Why do I get a stack overflow exception?

Question

Below is a simple test program that throws a StackOverflowException when Equals is called. I expected the generic Equals that I got from object to call my IEquatable<MyClass>.Equals, but it does not, it calls itself instead. Why? The parameter type seems ok. Why does it call the generic version in the first place? I am confused.

using System;

namespace consapp
{
    class Program
    {
        static void Main(string[] args)
        {
            MyClass x0 = new MyClass("x0");
            MyClass x1 = new MyClass("x");

            Console.WriteLine(x1.Equals(x0));
        }

    }

    internal class MyClass : IEquatable<MyClass>
    {
        public string Name { get; set; }
        public MyClass(string s) { this.Name = s; }
        public override bool Equals(object x) { return this.Equals(x as MyClass); }
        public override int GetHashCode() { return this.Name.ToLowerInvariant().GetHashCode(); }
        bool IEquatable<MyClass>.Equals(MyClass x) { return x != null && this.Name == x.Name; }
    }
}

Answer 1

IEquatable.Equals is implemented explicitly. You have to cast the class to the interface first to use an explicit implementation:

public override bool Equals(object x) 
{ 
  return (this as IEquatable).Equals(x as MyClass); 
}

Otherwise it will keep on calling itself in an infinite recursion, yielding in a StackoverflowException eventually.

Answer 2

If you want to call Object's Equals in your Equal override, you should call base.Equals, not this.Equals.

And note that Object's Equals just compares the references, not the contents.

Answer 3

Try

public override bool Equals(object x) {
    Console.Write("equals! ");
    return this.Equals(x as MyClass);

}

Answer 4

public override bool Equals(object x) { return this.Equals(x as MyClass); }
The above line is causing it.

You will have to change it to

public override bool Equals(object x) 
{ 
    return ((IEquatable<MyClass>)this).Equals(x as MyClass); 
}