert.That(obj[0, 1]["x"].ConvertToDoubleArray(), Is.EqualTo(new[] { -2.0 }));
+            Assert.That(obj[2]["x"].ConvertToDoubleArray(), Is.EqualTo(new[] { -2.0 }));
         }
 
         private static AbstractTestDataFactory<IMatFile> GetTests(string factoryName) =>

MatFileHandler/ArrayFlags.cs

@@ -85,9 +85,9 @@ namespace MatFileHandler
         MxUInt64 = 15,
 
         /// <summary>
-        /// Undocumented object (?) array type.
+        /// Undocumented opaque object type.
         /// </summary>
-        MxNewObject = 17,
+        MxOpaque = 17,
     }
 
     /// <summary>

MatFileHandler/DataElementReader.cs

@@ -12,14 +12,25 @@ namespace MatFileHandler
     /// <summary>
     /// Functions for reading data elements from a .mat file.
     /// </summary>
-    internal static class DataElementReader
+    internal class DataElementReader
     {
+        private readonly SubsystemData subsystemData;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="DataElementReader"/> class.
+        /// </summary>
+        /// <param name="subsystemData">Reference to file's SubsystemData.</param>
+        public DataElementReader(SubsystemData subsystemData)
+        {
+            this.subsystemData = subsystemData ?? throw new ArgumentNullException(nameof(subsystemData));
+        }
+
         /// <summary>
         /// Read a data element.
         /// </summary>
         /// <param name="reader">Input reader.</param>
         /// <returns>Data element.</returns>
-        public static DataElement Read(BinaryReader reader)
+        public DataElement Read(BinaryReader reader)
         {
             var (dataReader, tag) = ReadTag(reader);
             DataElement result;
@@ -66,6 +77,7 @@ namespace MatFileHandler
                 default:
                     throw new NotSupportedException("Unknown element.");
             }
+
             if (tag.Type != DataType.MiCompressed)
             {
                 var position = reader.BaseStream.Position;
@@ -74,9 +86,109 @@ namespace MatFileHandler
                     reader.ReadBytes(8 - (int)(position % 8));
                 }
             }
+
             return result;
         }
 
+        private static (int[] dimensions, int[] links, int classIndex) ParseOpaqueData(MatNumericalArrayOf<uint> data)
+        {
+            var nDims = data.Data[1];
+            var dimensions = new int[nDims];
+            var position = 2;
+            for (var i = 0; i < nDims; i++)
+            {
+                dimensions[i] = (int)data.Data[position];
+                position++;
+            }
+
+            var count = dimensions.NumberOfElements();
+            var links = new int[count];
+            for (var i = 0; i < count; i++)
+            {
+                links[i] = (int)data.Data[position];
+                position++;
+            }
+
+            var classIndex = (int)data.Data[position];
+
+            return (dimensions, links, classIndex);
+        }
+
+        private static ArrayFlags ReadArrayFlags(DataElement element)
+        {
+            var flagData = (element as MiNum<uint>)?.Data ??
+                           throw new HandlerException("Unexpected type in array flags.");
+            var class_ = (ArrayType)(flagData[0] & 0xff);
+            var variableFlags = (flagData[0] >> 8) & 0x0e;
+            return new ArrayFlags
+            {
+                Class = class_,
+                Variable = (Variable)variableFlags,
+            };
+        }
+
+        private static DataElement ReadData(DataElement element)
+        {
+            return element;
+        }
+
+        private static int[] ReadDimensionsArray(MiNum<int> element)
+        {
+            return element.Data;
+        }
+
+        private static string[] ReadFieldNames(MiNum<sbyte> element, int fieldNameLength)
+        {
+            var numberOfFields = element.Data.Length / fieldNameLength;
+            var result = new string[numberOfFields];
+            for (var i = 0; i < numberOfFields; i++)
+            {
+                var list = new List<byte>();
+                var position = i * fieldNameLength;
+                while (element.Data[position] != 0)
+                {
+                    list.Add((byte)element.Data[position]);
+                    position++;
+                }
+
+                result[i] = Encoding.ASCII.GetString(list.ToArray());
+            }
+
+            return result;
+        }
+
+        private static string ReadName(MiNum<sbyte> element)
+        {
+            return Encoding.ASCII.GetString(element.Data.Select(x => (byte)x).ToArray());
+        }
+
+        private static DataElement ReadNum<T>(Tag tag, BinaryReader reader)
+            where T : struct
+        {
+            var bytes = reader.ReadBytes(tag.Length);
+            if (tag.Type == DataType.MiUInt8)
+            {
+                return new MiNum<byte>(bytes);
+            }
+
+            var result = new T[bytes.Length / tag.ElementSize];
+            Buffer.BlockCopy(bytes, 0, result, 0, bytes.Length);
+            return new MiNum<T>(result);
+        }
+
+        private static SparseArrayFlags ReadSparseArrayFlags(DataElement element)
+        {
+            var arrayFlags = ReadArrayFlags(element);
+            var flagData = (element as MiNum<uint>)?.Data ??
+                           throw new HandlerException("Unexpected type in sparse array flags.");
+            var nzMax = flagData[1];
+            return new SparseArrayFlags
+            {
+                ArrayFlags = arrayFlags,
+                NzMax = nzMax,
+            };
+        }
+
         private static (BinaryReader, Tag) ReadTag(BinaryReader reader)
         {
             var type = reader.ReadInt32();
@@ -95,87 +207,66 @@ namespace MatFileHandler
             }
         }
 
