How to check for null in the operator== method?

Question

Consider the following class:

public class Code : IEquatable<Code>  {     public string Value { get; set; }      public override bool Equals(object obj)     {          return Equals(obj as Code);     }      public override bool Equals(Code code)     {          if (code == null) return false;          return this.Value == code.Value;     }      public static bool operator ==(Code a, Code b)     {          if (a == null) return b == null;          return a.Equals(b);     }      public static bool operator !=(Code a, Code b)     {          if (a == null) return b!= null;          return !a.Equals(b);     }      // rest of the class here } 

Now try using the == method:

Code a = new Code(); Code b = new Code(); Console.WriteLine("The same? {0}", a==b); 

The result is a StackOverflowException because the == method calls itself when it checks for null.

But if I take out the null check:

public static bool operator ==(Code a, Code b) {     return a.Equals(b); } 

I get a NullReferenceException!

What's the correct way to define these methods?

Answer 1

You can also use (object)a == null