Reading a C/C++ data structure in C# from a byte array

Question

What would be the best way to fill a C# struct from a byte[] array where the data was from a C/C++ struct? The C struct would look something like this (my C is very rusty):

typedef OldStuff {
    CHAR Name[8];
    UInt32 User;
    CHAR Location[8];
    UInt32 TimeStamp;
    UInt32 Sequence;
    CHAR Tracking[16];
    CHAR Filler[12];
}

And would fill something like this:

[StructLayout(LayoutKind.Explicit, Size = 56, Pack = 1)]
public struct NewStuff
{
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 8)]
    [FieldOffset(0)]
    public string Name;

    [MarshalAs(UnmanagedType.U4)]
    [FieldOffset(8)]
    public uint User;

    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 8)]
    [FieldOffset(12)]
    public string Location;

    [MarshalAs(UnmanagedType.U4)]
    [FieldOffset(20)]
    public uint TimeStamp;

    [MarshalAs(UnmanagedType.U4)]
    [FieldOffset(24)]
    public uint Sequence;

    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 16)]
    [FieldOffset(28)]
    public string Tracking;
}

What is best way to copy OldStuff to NewStuff, if OldStuff was passed as byte[] array?

I'm currently doing something like the following, but it feels kind of clunky.

GCHandle handle;
NewStuff MyStuff;

int BufferSize = Marshal.SizeOf(typeof(NewStuff));
byte[] buff = new byte[BufferSize];

Array.Copy(SomeByteArray, 0, buff, 0, BufferSize);

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

MyStuff = (NewStuff)Marshal.PtrToStructure(handle.AddrOfPinnedObject(), typeof(NewStuff));

handle.Free();

Is there better way to accomplish this?

---

Would using the BinaryReader class offer any performance gains over pinning the memory and using Marshal.PtrStructure?

Solution 1

From what I can see in that context, you don't need to copy SomeByteArray into a buffer. You simply need to get the handle from SomeByteArray, pin it, copy the IntPtr data using PtrToStructure and then release. No need for a copy.

That would be:

NewStuff ByteArrayToNewStuff(byte[] bytes)
{
    GCHandle handle = GCHandle.Alloc(bytes, GCHandleType.Pinned);
    try
    {
        NewStuff stuff = (NewStuff)Marshal.PtrToStructure(handle.AddrOfPinnedObject(), typeof(NewStuff));
    }
    finally
    {
        handle.Free();
    }
    return stuff;
}

Generic version:

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

Simpler version (requires unsafe switch):

unsafe T ByteArrayToStructure<T>(byte[] bytes) where T : struct
{
    fixed (byte* ptr = &bytes[0])
    {
        return (T)Marshal.PtrToStructure((IntPtr)ptr, typeof(T));
    }
}

Solution 2

Here is an exception safe version of the accepted answer:

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

Solution 3

Watch out for packing issues. In the example you gave all fields are at the obvious offsets because everything is on 4 byte boundaries but this will not always be the case. Visual C++ packs on 8 byte boundaries by default.

Solution 4

object ByteArrayToStructure(byte[] bytearray, object structureObj, int position)
{
    int length = Marshal.SizeOf(structureObj);
    IntPtr ptr = Marshal.AllocHGlobal(length);
    Marshal.Copy(bytearray, 0, ptr, length);
    structureObj = Marshal.PtrToStructure(Marshal.UnsafeAddrOfPinnedArrayElement(bytearray, position), structureObj.GetType());
    Marshal.FreeHGlobal(ptr);
    return structureObj;
}   

Have this

View more solutions

Comments

[Chris L]

"Visual C++ packs on 8 byte boundaries by default." This solved my issue, thanks a lot!

[SSpoke]

CS0411 The type arguments for method 'ByteArrayToStructure<T>(byte[], int)' cannot be inferred from the usage. Try specifying the type arguments explicitly. (I added int index of byte array) to it.

[cdiggins]

Will leak memory in the presence of exceptions. See: stackoverflow.com/a/41836532/184528 for a safer version.

[Admin]

As of 4.5.1, there's a generic version of PtrToStructure, so the second line in the generic version, above, can become: var stuff = Marshal.PtrToStructure<T>(handle.AddrOfPinnedObject());

[cdiggins]

@Ben-Collins The accepted answer was edited after I added my answer.