Why can't I retrieve the value for parameters of type out or ref using Type.InvokeMember?

Question

A long title, but I wanted it to be specific. The title is really the question. Even though the method that InvokeMember is calling has an out parameter and is assigning a value to to that parameter I can't grab that value. Here is the code I was initially using:

string parameter = "";
int result = Convert.ToInt32(typeof(Ability).InvokeMember(selectedMove, BindingFlags.InvokeMethod | BindingFlags.Public | BindingFlags.Static, null, null, new object[] { parameter }));

I changed it this, which now makes it work as intended but I don't know why:

object[] args = new object[1];      //necessary to retrieve ref/out parameter
int result = Convert.ToInt32(typeof(Ability).InvokeMember(selectedMove, BindingFlags.InvokeMethod | BindingFlags.Public | BindingFlags.Static, null, null, args));

Answer 1

I just wanted to help someone who is struggling(I did) with unmanaged(COM) and getting ref-parameter back. So, when using InvokeMember against COM-method, you have to tell which arguments are ref-type. This is achieved by using ParameterModifier-class, For example:

object[] args = new object[3] { param1, param2, errorStr };
ParameterModifier pMod = new ParameterModifier(3);
pMod[2] = true;            
ParameterModifier[] mods = { pMod };

object tempObj = myCOMObject.GetType().InvokeMember("MyCOMMethod", BindingFlags.InvokeMethod | BindingFlags.NonPublic | BindingFlags.Public, null, myCOMObject, args, mods, null, null);

In the code above, the 3rd argument is set to be a reference (pMod[2] = true;)

Answer 2

Your second snippet is missing a rather essential line of code. It should look like this, assuming the out argument is of type string:

object[] args = new object[1];      //necessary to retrieve ref/out parameter
int result = Convert.ToInt32(typeof(Ability).InvokeMember(selectedMove, 
    BindingFlags.InvokeMethod | BindingFlags.Public | BindingFlags.Static, 
    null, null, args));
string outValue = (string)args[0];  // <===  here!

It should now also be obvious why your 1st snippet cannot work, you don't have a reference to the object[] array that you pass so you can never retrieve the modified argument.

Answer 3

In your first code example, the call to InvokeMember doesn't modify the value of the parameter variable, it just replaces the first item in the parameter array (which now points to a different string instance). Since you didn't keep a reference to this array, you can't retrieve the value of the output parameter.

In other words: the array initially contains a copy of the parameter variable (i.e. a copy of the reference to an empty string). After the call, parameter and the value in the array refer to 2 different string instances.