I've stumbled upon this "feature" of C# - the base class that implements interface methods does not have to derive from it.

Example:

public interface IContract
{
    void Func();
}

// Note that Base does **not** derive from IContract
public abstract class Base
{
    public void Func()
    {
        Console.WriteLine("Base.Func");
    }
}

// Note that Derived does *not* provide implementation for IContract
public class Derived : Base, IContract
{
}

What happens is that Derived magically picks-up a public method Base.Func and decides that it will implement IContract.Func.

What is the reason behind this magic?

IMHO: this "quasi-implementation" feature is very-unintuitive and make code-inspection much harder. What do you think?