Скачать или смотреть How to Use Reflection on a COM Object Exposed through the RCW

Discover the step-by-step guide on how to effectively utilize `reflection` with `COM` objects using `RCW` in C# . Learn how to create expression trees for optimized method calls.
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How to Use Reflection on a COM Object Exposed through the RCW

Working with COM (Component Object Model) objects in C# can be a complex task, especially when trying to leverage reflection for dynamic method invocation. In particular, if you are using the Runtime Callable Wrapper (RCW), issues may arise while trying to get the method information needed for your operations. In this post, we'll walk through a common problem and its solution, demonstrating how to effectively use reflection on a COM object exposed through the RCW.

The Problem Overview

When interfacing with COM objects via the interop assembly generated by tools like TlbImp.exe, you might want to build and cache expression trees that call methods like SetValue(T value) across multiple interfaces with varying parameter types. While the generated assembly provides strongly typed interfaces, obtaining the MethodInfo using reflection can be challenging, especially when the wrapper type is System.__ComObject.

A central dilemma is whether invoking a method through InvokeMember will significantly impede performance compared to invoking the method directly via the interop interfaces.

The Approach to a Solution

After experimenting with a simple console application, a successful solution emerged. The key was to correct a couple of mistakes in the reflection calls:

Using the Full Type Name: The interface's full type name (including its namespace) must be specified when using Assembly.GetType.

Setting Correct Binding Flags: Ensure that the appropriate BindingFlags are set for method retrieval.

Step-by-Step Implementation

Here’s a clear step-by-step implementation that demonstrates how to set up an expression tree to call the SetValue method of a COM interface.

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Code

Parameter Expression Creation: We create ParameterExpression instances for both the instance of the COM object and the value to be passed.

Method Call Expression: By utilizing Expression.Call, we create a callable expression that will invoke the SetRequestedValue method on the COM interface.

Generating the Delegate: Finally, we compile our expression tree into an Action<object, T> delegate. This allows for efficient method calling without repeatedly accessing COM interfaces via reflection after the initial setup.

Conclusion

Using reflection with COM objects in C# doesn't have to be daunting. By leveraging the structure of your interop interfaces properly and ensuring the correct use of reflection, you can efficiently call methods like SetValue without suffering major performance hits. The provided solution illustrates how to create a robust reflection-based method invocation that can save time and enhance performance in application development.

Experiment with the example code and adapt it to your specific case to unlock the full potential of reflection in COM interop environments!