-        private static ArrayFlags ReadArrayFlags(DataElement element)
+        private DataElement ContinueReadingCellArray(
+            BinaryReader reader,
+            ArrayFlags flags,
+            int[] dimensions,
+            string name)
         {
-            var flagData = (element as MiNum<uint>)?.Data ??
+            var numberOfElements = dimensions.NumberOfElements();
-                           throw new HandlerException("Unexpected type in array flags.");
+            var elements = new List<IArray>();
-            var class_ = (ArrayType)(flagData[0] & 0xff);
+            for (var i = 0; i < numberOfElements; i++)
-            var variableFlags = (flagData[0] >> 8) & 0x0e;
-            return new ArrayFlags
             {
-                Class = class_,
+                var element = Read(reader) as IArray;
-                Variable = (Variable)variableFlags,
+                elements.Add(element);
-            };
-        }
-
-        private static SparseArrayFlags ReadSparseArrayFlags(DataElement element)
-        {
-            var arrayFlags = ReadArrayFlags(element);
-            var flagData = (element as MiNum<uint>)?.Data ??
-                           throw new HandlerException("Unexpected type in sparse array flags.");
-            var nzMax = flagData[1];
-            return new SparseArrayFlags
-            {
-                ArrayFlags = arrayFlags,
-                NzMax = nzMax,
-            };
-        }
-
-        private static int[] ReadDimensionsArray(MiNum<int> element)
-        {
-            return element.Data;
-        }
-
-        private static DataElement ReadData(DataElement element)
-        {
-            return element;
-        }
-
-        private static string ReadName(MiNum<sbyte> element)
-        {
-            return Encoding.ASCII.GetString(element.Data.Select(x => (byte)x).ToArray());
-        }
-
-        private static DataElement ReadNum<T>(Tag tag, BinaryReader reader)
-          where T : struct
-        {
-            var bytes = reader.ReadBytes(tag.Length);
-            if (tag.Type == DataType.MiUInt8)
-            {
-                return new MiNum<byte>(bytes);
             }
-            var result = new T[bytes.Length / tag.ElementSize];
+
-            Buffer.BlockCopy(bytes, 0, result, 0, bytes.Length);
+            return new MatCellArray(flags, dimensions, name, elements);
-            return new MiNum<T>(result);
         }
 
-        private static string[] ReadFieldNames(MiNum<sbyte> element, int fieldNameLength)
+        private DataElement ContinueReadingOpaque(BinaryReader reader)
         {
-            var numberOfFields = element.Data.Length / fieldNameLength;
+            var nameElement = Read(reader) as MiNum<sbyte> ??
-            var result = new string[numberOfFields];
+                              throw new HandlerException("Unexpected type in object name.");
-            for (var i = 0; i < numberOfFields; i++)
+            var name = ReadName(nameElement);
+            var anotherElement = Read(reader) as MiNum<sbyte> ??
+                                 throw new HandlerException("Unexpected type in object type description.");
+            var typeDescription = ReadName(anotherElement);
+            var classNameElement = Read(reader) as MiNum<sbyte> ??
+                                   throw new HandlerException("Unexpected type in class name.");
+            var className = ReadName(classNameElement);
+            var dataElement = Read(reader);
+            var data = ReadData(dataElement);
+            if (data is MatNumericalArrayOf<uint> linkElement)
             {
-                var list = new List<byte>();
+                var (dimensions, links, classIndex) = ParseOpaqueData(linkElement);
-                var position = i * fieldNameLength;
+                return new OpaqueLink(
-                while (element.Data[position] != 0)
+                    name,
-                {
+                    typeDescription,
-                    list.Add((byte)element.Data[position]);
+                    className,
-                    position++;
+                    dimensions,
-                }
+                    data,
-                result[i] = Encoding.ASCII.GetString(list.ToArray());
+                    links,
+                    classIndex,
+                    subsystemData);
+            }
+            else
+            {
+                return new Opaque(name, typeDescription, className, new int[] { }, data);
             }
-            return result;
         }
 
-        private static DataElement ContinueReadingSparseArray(
+        private DataElement ContinueReadingSparseArray(
             BinaryReader reader,
             DataElement firstElement,
             int[] dimensions,
             string name)
         {
             var sparseArrayFlags = ReadSparseArrayFlags(firstElement);
-            var rowIndex = Read(reader) as MiNum<int> ?? throw new HandlerException("Unexpected type in row indices of a sparse array.");
+            var rowIndex = Read(reader) as MiNum<int> ??
-            var columnIndex = Read(reader) as MiNum<int> ?? throw new HandlerException("Unexpected type in column indices of a sparse array.");
+                           throw new HandlerException("Unexpected type in row indices of a sparse array.");
+            var columnIndex = Read(reader) as MiNum<int> ??
+                              throw new HandlerException("Unexpected type in column indices of a sparse array.");
             var data = Read(reader);
             if (sparseArrayFlags.ArrayFlags.Variable.HasFlag(Variable.IsLogical))
             {
@@ -187,6 +278,7 @@ namespace MatFileHandler
                     columnIndex.Data,
                     data);
             }
+
             if (sparseArrayFlags.ArrayFlags.Variable.HasFlag(Variable.IsComplex))
             {
                 var imaginaryData = Read(reader);
@@ -199,6 +291,7 @@ namespace MatFileHandler
                     data,
                     imaginaryData);
             }
+
             switch (data)
             {
                 case MiNum<double> _:
@@ -214,23 +307,7 @@ namespace MatFileHandler
             }
         }
 
