Getting byte array from C into C#

Question

I have the following C function that I need to call from C#:

__declspec(dllexport) int receive_message(char* ret_buf, int buffer_size);

I've declared the following on the C# side:

[DllImport("MyCLibrary", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Auto, EntryPoint = "receive_message")]
public static extern int ReceiveMessage([MarshalAs(UnmanagedType.LPStr)]StringBuilder retBuf, int bufferSize);

I'm calling the function like so:

StringBuilder sb = new StringBuilder();
int len = ReceiveMessage(sb, 512);

This works fine with my initial tests where I was receiving "string" messages. But, now I want to receive packed messages (array of chars/bytes). The problem is that the array of chars/bytes will have 0s and will terminate the string so I don't get back the whole message. Any ideas how I can refactor to get array of bytes back?

Use Marshal.Copy(IntPtr source, byte[] destination, int startIndex, int length). — jdweng
– jdweng, Commented Apr 26, 2016 at 16:36
@jdweng, I'm not following. Do I need to change my extern declaration in my C#? — bsh152s
– bsh152s, Commented Apr 26, 2016 at 17:48
You are fibbing about the bufferSize argument, the Capacity of that StringBuilder is 0. The heap corruption that this can cause can be very unpleasant. If it is not actually a string then you must declare the 1st argument as byte[] instead. And same thing, pass its Length property as the 2nd argument. — Hans Passant
– Hans Passant, Commented Apr 26, 2016 at 17:57
I did notice my memory was slowly growing...maybe that was why. I think I figured out the Marshall.Copy that jdweng recommended. I'll post full answer once I confirm. — bsh152s
– bsh152s, Commented Apr 26, 2016 at 19:16

bsh152s · Accepted Answer · 2016-05-12 14:38:22Z

2

With jdweng help, I've changed the declaration to:

[DllImport("MyCLibrary", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Auto, EntryPoint = "receive_message")]
public static extern int ReceiveMessage(IntPtr retBuf, int bufferSize);

And, I'm allocating and freeing the memory on the C# side along with marshalling the data.

IntPtr pnt = Marshall.AllocHGlobal(512);
try
{
   int len = ReceiveMessage(pnt, 512);
   ...
   byte[] bytes = new byte[len];
   Marshal.Copy(pnt, bytes, 0, len);
   ...
}
finally
{
   Marshal.FreeHGlobal(pnt);
}

answered May 12, 2016 at 14:38

bsh152s

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