Windows Live Logout Failure

A little digression here. This morning, I clicked on the “Logout” button on Windows Live, and received this message:

WindowsLiveLogoutFail

Not only it asks me to delete the cookies from my browser myself, but the list of site he successfully logged me out off is weird, to say the least.

Oh well…

December 9, 2010 | Tags: Tags: | Leave a Comment 

Overload Resolution and Covariance

As of C# 4.0, generics support covariance and contravariance. I won’t talke about contravariance in the post, however.

To sum it up quickly, covariance enables implicit conversion of a generic collection of type T to the same generic collection of a type that derives from T. In short:

IEnumerable<Object> list = new List<String>().AsEnumerable();

This means that as of now, any method that accepts something like IEnumerable<Object> can accept an IEnumerable<String> as well. This changes the overload resolution mechanism:

class A { }
class B : A { }

class T
{
    internal void Foo(IEnumerable<B> sequence)
    {
        Console.WriteLine("In T.Foo.B");
    }
}

class U : T
{
    internal void Foo(IEnumerable<A> sequence)
    {
        Console.WriteLine("In U.Foo.A");
    }
}

In the following code, how does the method resolution changes?

U u = new U();

var l = new List<A>();
var m = new List<B>();

u.Foo(l);
u.Foo(m);

If ran in Visual Studio as a .NET 3.5 application, here is the result:

Overload35

If ran in Visual Studio as a .NET 4.0 application, here is the result:

Overload40

Prior to C# 4.0, U.Foo was not a candidate for a call using a generic collection of B. However, with covariance, it is, hence the different result. So, this is no breaking change, but the behavior of an application might be affected.

December 1, 2010 | Tags: Tags: , , | Leave a Comment