How to read string from pointer to buffer in C#

Question

Hi,

How can I read the error string in C# from this C++ dll call?

//
//  PARAMETERS:
//   objptr
//    Pointer to class instance.
//
//   pBuffer
//    Pointer to buffer receiving NULL terminated error message string. 
//    If this value is zero, the function returns the required buffer size, in bytes,
//    and makes no use of the pBuffer. 
//
//   nSize
//    Size of receiving buffer.
//    If this value is zero, the function returns the required buffer size, in bytes,
//    and makes no use of the pBuffer. 
//
//  RETURN VALUES:
//   If pBuffer or nSize is zero, the function returns the required buffer size, in bytes.
//   If the function succeeds, the return value is number of bytes copied into pBuffer.
//   If function fails return value is 0.
//
extern unsafe int GetError(uint objptr, byte* pBuffer, int nSize);

thanks!

Answer 1

If you change the data type from byte* to IntPtr this might work:

Marshal.PtrToStringAnsi

Or one of the string constructors (one of them also includes an Encoding parameter):

String Constructors

Answer 2

Marshal.AllocHGlobal(int) plus Marshal.PtrToStringAuto(IntPtr, int) perhaps?

Some code context would be nice, as I'm assuming that you're already coercing byte* into an IntPtr using p/invoke or some other trickery.

Basically, call your function to get the buffer size, allocate a buffer using AllocHGlobal(), call it again, read the string in, then free the buffer (using Marshall.FreeHGlobal(IntPtr)). This is assuming you can use Marshall's allocator, of course.

Answer 3

byte[] buffer = new byte[1000];
int size;
unsafe
{
  fixed ( byte* p = buffer )
  {
    size = GetError( ???, p, buffer.Length ); 
  }
}
string result = System.Text.Encoding.Default.GetString( buffer, 0, size );