Question

I’m using a C++ dll containing a method with this signature:

int GetEnumerationString(int type, int value, const char** s ); /* ALLOCATES */

As mentioned in the header comment, the method allocates the pointer. I tried the following signature in C#:

[DllImport("thedll.dll", EntryPoint="GetEnumerationString")]
private static extern int GetEnumerationString(int type, int value, ref string s );

but when I run my program, I get an AccessViolationException.

What is the right signature for the method?

Answer 1

If an unmanaged function allocates a buffer, it must use the CLR allocator in order for the CLR to pick up the memory and free it when needed.

If that is not possible, you must return a pointer:

[DllImport("thedll.dll", EntryPoint="GetEnumerationString")]
private static extern int GetEnumerationString(int type, int value, out IntPtr s);

Then manually figure a string out of it and dispose the pointed to memory (for which you must know what the allocator was).