C# Puzzle No.24 (intermediate)

Given a generic interface definition

public interface IDoSomething : IDisposable { }

please explain why this is possible

class DoesSomething<String> : IDoSomething<String>
{
    public void Dispose()
    {
    }
}

and this is not possible (doesn't compile)

class DoesSomething<string> : IDoSomething<string>
{
    public void Dispose()
    {
    }
}

An explanation can be found here.