IKVM.NET Weblog

I came across an interesting Whidbey beta 1 CLR feature today. As far as I can tell this hasn't been documented anywhere yet, but the Whidbey beta 1 CLR supports covariant and contravariant generic type parameters.

This is a really interesting feature. If C# would support it (at the moment it doesn't), you'd be able to do something like this:

interface IReader<T>
{
  T Get(); 
}

interface IWriter<T>
{
  void Put(T t);
}

class Test<T> : IReader<T>, IWriter<T>
{
  T t;
  public T Get() { return t; }
  public void Put(T t) { this.t = t; }
}
class Driver
{
  static void Main()
  {
    Test<string> t1 = new Test<string>();
    t1.Put("covariance");
    IReader<object> rdr = t1;
    Console.WriteLine(rdr.Get());
    Test<object> t2 = new Test<object>();
    IWriter<string> wrtr = t1;
    wrtr.Put("contravariance");
    Console.WriteLine(t2.Get());
  }
}

Notice that in the first highlighted line we're assigning an object reference that implements IReader<string> to a local variable of type IReader<object>. This is covariance and it is type safe because the IReader<T> interface only returns T. In the second highlighted line we assign a reference that implements IWriter<object> to a local of type IWriter<string>, this is contravariance and this is also type safe, because IWriter<T> only accept arguments of type T.

Unfortunately, C# doesn't yet seem to support this feature, but if we manually write the IL, it'll run on the Whidbey beta 1 CLR (and it is fully verifiable):

Notice the plus and minus signs in the interface declarations to signal that a generic type parameter is covariant or contravariant.