Question

I have an interface, IProxy, and an implemenation Proxy. The purpose of the proxy is to wrap some extern functions which call into a C library. For the extern declaritions this requires that the structs be passed using the ref keyword. Here’s a sample:

namespace CFuncs {
  [DllImport("cLibrary.dll", CallingConvention = CallingConvention.Cdecl)]
  public static extern int MyCFunc(ref MyStruct result);
}

public interface IProxy { public int MyFunc(MyStruct result); }

public class Proxy : IProxy { 
  public int MyFunc(MyStruct result) { 
    return CFuncs.MyCFunc(ref result);
  }
}

The function declartion from the C header file:

int MyCFunc (MY_STRUCT* result);

typedef struct  { 
  // some fields
} MY_STRUCT;

And the calling code:

var proxy = new Proxy();
var result = new MyStruct();
proxy.MyFunc(result);

My question is should I be using ref in the wrapper interface and implemenation as well, or will the struct be properly filled with data by the C function?

EDIT: The code presented as a sample DOES seem to work. Could that be that even though the C library wants a pointer it never actually does change it? Is ref a requirement for interop-ing with functions that take pointers? The sample C code that comes with the library always has the caller allocating memory for the structs it takes, if that means anything.

Answer 1

.NET uses call by value, and moreover struct is a value type — hence, if you want it changed by an external function, it should be a ref parameter.