How to read string from pointer to buffer in C#

Question

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!

score 4 · Answer 1 · answered Dec 27 '09 at 10:20

4

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

answered Dec 27 '09 at 10:20

Aviad P.

32,036
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Dan Byström · Accepted Answer · 2009-12-27T10:58:10.373

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 );

edited Dec 27 '09 at 10:58

answered Dec 27 '09 at 10:50

Dan Byström

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I used GetError(???, null, 0) to get the message size in order to create the buffer and worked also perfect. Thanks! – Santiago Corredoira Dec 27 '09 at 11:42
You could also try to change byte* to StringBuilder and pass that instead and avoid the unsafe code. – Mikael Svenson Dec 27 '09 at 12:06
@Mikael: how can I do it? The dll method expects byte* – Santiago Corredoira Dec 27 '09 at 15:44
@Dawkins: what the method really expects is a 32 bit pointer. What it points to doesn't really matter as long as it points to something that it can write to without causing a protection fault. Using a StringBuilder will work, just as Mikael says - as long as you remember to allocate sufficient memory in the StringBuilder. Since your question specifically stated a byte* and unsafe code - I just went down that road in my answer. There exists quite a few more possible solutions. – Dan Byström Dec 27 '09 at 18:26
thanks danbystrom, but how should I pass the Stringbuilder? The compiler throws an exception if I just replace the byte* with it. – Santiago Corredoira Dec 28 '09 at 11:38
It should not. Not if you change the declaration and then pass it a StringBuilder. – Dan Byström Dec 28 '09 at 13:11

score 0 · Answer 3 · answered Dec 27 '09 at 10:24

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.

How to read string from pointer to buffer in C#

3 Answers3