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!

Aviad P. · Accepted Answer · 2009-12-27 10:20:40Z

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, 2009 at 10:20

Aviad P.

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Dan Byström · Accepted Answer · 2009-12-27 10:58:10Z

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, 2009 at 10:58

answered Dec 27, 2009 at 10:50

Dan Byström

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6 Comments

Santiago Corredoira Over a year ago

I used GetError(???, null, 0) to get the message size in order to create the buffer and worked also perfect. Thanks!

Mikael Svenson Over a year ago

You could also try to change byte* to StringBuilder and pass that instead and avoid the unsafe code.

Santiago Corredoira Over a year ago

@Mikael: how can I do it? The dll method expects byte*

Dan Byström Over a year ago

@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.

Santiago Corredoira Over a year ago

thanks danbystrom, but how should I pass the Stringbuilder? The compiler throws an exception if I just replace the byte* with it.

|

Kevin Montrose · Accepted Answer · 2009-12-27 10:24:41Z

0

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.

answered Dec 27, 2009 at 10:24

Kevin Montrose

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