-        private static DataElement ContinueReadingCellArray(
+        private DataElement ContinueReadingStructure(
-            BinaryReader reader,
-            ArrayFlags flags,
-            int[] dimensions,
-            string name)
-        {
-            var numberOfElements = dimensions.NumberOfElements();
-            var elements = new List<IArray>();
-            for (var i = 0; i < numberOfElements; i++)
-            {
-                var element = Read(reader) as IArray;
-                elements.Add(element);
-            }
-            return new MatCellArray(flags, dimensions, name, elements);
-        }
-
-        private static DataElement ContinueReadingStructure(
             BinaryReader reader,
             ArrayFlags flags,
             int[] dimensions,
@@ -244,6 +321,7 @@ namespace MatFileHandler
             {
                 fields[fieldName] = new List<IArray>();
             }
+
             var numberOfElements = dimensions.NumberOfElements();
             for (var i = 0; i < numberOfElements; i++)
             {
@@ -253,27 +331,53 @@ namespace MatFileHandler
                     fields[fieldName].Add(field);
                 }
             }
+
             return new MatStructureArray(flags, dimensions, name, fields);
         }
 
-        private static DataElement ContinueReadingNewObject()
+        private DataElement Read(Stream stream)
         {
-            throw new HandlerException("Cannot read objects.");
+            using (var reader = new BinaryReader(stream))
+            {
+                return Read(reader);
+            }
         }
 
-        private static DataElement ReadMatrix(Tag tag, BinaryReader reader)
+        private DataElement ReadCompressed(Tag tag, BinaryReader reader)
+        {
+            reader.ReadBytes(2);
+            var compressedData = new byte[tag.Length - 6];
+            reader.BaseStream.Read(compressedData, 0, tag.Length - 6);
+            reader.ReadBytes(4);
+            var resultStream = new MemoryStream();
+            using (var compressedStream = new MemoryStream(compressedData))
+            {
+                using (var stream = new DeflateStream(compressedStream, CompressionMode.Decompress, leaveOpen: true))
+                {
+                    stream.CopyTo(resultStream);
+                }
+            }
+
+            resultStream.Position = 0;
+            return Read(resultStream);
+        }
+
+        private DataElement ReadMatrix(Tag tag, BinaryReader reader)
         {
             if (tag.Length == 0)
             {
                 return MatArray.Empty();
             }
+
             var element1 = Read(reader);
             var flags = ReadArrayFlags(element1);
-            if (flags.Class == ArrayType.MxNewObject)
+            if (flags.Class == ArrayType.MxOpaque)
             {
-                return ContinueReadingNewObject();
+                return ContinueReadingOpaque(reader);
             }
-            var element2 = Read(reader) as MiNum<int> ?? throw new HandlerException("Unexpected type in array dimensions data.");
+
+            var element2 = Read(reader) as MiNum<int> ??
+                           throw new HandlerException("Unexpected type in array dimensions data.");
             var dimensions = ReadDimensionsArray(element2);
             var element3 = Read(reader) as MiNum<sbyte> ?? throw new HandlerException("Unexpected type in array name.");
             var name = ReadName(element3);
@@ -281,10 +385,12 @@ namespace MatFileHandler
             {
                 return ContinueReadingCellArray(reader, flags, dimensions, name);
             }
+
             if (flags.Class == ArrayType.MxSparse)
             {
                 return ContinueReadingSparseArray(reader, element1, dimensions, name);
             }
+
             var element4 = Read(reader);
             var data = ReadData(element4);
             DataElement imaginaryData = null;
@@ -293,12 +399,15 @@ namespace MatFileHandler
                 var element5 = Read(reader);
                 imaginaryData = ReadData(element5);
             }
+
             if (flags.Class == ArrayType.MxStruct)
             {
                 var fieldNameLengthElement = data as MiNum<int> ??
-                                             throw new HandlerException("Unexpected type in structure field name length.");
+                                             throw new HandlerException(
+                                                 "Unexpected type in structure field name length.");
                 return ContinueReadingStructure(reader, flags, dimensions, name, fieldNameLengthElement.Data[0]);
             }
+
             switch (flags.Class)
             {
                 case ArrayType.MxChar:
@@ -339,6 +448,7 @@ namespace MatFileHandler
                             data,
                             imaginaryData);
                     }
+
                     return DataElementConverter.ConvertToMatNumericalArrayOf<byte>(
                         flags,
                         dimensions,
@@ -405,31 +515,5 @@ namespace MatFileHandler
                     throw new HandlerException("Unknown data type.");
             }
         }
-
-        private static DataElement ReadCompressed(Tag tag, BinaryReader reader)
-        {
-            reader.ReadBytes(2);
-            var compressedData = new byte[tag.Length - 6];
-            reader.BaseStream.Read(compressedData, 0, tag.Length - 6);
-            reader.ReadBytes(4);
-            var resultStream = new MemoryStream();
-            using (var compressedStream = new MemoryStream(compressedData))
-            {
-                using (var stream = new DeflateStream(compressedStream, CompressionMode.Decompress, leaveOpen: true))
-                {
-                    stream.CopyTo(resultStream);
-                }
-            }
-            resultStream.Position = 0;
-            return Read(resultStream);
-        }
-
-        private static DataElement Read(Stream stream)
-        {
-            using (var reader = new BinaryReader(stream))
-            {
-                return Read(reader);
-            }
-        }
     }
 }

