Marshalling a big-endian byte collection into a struct in order to pull out values

Question

There is an insightful question about reading a C/C++ data structure in C# from a byte array, but I cannot get the code to work for my collection of big-endian (network byte order) bytes. (EDIT: Note that my real struct has more than just one field.) Is there a way to marshal the bytes into a big-endian version of the structure and then pull out the values in the endianness of the framework (that of the host, which is usually little-endian)?

(Note, reversing the array of bytes will not work - each value's bytes must be reversed, which does not give you the same collection as reversing all of the bytes.)

This should summarize what I'm looking for (LE=LittleEndian, BE=BigEndian):

void Main()
{
    var leBytes = new byte[] {1, 0, 2, 0};
    var beBytes = new byte[] {0, 1, 0, 2};
    Foo fooLe = ByteArrayToStructure<Foo>(leBytes);
    Foo fooBe = ByteArrayToStructureBigEndian<Foo>(beBytes);
    Assert.AreEqual(fooLe, fooBe);
}

[StructLayout(LayoutKind.Explicit, Size=4)]
public struct Foo  {
    [FieldOffset(0)] 
    public ushort firstUshort;
    [FieldOffset(2)] 
    public ushort secondUshort;
}

T ByteArrayToStructure<T>(byte[] bytes) where T: struct 
{
    GCHandle handle = GCHandle.Alloc(bytes, GCHandleType.Pinned);
    T stuff = (T)Marshal.PtrToStructure(handle.AddrOfPinnedObject(),typeof(T));
    handle.Free();
    return stuff;
}

T ByteArrayToStructureBigEndian<T>(byte[] bytes) where T: struct 
{
    ???
}

Other helpful links:

Byte of a struct and onto endian concerns

A little more on bytes and endianness (byte order)

Read binary files more efficiently using C#

Unsafe and reading from files

Mono's contribution to the issue

Mastering C# structs

Answer 1

Here's another solution for swapping endianness.

It's adjusted from Adam Robinsons solution here: https://stackoverflow.com/a/2624377/1254743

It's even capable of handling nested structs.

public static class FooTest
{
    [StructLayout(LayoutKind.Sequential, Pack = 1)]
    public struct Foo2
    {
        public byte b1;
        public short s;
        public ushort S;
        public int i;
        public uint I;
        public long l;
        public ulong L;
        public float f;
        public double d;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 10)]
        public string MyString;
    }

    [StructLayout(LayoutKind.Sequential, Pack = 1)]
    public struct Foo
    {
        public byte b1;
        public short s;
        public ushort S;
        public int i;
        public uint I;
        public long l;
        public ulong L;
        public float f;
        public double d;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 10)]
        public string MyString;
        public Foo2 foo2;
    }

    public static void test()
    {
        Foo2 sample2 = new Foo2()
        {
            b1 = 0x01,
            s = 0x0203,
            S = 0x0405,
            i = 0x06070809,
            I = 0x0a0b0c0d,
            l = 0xe0f101112131415,
            L = 0x161718191a1b1c,
            f = 1.234f,
            d = 4.56789,
            MyString = @"123456789", // null terminated => only 9 characters!
        };

        Foo sample = new Foo()
        {
            b1 = 0x01,
            s = 0x0203,
            S = 0x0405,
            i = 0x06070809,
            I = 0x0a0b0c0d,
            l = 0xe0f101112131415,
            L = 0x161718191a1b1c,
            f = 1.234f,
            d = 4.56789,
            MyString = @"123456789", // null terminated => only 9 characters!
            foo2 = sample2,
        };

        var bytes_LE = Dummy.StructToBytes(sample, Endianness.LittleEndian);
        var restoredLEAsLE = Dummy.BytesToStruct<Foo>(bytes_LE, Endianness.LittleEndian);
        var restoredLEAsBE = Dummy.BytesToStruct<Foo>(bytes_LE, Endianness.BigEndian);

        var bytes_BE = Dummy.StructToBytes(sample, Endianness.BigEndian);
        var restoredBEAsLE = Dummy.BytesToStruct<Foo>(bytes_BE, Endianness.LittleEndian);
        var restoredBEAsBE = Dummy.BytesToStruct<Foo>(bytes_BE, Endianness.BigEndian);

        Debug.Assert(sample.Equals(restoredLEAsLE));
        Debug.Assert(sample.Equals(restoredBEAsBE));
        Debug.Assert(restoredBEAsLE.Equals(restoredLEAsBE));
    }

    public enum Endianness
    {
        BigEndian,
        LittleEndian
    }

    private static void MaybeAdjustEndianness(Type type, byte[] data, Endianness endianness, int startOffset = 0)
    {
        if ((BitConverter.IsLittleEndian) == (endianness == Endianness.LittleEndian))
        {
            // nothing to change => return
            return;
        }

        foreach (var field in type.GetFields())
        {
            var fieldType = field.FieldType;
            if (field.IsStatic)
                // don't process static fields
                continue;

            if (fieldType == typeof(string)) 
                // don't swap bytes for strings
                continue;

            var offset = Marshal.OffsetOf(type, field.Name).ToInt32();

            // handle enums
            if (fieldType.IsEnum)
                fieldType = Enum.GetUnderlyingType(fieldType);

