Question

I asked a question here : When To Use IEquatable And Why about using IEquatable.

From the msdn:

The IEquatable(T) interface is used by generic collection objects such as Dictionary(TKey, TValue), List(T), and LinkedList(T) when testing for equality in such methods as Contains, IndexOf, LastIndexOf, and Remove.

If you dont implement that interface what exactly happens?? Exception / default object equals / ref equals?

Answer 1

Here’s one example:

public class Foo : IEquatable<Foo>
{
    public string Bar { get; set; }

    public bool Equals(Foo other)
    {
        return string.Equals(other.Bar, Bar);
    }
}

class Program
{

    public static void Main(string[] args)
    {
        var foos = new List<Foo>(new[] 
        { 
            new Foo { Bar = "b1" },
            new Foo { Bar = "b2" },
            new Foo { Bar = "b3" },
        });

        var foo = new Foo { Bar = "b2" };
        Console.WriteLine(foos.IndexOf(foo)); // prints 1
    }
}

Now comment the IEquatable<Foo> implementation and run the program again.