Passing struct with char* to C++ dll function in C#

Question

I'm using a C++ dll in C# and have problems with passing char* to it and back.

First of all the function looks the following:

[DllImport("xyz.dll")]
public static extern int ReadWrite_VCP(IntPtr handle, ref DATA_BUFFER readBuffer, ref DATA_BUFFER writeBuffer, int timeout);

The struct is defined as:

public struct DATA_BUFFER
{
    public string buffer;
    [MarshalAs(UnmanagedType.U4)]
    public int length;
    [MarshalAs(UnmanagedType.U4)]
    public int transferCount;
}

The function call is:

ReadWrite_VCP(handle, ref rBuffer, ref wrBuffer, 5000);

In general this works fine, but the problem is, that the ReadWrite_VCP function writes a char array (including '\\0' characters). This results in the problem that if the data is '\\0' '1' '2' '3' , The string is empty as it is terminated by '\\0'.

I've already tried to use Stringbuilder or byte array, without any success.

On C++ Side the function looks like:

typedef struct 
{
    UCHAR *buffer;
    UINT32 length; 
    UINT32 transferCount; 
} DATA_BUFFER ;

And the Function:

unsigned int ReadWrite_VCP (HANDLE handle, DATA_BUFFER* ReadBuffer, DATA_BUFFER* writeBuffer, UINT32 timeout);

I hope anyone can help me!

Thank you!

Answer 1

Yes you were totally right!

my struct changed to:

        public struct DATA_BUFFER
    {
        public IntPtr  buffer;
        [MarshalAs(UnmanagedType.U4)]
        public int length;
        [MarshalAs(UnmanagedType.U4)]
        public int transferCount;
    }

and before I call the function a allocate a Byte array and set the pointer to it

 buf.length = 10;
 byte[] input = new byte[buf.length];
 buf.buffer = Marshal.UnsafeAddrOfPinnedArrayElement(input, 0);

Thank you!