            // check for sub-fields to recurse if necessary
            var subFields = fieldType.GetFields().Where(subField => subField.IsStatic == false).ToArray();

            var effectiveOffset = startOffset + offset;

            if (subFields.Length == 0)
            {
                Array.Reverse(data, effectiveOffset, Marshal.SizeOf(fieldType));
            }
            else
            {
                // recurse
                MaybeAdjustEndianness(fieldType, data, endianness, effectiveOffset);
            }
        }
    }

    internal static T BytesToStruct<T>(byte[] rawData, Endianness endianness) where T : struct
    {
        T result = default(T);

        MaybeAdjustEndianness(typeof(T), rawData, endianness);

        GCHandle handle = GCHandle.Alloc(rawData, GCHandleType.Pinned);

        try
        {
            IntPtr rawDataPtr = handle.AddrOfPinnedObject();
            result = (T)Marshal.PtrToStructure(rawDataPtr, typeof(T));
        }
        finally
        {
            handle.Free();
        }

        return result;
    }

    internal static byte[] StructToBytes<T>(T data, Endianness endianness) where T : struct
    {
        byte[] rawData = new byte[Marshal.SizeOf(data)];
        GCHandle handle = GCHandle.Alloc(rawData, GCHandleType.Pinned);
        try
        {
            IntPtr rawDataPtr = handle.AddrOfPinnedObject();
            Marshal.StructureToPtr(data, rawDataPtr, false);
        }
        finally
        {
            handle.Free();
        }

        MaybeAdjustEndianness(typeof(T), rawData, endianness);

        return rawData;
    }

}

Answer 2

As alluded to in my comment on @weismat's answer, there is an easy way to achieve big-endian structuring. It involves a double-reversal: the original bytes are reversed entirely, then the struct itself is the reversal of the original (big-endian) data format.

The fooLe and fooBe in Main will have the same values for all fields. (Normally, the little-endian struct and bytes wouldn't be present, of course, but this clearly shows the relationship between the byte orders.)

NOTE: See updated code including how to get bytes back out of the struct.

public void Main()
{
    var beBytes = new byte[] {
        0x80, 
        0x80,0, 
        0x80,0, 
        0x80,0,0,0, 
        0x80,0,0,0,
        0x80,0,0,0,0,0,0,0, 
        0x80,0,0,0,0,0,0,0, 
        0x3F,0X80,0,0, // float of 1 (see http://en.wikipedia.org/wiki/Endianness#Floating-point_and_endianness)
        0x3F,0xF0,0,0,0,0,0,0, // double of 1
        0,0,0,0x67,0x6E,0x69,0x74,0x73,0x65,0x54 // Testing\0\0\0
    };
    var leBytes = new byte[] {
        0x80, 
        0,0x80,
        0,0x80, 
        0,0,0,0x80,
        0,0,0,0x80, 
        0,0,0,0,0,0,0,0x80, 
        0,0,0,0,0,0,0,0x80, 
        0,0,0x80,0x3F, // float of 1
        0,0,0,0,0,0,0xF0,0x3F, // double of 1
        0x54,0x65,0x73,0x74,0x69,0x6E,0x67,0,0,0 // Testing\0\0\0
    };
    Foo fooLe = ByteArrayToStructure<Foo>(leBytes).Dump("LE");
    FooReversed fooBe = ByteArrayToStructure<FooReversed>(beBytes.Reverse().ToArray()).Dump("BE");  
}

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct Foo  {
    public byte b1;
    public short s;
    public ushort S;
    public int i;
    public uint I;
    public long l;
    public ulong L;
    public float f;
    public double d;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 10)]
    public string MyString;
}

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct FooReversed  {
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 10)]
    public string MyString;
    public double d;
    public float f;
    public ulong L;
    public long l;
    public uint I;
    public int i;
    public ushort S;
    public short s;
    public byte b1;
}

T ByteArrayToStructure<T>(byte[] bytes) where T: struct 
{
    GCHandle handle = GCHandle.Alloc(bytes, GCHandleType.Pinned);
    T stuff = (T)Marshal.PtrToStructure(handle.AddrOfPinnedObject(),typeof(T));
    handle.Free();
    return stuff;
}

Answer 3

It seems there must be a more elegant solution, but this should at least get you going:

    static T ByteArrayToStructureBigEndian<T>(byte[] bytes) where T : struct
    {
        GCHandle handle = GCHandle.Alloc(bytes, GCHandleType.Pinned);
        T stuff = (T)Marshal.PtrToStructure(handle.AddrOfPinnedObject(), typeof(T));
        handle.Free();
        System.Type t = stuff.GetType();
        FieldInfo[] fieldInfo = t.GetFields();
        foreach (FieldInfo fi in fieldInfo)
        {                 
            if (fi.FieldType == typeof(System.Int16))
            {
                // TODO
            }
            else if (fi.FieldType == typeof(System.Int32))
            {
                // TODO
            }
            else if (fi.FieldType == typeof(System.Int64))
            {
                // TODO
            }
            else if (fi.FieldType == typeof(System.UInt16))
            {
                UInt16 i16 = (UInt16)fi.GetValue(stuff);
                byte[] b16 = BitConverter.GetBytes(i16);
                byte[] b16r = b16.Reverse().ToArray();
                fi.SetValueDirect(__makeref(stuff), BitConverter.ToUInt16(b16r, 0);
            }
            else if (fi.FieldType == typeof(System.UInt32))
            {
                // TODO
            }
            else if (fi.FieldType == typeof(System.UInt64))
            {
                // TODO
            }
        }
        return stuff;
    }