ExplicitInterfaceImplementationsAreReplaceable.cs

using System;

class Program
{
    static void Main()
    {
        var b = new Base();

        b.DoSomething();
        // The answer is 42!

        var s = new Sub();

        s.DoSomething();
        // Sub observed secret value: 21
        // The answer is 0!
    }
}

interface IBlah
{
    int GetSomeNumber(
        int someSecret);
}

class Base : IBlah
{
    public void DoSomething()
    {
        // Get explicit implementation.
        IBlah blah = this;
        var number = blah.GetSomeNumber(21);
        Console.WriteLine($"The answer is {number}!");
    }

    int IBlah.GetSomeNumber(int someSecret) => 2 * someSecret;
}

class Sub : Base, IBlah
{
    // Change a behavior of the base class by replacing its
    // IBlah implementation.
    int IBlah.GetSomeNumber(int someSecret)
    {
        Console.WriteLine($"{GetType().Name} observed secret value: {someSecret}");
        return 0;
    }
}

Mayhaps this is an argument that classes be sealed by default?

Very nice example