How to pass this pointer as context parameter for callback in c#

Question

In C++ it's common to pass the 'this' pointer to a callback registration method so that the callback will receive this pointer back. I have a 3rd party library that i am trying to interop with from C# that uses this pattern.

Ex.

typedef HRESULT (WINAPI PFNCALLBACK*)(CALLBACKDATA *data, void *context);

HRESULT SetCallback (PFNCALLBACK *callback, void *context);

I've defined a delegate for the callback function:

public delegate int Callback(IntPtr context, CallbackData sample);

and call the SetCallback function

void SetupCallback()
{
  // My stab at passing the this pointer as context for the callback
  IntPtr pThis = GCHandle.ToIntPtr(GCHandle.Alloc(this));
  hr = obj.SetCallback(OnCallback, pThis);
}

public static int OnCallback(CallbackData data, IntPtr context)
{
   // HOW do I reconstitute the This pointer from the context parameter
}

My question is how do I pass a C# 'this' pointer to the SetCallback method and reconstitute it in the callback function?

`OnCallback` is an instance method. What's wrong with the `this` reference it's already got? — 15ee8f99-57ff-4f92-890c-b56153, Aug 25 '16 at 18:55
Fixed. OnCallback is static because it is not passed the implicit 'this' pointer. — wta, Aug 25 '16 at 18:58
Makes sense. Wouldn't expect a C function to be able to invoke a .NET delegate properly when all it's expecting is a pointer to a C function with whatever calling convention. — 15ee8f99-57ff-4f92-890c-b56153, Aug 25 '16 at 19:00

score 1 · Accepted Answer · answered Aug 25 '16 at 20:27

Ugh, you are asking for an interop nightmare, especially since you are dealing with a "black box" third party DLL.

Having said that, theoretically this should be possible by utilizing the UnmanagedFunctionPointer attribute.

[UnmanagedFunctionPointer(CallingConvention.StdCall)]
public delegate int Callback(IntPtr context, CallbackData sample);

Then store off your delegate

static Callback _someCallBack = null;
_someCallBack += new Callback(OnCallback);

Finally pass in the delegate when calling the C code

IntPtr pThis = GCHandle.ToIntPtr(GCHandle.Alloc(this));
hr = obj.SetCallback(_someCallBack, pThis);

I also don't think you even really need to mess with the this pointer, unless you really need it in the callback honestly I would try passing in null. It's not safe in .Net land to marshall and marshall references back and re-use it.

Actually, I am dealing with a black box .lib file for which I created a dll that wraps it. The .lib has a function that creates an unregistered COM object. The COM object has an interface with the callback registraton method. — wta, Aug 26 '16 at 02:10

How to pass this pointer as context parameter for callback in c#

1 Answers1