C# - How to pin the StringBuilder Object when passing to Unmanaged Code

[firstquestion]

Hello,

I need to pass a StringBuilder object to a custom unmanaged DLL. The code most of the time works great, however once in a while I get "Object reference not set to an instance of an object" which means the GC moved the address of StringBuilder object while managed code is writing to it.

I have tried GC.keepalive already. GCHandle.allc and Marshal.alloc doesn't seem to be possible with this type of object. I need the equivalent of (__pin *) in managed C++ to pin the object somehow.

UpdateStatus(DWORD type, DWORD action, LPTSTR buffer, DWORD bufferSize);
 
DLL Declare
private static extern int UpdateStatus(int type, int action, StringBuilder buffer, int bufferSize);
 

StringBuilder buffer = new StringBuilder(256);
// error happens here.
UpdateStatus(1, 1, buffer, buffer.Capacity); 
 

Note that I am careful inside the DLL not to write passed the end of size passed.

StringBuilder buffer = new StringBuilder(256);
// this obviously always works 
UpdateStatus(1, 1, null, 0); 
 

Any suggestions is greatly appreciated.
Thanks.