MatFileHandler/Header.cs

@@ -13,7 +13,7 @@ namespace MatFileHandler
     /// </summary>
     internal class Header
     {
-        private Header(string text, byte[] subsystemDataOffset, int version)
+        private Header(string text, long subsystemDataOffset, int version)
         {
             Text = text;
             SubsystemDataOffset = subsystemDataOffset;
@@ -28,7 +28,7 @@ namespace MatFileHandler
         /// <summary>
         /// Gets subsystem data offset.
         /// </summary>
-        public byte[] SubsystemDataOffset { get; }
+        public long SubsystemDataOffset { get; }
 
         /// <summary>
         /// Gets file version.
@@ -55,7 +55,7 @@ namespace MatFileHandler
                 platform = platform.Remove(length);
             }
             var text = $"MATLAB 5.0 MAT-file, Platform: {platform}, Created on: {dateTime}{padding}";
-            return new Header(text, subsystemDataOffset, 256);
+            return new Header(text, 0, 256);
         }
 
         /// <summary>
@@ -67,7 +67,8 @@ namespace MatFileHandler
         {
             var textBytes = reader.ReadBytes(116);
             var text = System.Text.Encoding.UTF8.GetString(textBytes);
-            var subsystemDataOffset = reader.ReadBytes(8);
+            var subsystemDataOffsetBytes = reader.ReadBytes(8);
+            var subsystemDataOffset = BitConverter.ToInt64(subsystemDataOffsetBytes, 0);
             var version = reader.ReadInt16();
             var endian = reader.ReadInt16();
             var isLittleEndian = endian == 19785;

MatFileHandler/IMatObject.cs

@@ -0,0 +1,34 @@
+// Copyright 2017-2018 Alexander Luzgarev
+
+using System.Collections.Generic;
+
+namespace MatFileHandler
+{
+    /// <summary>
+    /// An interface to access Matlab objects (more precisely, "object arrays").
+    /// This is very similar to the <see cref="IStructureArray"/> interface:
+    /// an object holds fields that you can access, and the name of its class.
+    /// Additionally, you can treat is as an array of dictionaries mapping
+    /// field names to contents of fields.
+    /// </summary>
+    public interface IMatObject : IArrayOf<IReadOnlyDictionary<string, IArray>>
+    {
+        /// <summary>
+        /// Gets the name of object's class.
+        /// </summary>
+        string ClassName { get; }
+
+        /// <summary>
+        /// Gets the names of object's fields.
+        /// </summary>
+        IEnumerable<string> FieldNames { get; }
+
+        /// <summary>
+        /// Access a given field of a given object in the array.
+        /// </summary>
+        /// <param name="field">Field name.</param>
+        /// <param name="list">Index of the object to access.</param>
+        /// <returns>The value of the field in the selected object.</returns>
+        IArray this[string field, params int[] list] { get; set; }
+    }
+}

MatFileHandler/MatFileReader.cs

@@ -1,5 +1,6 @@
 // Copyright 2017-2018 Alexander Luzgarev
 
+using System;
 using System.Collections.Generic;
 using System.IO;
 
