c# - Use new keyword if hiding was intended

Question

I have the following snippet of code that's generating the "Use new keyword if hiding was intended" warning in VS2008:

public double Foo(double param)
{
   return base.Foo(param);
}

The Foo() function in the base class is protected and I want to expose it to a unit test by putting it in wrapper class solely for the purpose of unit testing. I.e. the wrapper class will not be used for anything else. So one question I have is: is this accepted practice?

Back to the new warning. Why would I have to new the overriding function in this scenario?

Answer 1

The new just makes it absolutely clear that you know you are stomping over an existing method. Since the existing code was protected, it isn't as big a deal - you can safely add the new to stop it moaning.

The difference comes when your method does something different; any variable that references the derived class and calls Foo() would do something different (even with the same object) as one that references the base class and calls Foo():

SomeDerived obj = new SomeDerived();
obj.Foo(); // runs the new code
SomeBase objBase = obj; // still the same object
objBase.Foo(); // runs the old code

This could obviously have an impact on any existing code that knows about SomeDerived and calls Foo() - i.e. it is now running a completely different method.

Also, note that you could mark it protected internal, and use [InternalsVisibleTo] to provide access to your unit test (this is the most common use of [InternalsVisibleTo]; then your unit-tests can access it directly without the derived class.