Question

This is the code I use:

Type type = /* retrieved Type */
object arg = /* something that evaluates to null */
MyClass obj = (MyClass)Activator.CreateInstance(type, arg);

I get a crash, that given constructor doesn’t exist on type type.

However, when I put this in Watch in Visual Studio 2008:

(MyClass)System.Activator.CreateInstance(type, null)

it creates the object as usual.

I even tried replacing my code with the one I put in the Watch. It works – object gets created.

My question: what’s up with that?

Edit: MyClass doesn’t have any constructors – apart from pregenerated parameterless constructor.

Edit 2: Using new object[0] instead of null still causes the same exception.

Answer 1

You’ve run into an issue with the params keyword.

The actual signature of the function is CreateInstance(Type, object[]). However, the fact that the object[] parameter is declared as params means that you can pass a variable number of arguments to the function and those arguments will get rolled into a new array, or you can directly pass an object array.

When the compiler performs overload resolution on the version where you pass null directly into the function, it does not convert the parameter into an array since null is a valid value for this. However, when you pass in a null-valued object variable, overload resolution has to turn that into an object array. This means that you are passing an object array with one value, which is null. The runtime then looks for a constructor with one argument, which it would then pass null to.

This is why the resolution fails at runtime.