In C#3:

var command = new AuthenticationCommand();
IResult result = command.Process();

Which would assign the type "AuthenticationCommand" to command -- not
exactly wildcarding but syntactically similar in this case. In C#2 you
have lost with your exact code.

You can use the explicit type in the declaration of command:

 ICommand<AuthenticationResult> command = new AuthenticationCommand();

You might consider making ICommand unparametrised:

 interface ICommand { IResult Process(); }

if you really need to uniformly treat results.

An alternative approach, which I don't really like too much is:

   interface IResult { }
   interface ICommand { IResult Process(); }
   interface ICommand<T>: ICommand where T: IResult { new T Process(); }
   class Foo: IResult {}
   class X : ICommand<Foo> {
       IResult ICommand.Process() { return Process(); }
       public Foo Process() { return new Foo(); }
   }
   class Program
   {
       static void Main(string[] args)
       {
           ICommand command = new X();
           IResult r = command.Process();
       }
   }