The expression tree may not contain a call or a call that uses optional arguments

Question

The expression tree may not contain a call or a call that uses optional arguments

return this.RedirectToAction<MerchantController>(x => x.Edit(merchantId)); 

If editing had a second argument with a null value.

Why is this?

Answer 1

The underlying expression tree API does not support optional arguments.

For IL-compiled code, the C # compiler inserts default values ​​at compile time (hard-coded) because the CLR does not support calls with optional arguments when the arguments are not explicitly specified.

Answer 2

There was the same message when trying to use Mock.setup for a method layout with several default parameters. I just needed to add additional parameters to the lambda.

 void someMethod(string arg1 = "", string arg2 = "") mockedObject.Setup(x => x.someMethod(It.IsAny<string>(), It.IsAny<string>()))