@@ -33,35 +34,72 @@ namespace MatFileHandler
             }
         }
 
-        private static void ReadHeader(BinaryReader reader)
+        /// <summary>
+        /// Read raw variables from a .mat file.
+        /// </summary>
+        /// <param name="reader">Binary reader.</param>
+        /// <param name="subsystemDataOffset">Offset to the subsystem data to use (read from the file header).</param>
+        /// <returns>Raw variables read.</returns>
+        internal static List<RawVariable> ReadRawVariables(BinaryReader reader, long subsystemDataOffset)
         {
-            Header.Read(reader);
+            var variables = new List<RawVariable>();
-        }
+            var subsystemData = new SubsystemData();
-
+            var dataElementReader = new DataElementReader(subsystemData);
-        private static IMatFile Read(BinaryReader reader)
-        {
-            ReadHeader(reader);
-            var variables = new List<IVariable>();
             while (true)
             {
                 try
                 {
-                    var dataElement = DataElementReader.Read(reader) as MatArray;
+                    var position = reader.BaseStream.Position;
-                    if (dataElement == null)
+                    var dataElement = dataElementReader.Read(reader);
+                    if (position == subsystemDataOffset)
                     {
-                        continue;
+                        var subsystemDataElement = dataElement as IArrayOf<byte>;
+                        subsystemData.Set(ReadSubsystemData(subsystemDataElement.Data));
+                    }
+                    else
+                    {
+                        variables.Add(new RawVariable(position, dataElement));
                     }
-                    variables.Add(new MatVariable(
-                        dataElement,
-                        dataElement.Name,
-                        dataElement.Flags.Variable.HasFlag(Variable.IsGlobal)));
                 }
                 catch (EndOfStreamException)
                 {
                     break;
                 }
             }
+
+            return variables;
+        }
+
+        private static IMatFile Read(BinaryReader reader)
+        {
+            var header = ReadHeader(reader);
+            var rawVariables = ReadRawVariables(reader, header.SubsystemDataOffset);
+            var variables = new List<IVariable>();
+            foreach (var variable in rawVariables)
+            {
+                var array = variable.DataElement as MatArray;
+                if (array is null)
+                {
+                    continue;
+                }
+
+                variables.Add(new MatVariable(
+                    array,
+                    array.Name,
+                    array.Flags.Variable.HasFlag(Variable.IsGlobal)));
+            }
+
             return new MatFile(variables);
         }
+
+        private static Header ReadHeader(BinaryReader reader)
+        {
+            return Header.Read(reader);
+        }
+
+        private static SubsystemData ReadSubsystemData(byte[] bytes)
+        {
+            return SubsystemDataReader.Read(bytes);
+        }
     }
 }

MatFileHandler/Opaque.cs

@@ -0,0 +1,58 @@
+// Copyright 2017-2018 Alexander Luzgarev
+
+using System;
+using System.Numerics;
+
+namespace MatFileHandler
+{
+    /// <summary>
+    /// Matlab "opaque object" structure.
+    /// If this object appears in the "main" section of the .mat file,
+    /// it just contains a small data structure pointing to the object's
+    /// storage in the "subsystem data" portion of the file.
+    /// In this case, an instance of <see cref="OpaqueLink"/> class
+    /// will be created.
+    /// If this object appears in the "subsystem data" part, it contains
+    /// the data of all opaque objects in the file, and that is what we
+    /// put into <see cref="RawData"/> property.
+    /// </summary>
+    internal class Opaque : MatArray, IArray
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Opaque"/> class.
+        /// </summary>
+        /// <param name="name">Name of the object.</param>
+        /// <param name="typeDescription">Type description.</param>
+        /// <param name="className">Class name.</param>
+        /// <param name="dimensions">Dimensions of the object.</param>
+        /// <param name="rawData">Raw object's data.</param>
+        public Opaque(string name, string typeDescription, string className, int[] dimensions, DataElement rawData)
+            : base(new ArrayFlags(ArrayType.MxOpaque, 0), dimensions, name)
+        {
+            TypeDescription = typeDescription ?? throw new ArgumentNullException(nameof(typeDescription));
+            ClassName = className ?? throw new ArgumentNullException(nameof(className));
+            RawData = rawData ?? throw new ArgumentNullException(nameof(rawData));
+        }
+
+        /// <summary>
+        /// Gets class name of the opaque object.
+        /// </summary>
+        public string ClassName { get; }
+
+        /// <summary>
+        /// Gets raw object's data: either links to subsystem data, or actual data.
+        /// </summary>
+        public DataElement RawData { get; }
+
+        /// <summary>
+        /// Gets "type description" of the opaque object.
+        /// </summary>
+        public string TypeDescription { get; }
+
+        /// <inheritdoc />
+        public override Complex[] ConvertToComplexArray() => null;
+
+        /// <inheritdoc />
+        public override double[] ConvertToDoubleArray() => null;
+    }
+}

