Method with Object argument won't accept anything other than an Object

Question

We have a VB.net function with the following signature in class InitializerFactory:

Public Shared Function Create(ByRef ui As Object) As IModeInitializer

I'm attempting to test this function by passing in a mock of ui (using Rhino Mocks):

MainForm ui = mocks.StrictMock<MainForm>();
IModeInitializer item = InitializerFactory.Create(ref ui);

When attempting to pass ui as a parameter, I get the following errors:

• The best overloaded method match for 'InitializerFactory.Create(ref object)' has some invalid arguments
• Argument '1': cannot convert from 'ref MainForm' to 'ref object'

Ideally, the solution would be to extract an interface on UI (or its class, MainForm) but that's not doable by any means - it's an extremely bloated class.

I also can't declare ui as an Object, or else I can't mock the methods inside of it since the methods don't belong to an Object type.

My question is -- what am I doing wrong?

Answer 1

This is just because of the ref parameter syntax. The problem is that the function has to be able to set ANY type of object, since it's a by ref parameter. You can't just pass it a reference to a MainForm, which is what you're attempting.

Unfortunately, this is a pretty difficult API to work with.

You can handle this by assigning your instance to an object first:

MainForm ui = mocks.StrictMock<MainForm>();
object uiObj = ui;
IModeInitializer item = InitializerFactory.Create(ref uiObj);
if (uiObj != ui) { 
    // Handle the case where the reference changed!
    ui = uiObj as MainForm; // May be null, if it's no longer a "MainForm"
}

If you want to fully understand this, you can read up on Covariance and Contravariance.

Answer 2

This is a duplicate of:

http://stackoverflow.com/questions/1207144/c-why-doesnt-ref-and-out-support-polymorphism/1207302#1207302

I answered that question a couple weeks ago on my blog. Here's a link:

http://blogs.msdn.com/ericlippert/archive/2009/09/21/why-do-ref-and-out-parameters-not-allow-type-variation.aspx