COM-Interop中的十进制封送处理

Question

I have COM object method with signature

HRESULT _stdcall Method1([in] int ms);

Next, i call this method from C# by reflection:

...

decimal ms = 100.5m;
comType.InvokeMember("Method1", flags, null, comObject, new object[] { ms });

 ...

Is this call correct ? I mean how decimal ms will be marshaled to int ?

This code works if i create instance by Activator

var comType= Type.GetTypeFromProgID("MyCom.Server", false);
var comObject= Activator.CreateInstance(comType);

Thanks!

Answer 1

The first snippet makes an early-bound call, using the Runtime Callable Wrapper that was created when you added a reference to the COM server. It is not going to enjoy you passing a decimal when the argument type is int, Reflection won't convert argument values for you.

The second snippet makes a late-bound call, using IDispatch::Invoke(). The decimal is converted to a VARIANT of type VT_DEC. The IDispatch implementation in the COM server then converts that variant to the required argument type. The COM automation helper function VarI4FromDec() probably does that, depends if the server implemented IDispatch itself or left it up to the stock implementation.

Since you are already using an RCW, neither code snippet makes much sense. Just use comObject.Method1(ms). The C# compiler will coerce the decimal for you. If you favor using late binding, slow but safe, you really want to favor the dynamic keyword in C# version 4 and up.