How to pass a type from DLL into generic method?

Question

Hi I am trying to get my first generic method work. Here is what I want to do: Based on diffent type of users we need to use different DLL. The DLL is COM object wrapper which expose a function that can be called. The original code looks like the following: using System.Runtime.InteropServices; using AsynchWrapper1; using AsynchWrapper2; using AsynchWrapper3; .... namespace MyNameSpace { public class Unility { public static void CallAsynchWrapper(int clientType int clientId) { if (clientType ==0) { clsAsynchWrapper1 asynchWrapper = new clsAsynchWrapper1(); asynchWrapper.XYZ(clientId); Marshal.ReleaseComObject(asynchWrapper); asynchWrapper = null; } else if (clientType ==1) { clsAsynchWrapper2 asynchWrapper = new clsAsynchWrapper2(); asynchWrapper.XYZ(clientId); Marshal.ReleaseComObject(asynchWrapper); asynchWrapper = null; } else { clsAsynchWrapper3 asynchWrapper = new clsAsynchWrapper3(); asynchWrapper.XYZ(clientId); Marshal.ReleaseComObject(asynchWrapper); asynchWrapper = null; } } } Where XYZ is the function that all 3 dlld expose. I want to make a generic method by modifying the CallAsynchWrapper function doing the following: public static void CallAsynchWrapper(int clientType int clientId) { if (clientType ==0) { clsAsynchWrapper1 asynchWrapper = new clsAsynchWrapper1(); genericCallAsynchWrapper (asynchWrapper clientId); } else if (clientType ==1) { clsAsynchWrapper2 asynchWrapper = new clsAsynchWrapper2(); genericCallAsynchWrapper (asynchWrapper clientId); } else { clsAsynchWrapper3 asynchWrapper = new clsAsynchWrapper3(); genericCallAsynchWrapper (asynchWrapper clientId); } } and add a generic method: static void genericCallAsynchWrapper (T asynchWrapper int clientId) { asynchWrapper.XYZ(clientId); Marshal.ReleaseComObject(asynchWrapper); asynchWrapper = default(T); } When I compile it I got error message” T doesn’t have a definition of XYZ.” My first question is: Is that possible to make a generic method for what I am trying to do? If that’s possible how can I make compiler knows that T does have XYZ? Does anybody know the answer? Thanks in advance for any help!

Alan · Answer

IUnknown is a very basic interface which all COM objects implement. It contains just three methods: QueryInterface()AddRef() and Release(). The first of these returns a reference to an interface which the COM object implements given its ID and the other two are concerned with 'reference counting' which control object lifetime. However when a .NET program interacts with a COM object it's not the object itself which it accesses but what's called a Runtime Callable Wrapper (or RCW) which serves as a bridge between .NET and COM. The RCW handles calls to IUnknown and other low level interfaces so you won't see it actually exposed to .NET code. What I'd suggest you do is to examine the COM objects with the Visual Studio object browser to see which interfaces are exposed by the RCW and whether the XYZ() method in each object is a member of a common interface. If it is then you can use the name of this interface in the constraint for the generic method. Otherwise there's nothing you can do in this respect.

Vicky · Answer

clsAsynchWrapper123 are written in VBscript. They all use a DLL which is written in C. XYZ are defined in clsAsynchWrapper123. XYZ actually call the function CXYZ defined in C DLL. I didn't see where the IUnknown interface is implemented. I tried to add where clause for the generic method IUnknow wasn't popped up. If that's a default interface how do I let C# know that interface called IUnknown? Thank you for your help!

Alan · Answer

All COM objects implement interfaces (notably IUnknown) whatever language they're written in. However if the XYZ() method doesn't belong to a common interface then no I can't think of a way to do what you want using a C# generic method.

Vicky · Answer

Thank you for the reply! well all 3 COM objects are written in C. they don't have interface... Does that mean no any other solution any more???

Alan · Answer

The only way I can think of to get that to work is if all 3 COM objects implement the same interface (let's say IFace) which contains the XYZ method. You could then use a constraint to force the compiler to accept it: static void genericCallAsynchWrapper (T asynchWrapper int clientId) where T : IFace { asynchWrapper.XYZ(clientId); Marshal.ReleaseComObject(asynchWrapper); asynchWrapper = default(T); } Otherwise I think you'll have to stay with a non-generic solution.