MatFileHandler/OpaqueLink.cs

@@ -0,0 +1,169 @@
+// Copyright 2017-2018 Alexander Luzgarev
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace MatFileHandler
+{
+    /// <summary>
+    /// Implementation of Matlab's "opaque objects" via links to subsystem data.
+    /// </summary>
+    internal class OpaqueLink : Opaque, IMatObject
+    {
+        private readonly SubsystemData subsystemData;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="OpaqueLink"/> class.
+        /// </summary>
+        /// <param name="name">Name of the object.</param>
+        /// <param name="typeDescription">Description of object's class.</param>
+        /// <param name="className">Name of the object's class.</param>
+        /// <param name="dimensions">Dimensions of the object.</param>
+        /// <param name="data">Raw data containing links to object's storage.</param>
+        /// <param name="links">Links to object's storage.</param>
+        /// <param name="classIndex">Index of object's class.</param>
+        /// <param name="subsystemData">Reference to global subsystem data.</param>
+        public OpaqueLink(
+            string name,
+            string typeDescription,
+            string className,
+            int[] dimensions,
+            DataElement data,
+            int[] links,
+            int classIndex,
+            SubsystemData subsystemData)
+            : base(name, typeDescription, className, dimensions, data)
+        {
+            Links = links ?? throw new ArgumentNullException(nameof(links));
+            ClassIndex = classIndex;
+            this.subsystemData = subsystemData ?? throw new ArgumentNullException(nameof(subsystemData));
+        }
+
+        /// <summary>
+        /// Gets index of this object's class in subsystem data class list.
+        /// </summary>
+        public int ClassIndex { get; }
+
+        /// <inheritdoc />
+        public IReadOnlyDictionary<string, IArray>[] Data => null;
+
+        /// <inheritdoc />
+        public IEnumerable<string> FieldNames => FieldNamesArray;
+
+        /// <summary>
+        /// Gets links to the fields stored in subsystem data.
+        /// </summary>
+        public int[] Links { get; }
+
+        private string[] FieldNamesArray => subsystemData.ClassInformation[ClassIndex - 1].FieldNames.ToArray();
+
+        /// <inheritdoc />
+        public IArray this[string field, params int[] list]
+        {
+            get
+            {
+                if (TryGetValue(field, out var result, list))
+                {
+                    return result;
+                }
+
+                throw new IndexOutOfRangeException();
+            }
+            set => throw new NotImplementedException();
+        }
+
+        /// <inheritdoc />
+        public IReadOnlyDictionary<string, IArray> this[params int[] list]
+        {
+            get => ExtractObject(Dimensions.DimFlatten(list));
+            set => throw new NotImplementedException();
+        }
+
+        private IReadOnlyDictionary<string, IArray> ExtractObject(int i)
+        {
+            return new OpaqueObjectArrayElement(this, i);
+        }
+
+        private bool TryGetValue(string field, out IArray output, params int[] list)
+        {
+            var index = Dimensions.DimFlatten(list);
+            var maybeFieldIndex = subsystemData.ClassInformation[ClassIndex - 1].FindField(field);
+            if (!(maybeFieldIndex is int fieldIndex))
+            {
+                output = default(IArray);
+                return false;
+            }
+
+            if (index >= subsystemData.ObjectInformation.Length)
+            {
+                output = default(IArray);
+                return false;
+            }
+
+            var dataIndex = subsystemData.ObjectInformation[index].FieldLinks[fieldIndex + 1];
+            output = subsystemData.Data[dataIndex];
+            return true;
+        }
+
+        /// <summary>
+        /// Provides access to a single object in object array.
+        /// </summary>
+        internal class OpaqueObjectArrayElement : IReadOnlyDictionary<string, IArray>
+        {
+            private readonly int index;
+            private readonly OpaqueLink parent;
+
+            /// <summary>
+            /// Initializes a new instance of the <see cref="OpaqueObjectArrayElement"/> class.
+            /// </summary>
+            /// <param name="parent">Parent object array.</param>
+            /// <param name="index">Index of the object in the array.</param>
+            public OpaqueObjectArrayElement(OpaqueLink parent, int index)
+            {
+                this.parent = parent ?? throw new ArgumentNullException(nameof(parent));
+                this.index = index;
+            }
+
+            /// <inheritdoc />
+            public int Count => parent.FieldNamesArray.Length;
+
+            /// <inheritdoc />
+            public IEnumerable<string> Keys => parent.FieldNames;
+
+            /// <inheritdoc />
+            public IEnumerable<IArray> Values => parent.FieldNames.Select(fieldName => parent[fieldName, index]);
+
+            /// <inheritdoc />
+            public IArray this[string key] => parent[key, index];
+
+            /// <inheritdoc />
+            public bool ContainsKey(string key)
+            {
+                return parent.FieldNames.Contains(key);
+            }
+
+            /// <inheritdoc />
+            public IEnumerator<KeyValuePair<string, IArray>> GetEnumerator()
+            {
+                foreach (var field in parent.FieldNamesArray)
+                {
+                    yield return new KeyValuePair<string, IArray>(field, parent[field, index]);
+                }
+            }
+
+            /// <inheritdoc />
+            public bool TryGetValue(string key, out IArray value)
+            {
+                return parent.TryGetValue(key, out value, index);
+            }
+
+            /// <inheritdoc />
+            IEnumerator IEnumerable.GetEnumerator()
+            {
+                return GetEnumerator();
+            }
+        }
+    }
+}

MatFileHandler/RawVariable.cs

@@ -0,0 +1,36 @@
+// Copyright 2017-2018 Alexander Luzgarev
+
+using System;
+
+namespace MatFileHandler
+{
+    /// <summary>
+    /// Raw variable read from the file.
+    /// This gives a way to deal with "subsystem data" which looks like
+    /// a variable and can only be detected by comparing its offset with
+    /// the value stored in the file's header.
+    /// </summary>
+    internal class RawVariable
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="RawVariable"/> class.
+        /// </summary>
+        /// <param name="offset">Offset of the variable in the source file.</param>
+        /// <param name="dataElement">Data element parsed from the file.</param>
+        internal RawVariable(long offset, DataElement dataElement)
+        {
+            Offset = offset;
+            DataElement = dataElement ?? throw new ArgumentNullException(nameof(dataElement));
+        }
+
+        /// <summary>
+        /// Gets data element with the variable's contents.
+        /// </summary>
+        public DataElement DataElement { get; }
+
+        /// <summary>
+        /// Gets offset of the variable in the .mat file.
+        /// </summary>
+        public long Offset { get; }
+    }
+}

