ThrowException through reflection

Question

This is not a common scenario. I am trying to throw an exception through reflection. I have something like: testMethod is of type MethodBuilder

testMethod.GetILGenerator().ThrowException(typeof(CustomException)); 

My CustomException does not have a default constructor, so the above statement throws an error, throwing an ArgumentException. If there is a default constructor, this works fine.

So, is there a way this can work without a default constructor? Tried already 2 hours. :(

Any help is appreciated.

Thanks!

Answer 1

See the documentation:

 // This example uses the ThrowException method, which uses the default // constructor of the specified exception type to create the exception. If you // want to specify your own message, you must use a different constructor; // replace the ThrowException method call with code like that shown below, // which creates the exception and throws it. // // Load the message, which is the argument for the constructor, onto the // execution stack. Execute Newobj, with the OverflowException constructor // that takes a string. This pops the message off the stack, and pushes the // new exception onto the stack. The Throw instruction pops the exception off // the stack and throws it. //adderIL.Emit(OpCodes.Ldstr, "DoAdd does not accept values over 100."); //adderIL.Emit(OpCodes.Newobj, _ // overflowType.GetConstructor(new Type[] { typeof(String) })); //adderIL.Emit(OpCodes.Throw); 

Answer 2

The ThrowException method essentially boils down to the following

 Emit(OpCodes.NewObj, ...); Emit(OpCodes.Throw); 

The key here is to replace the first Emit calls Emit set of IL statements needed to instantiate your custom exception. Then add Emit(OpCodes.Throw)

for instance

 class MyException : Exception { public MyException(int p1) {} } var ctor = typeof(MyException).GetConstructor(new Type[] {typeof(int)}); var gen = builder.GetILGenerator(); gen.Emit(OpCodes.Ldc_I4, 42); gen.Emit(OpCodes.NewObj, ctor); gen.Emit(OpCodes.Throw);