My name
is
Jon Skeet

When implementing an interface in Generic, why it is not must implement the methods

public interface IMyTest<T>
{
    T Add(T i, T j);
}

public class MyContainer<T> where T : IComparable<T>, IMyTest<T> 
{

}

You're not implementing the interface. You're saying that the type argument supplied for the type parameter T must itself implement the interface. That's what the where T part means - it's specifying constraints on T.

This means that in your MyContainer class you can use the members of the interface:

public class MyContainer<T> where T : IComparable<T>, IMyTest<T> 
{
    public T SumBiggestAndSmallest(IEnumerable<T> items)
    {
        var ordered = items.OrderBy(x => x)
                           .ToList();
        return ordered.First().Add(ordered.First(), ordered.Last());
    }
}

(It's unclear why your Add method takes two T values, as well as being an instance method, but that's a different matter.)

Without the constraints on T, you wouldn't have an Add method to call.

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