MatFileHandler/SubsystemData.cs

@@ -0,0 +1,144 @@
+// Copyright 2017-2018 Alexander Luzgarev
+
+using System;
+using System.Collections.Generic;
+
+namespace MatFileHandler
+{
+    /// <summary>
+    /// "Subsystem data" of the .mat file.
+    /// Subsystem data stores the actual contents
+    /// of all the "opaque objects" in the file.
+    /// </summary>
+    internal class SubsystemData
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="SubsystemData"/> class.
+        /// Default constructor: initializes everything to null.
+        /// </summary>
+        public SubsystemData()
+        {
+            ClassInformation = null;
+            ObjectInformation = null;
+            Data = null;
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="SubsystemData"/> class.
+        /// The actual constructor.
+        /// </summary>
+        /// <param name="classInformation">Information about the classes.</param>
+        /// <param name="objectInformation">Information about the objects.</param>
+        /// <param name="data">Field values.</param>
+        public SubsystemData(ClassInfo[] classInformation, ObjectInfo[] objectInformation, Dictionary<int, IArray> data)
+        {
+            this.ClassInformation =
+                classInformation ?? throw new ArgumentNullException(nameof(classInformation));
+            this.ObjectInformation =
+                objectInformation ?? throw new ArgumentNullException(nameof(objectInformation));
+            this.Data = data ?? throw new ArgumentNullException(nameof(data));
+        }
+
+        /// <summary>
+        /// Gets or sets information about all the classes occurring in the file.
+        /// </summary>
+        public ClassInfo[] ClassInformation { get; set; }
+
+        /// <summary>
+        /// Gets or sets the actual data: mapping of "object field" indices to their values.
+        /// </summary>
+        public IReadOnlyDictionary<int, IArray> Data { get; set; }
+
+        /// <summary>
+        /// Gets or sets information about all the objects occurring in the file.
+        /// </summary>
+        public ObjectInfo[] ObjectInformation { get; set; }
+
+        /// <summary>
+        /// Initialize this object from another object.
+        /// This ugly hack allows us to read the opaque objects and store references to
+        /// the subsystem data in them before parsing the actual subsystem data (which
+        /// comes later in the file).
+        /// </summary>
+        /// <param name="data">Another subsystem data.</param>
+        public void Set(SubsystemData data)
+        {
+            this.ClassInformation = data.ClassInformation;
+            this.ObjectInformation = data.ObjectInformation;
+            this.Data = data.Data;
+        }
+
+        /// <summary>
+        /// Stores information about a class.
+        /// </summary>
+        internal class ClassInfo
+        {
+            private readonly string[] fieldNames;
+
+            private readonly Dictionary<string, int> fieldToIndex;
+
+            /// <summary>
+            /// Initializes a new instance of the <see cref="ClassInfo"/> class.
+            /// </summary>
+            /// <param name="name">Class name.</param>
+            /// <param name="fieldNames">Names of the fields.</param>
+            public ClassInfo(string name, string[] fieldNames)
+            {
+                Name = name ?? throw new ArgumentNullException(nameof(name));
+                this.fieldNames = fieldNames ?? throw new ArgumentNullException(nameof(fieldNames));
+                fieldToIndex = new Dictionary<string, int>();
+                for (var i = 0; i < fieldNames.Length; i++)
+                {
+                    fieldToIndex[fieldNames[i]] = i;
+                }
+            }
+
+            /// <summary>
+            /// Gets names of the fields in the class.
+            /// </summary>
+            public IReadOnlyCollection<string> FieldNames => fieldNames;
+
+            /// <summary>
+            /// Gets name of the class.
+            /// </summary>
+            public string Name { get; }
+
+            /// <summary>
+            /// Find a field index given its name.
+            /// </summary>
+            /// <param name="fieldName">Field name.</param>
+            /// <returns>Field index.</returns>
+            public int? FindField(string fieldName)
+            {
+                if (fieldToIndex.TryGetValue(fieldName, out var index))
+                {
+                    return index;
+                }
+
+                return null;
+            }
+        }
+
+        /// <summary>
+        /// Stores information about an object.
+        /// </summary>
+        internal class ObjectInfo
+        {
+            private readonly Dictionary<int, int> fieldLinks;
+
+            /// <summary>
+            /// Initializes a new instance of the <see cref="ObjectInfo"/> class.
+            /// </summary>
+            /// <param name="fieldLinks">A dictionary mapping the field indices to "field values" indices.</param>
+            public ObjectInfo(Dictionary<int, int> fieldLinks)
+            {
+                this.fieldLinks = fieldLinks ?? throw new ArgumentNullException(nameof(fieldLinks));
+            }
+
+            /// <summary>
+            /// Gets mapping between the field indices and "field values" indices.
+            /// </summary>
+            public IReadOnlyDictionary<int, int> FieldLinks => fieldLinks;
+        }
+    }
+}

