allgood.cs

using System;

interface IFoo
{
    void Do();
}
class A : IFoo
{
    public void Do() {}
}

class B : IFoo
{
    public void Do() {}
}

class Goodarray
{
    static void Main(string[] args) 
    {
        var x = new IFoo[2];
        x[0] = new A(); 
        x[1] = new B(); 
        Ohyes(x); 

        var y = new IFoo[1];
        y[0] = new B();
        Ohyes(y);
    }

    static void Ohyes(IFoo[] thisArrayContainsFoos)
    {
        thisArrayContainsFoos[0].Do();
        Console.WriteLine(thisArrayContainsFoos[0]);  
        thisArrayContainsFoos[0] = new A(); //nothing wrong here!
    }
}