MatFileHandler/SubsystemDataReader.cs

@@ -0,0 +1,168 @@
+// Copyright 2017-2018 Alexander Luzgarev
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+
+namespace MatFileHandler
+{
+    /// <summary>
+    /// Reader for "subsystem data" in .mat files.
+    /// </summary>
+    internal class SubsystemDataReader
+    {
+        /// <summary>
+        /// Read subsystem data from a given byte array.
+        /// </summary>
+        /// <param name="bytes">Byte array with the data.</param>
+        /// <returns>Subsystem data read.</returns>
+        public static SubsystemData Read(byte[] bytes)
+        {
+            List<RawVariable> rawVariables = null;
+            using (var stream = new MemoryStream(bytes))
+            {
+                using (var reader = new BinaryReader(stream))
+                {
+                    reader.ReadBytes(8);
+                    rawVariables = MatFileReader.ReadRawVariables(reader, -1);
+                }
+            }
+
+            // Parse subsystem data.
+            var mainVariable = rawVariables[0].DataElement as IStructureArray;
+            var mcosData = mainVariable["MCOS", 0] as Opaque;
+            var opaqueData = mcosData.RawData as ICellArray;
+            var info = (opaqueData[0] as IArrayOf<byte>).Data;
+            var (offsets, position) = ReadOffsets(info, 0);
+            var fieldNames = ReadFieldNames(info, position, offsets[1]);
+            var numberOfClasses = ((offsets[3] - offsets[2]) / 16) - 1;
+            SubsystemData.ClassInfo[] classInformation = null;
+            using (var stream = new MemoryStream(info, offsets[2], offsets[3] - offsets[2]))
+            {
+                using (var reader = new BinaryReader(stream))
+                {
+                    classInformation = ReadClassInformation(reader, fieldNames, numberOfClasses);
+                }
+            }
+            var numberOfObjects = ((offsets[5] - offsets[4]) / 24) - 1;
+            SubsystemData.ObjectInfo[] objectInformation = null;
+            using (var stream = new MemoryStream(info, offsets[5], offsets[6] - offsets[5]))
+            {
+                using (var reader = new BinaryReader(stream))
+                {
+                    objectInformation = ReadObjectInformation(reader, numberOfObjects);
+                }
+            }
+
+            var allFields = objectInformation.SelectMany(obj => obj.FieldLinks.Values);
+            var data = new Dictionary<int, IArray>();
+            foreach (var i in allFields)
+            {
+                data[i] = opaqueData[i + 2];
+            }
+            return new SubsystemData(classInformation, objectInformation, data);
+        }
+
+        private static SubsystemData.ObjectInfo ReadObjectInformation(BinaryReader reader)
+        {
+            var length = reader.ReadInt32();
+            var fieldLinks = new Dictionary<int, int>();
+            for (var i = 0; i < length; i++)
+            {
+                var x = reader.ReadInt32();
+                var y = reader.ReadInt32();
+                var index = x * y;
+                var link = reader.ReadInt32();
+                fieldLinks[index] = link;
+            }
+            return new SubsystemData.ObjectInfo(fieldLinks);
+        }
+
+        private static SubsystemData.ObjectInfo[] ReadObjectInformation(BinaryReader reader, int numberOfObjects)
+        {
+            var result = new SubsystemData.ObjectInfo[numberOfObjects];
+            reader.ReadBytes(8);
+            for (var objectIndex = 0; objectIndex < numberOfObjects; objectIndex++)
+            {
+                result[objectIndex] = ReadObjectInformation(reader);
+                var position = reader.BaseStream.Position;
+                if (position % 8 != 0)
+                {
+                    reader.ReadBytes(8 - (int)(position % 8));
+                }
+            }
+            return result;
+        }
+
+        private static SubsystemData.ClassInfo[] ReadClassInformation(
+            BinaryReader reader,
+            string[] fieldNames,
+            int numberOfClasses)
+        {
+            var result = new SubsystemData.ClassInfo[numberOfClasses];
+            var indices = new int[numberOfClasses + 1];
+            for (var i = 0; i <= numberOfClasses; i++)
+            {
+                reader.ReadInt32();
+                indices[i] = reader.ReadInt32();
+                reader.ReadInt32();
+                reader.ReadInt32();
+            }
+
+            for (var i = 0; i < numberOfClasses; i++)
+            {
+                var numberOfFields = indices[i + 1] - indices[i] - 1;
+                var names = new string[numberOfFields];
+                Array.Copy(fieldNames, indices[i], names, 0, numberOfFields);
+                var className = fieldNames[indices[i + 1] - 1];
+                result[i] = new SubsystemData.ClassInfo(className, names);
+            }
+
+            return result;
+        }
+
+        private static (int[] offsets, int newPosition) ReadOffsets(byte[] bytes, int startPosition)
+        {
+            var position = startPosition;
+            var offsets = new List<int>();
+            while (true)
+            {
+                var next = BitConverter.ToInt32(bytes, position);
+                position += 4;
+                if (next == 0)
+                {
+                    if (position % 8 != 0)
+                    {
+                        position += 4;
+                    }
+
+                    break;
+                }
+                offsets.Add(next);
+            }
+
+            return (offsets.ToArray(), position);
+        }
+
+        private static string[] ReadFieldNames(byte[] bytes, int startPosition, int numberOfFields)
+        {
+            var result = new string[numberOfFields];
+            var position = startPosition;
+            for (var i = 0; i < numberOfFields; i++)
+            {
+                var list = new List<byte>();
+                while (bytes[position] != 0)
+                {
+                    list.Add(bytes[position]);
+                    position++;
+                }
+                result[i] = Encoding.ASCII.GetString(list.ToArray());
+                position++;
+            }
+
+            return result;
+        }
+    }
+}