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?

score 34 · Accepted Answer · answered Dec 29 '10 at 15:57

34

You can also use (object)a == null

answered Dec 29 '10 at 15:57

Rex M

142,167
33
283
313

1

There is an alternative to this solution that currently has more upvotes, so let me note that `(object)a == null` results in faster code, to the extent that a `ReferenceEquals` op== implementation took more than twice as long as the equivalent using `(object)a == null`. If you use the same code also for `Equals`, this can have a knock-on effect in every `Distinct()`, `Dictionary<>`, `HashSet<>`, `ToLookup` etc. you use with this type as the key. – Eamon Nerbonne May 29 '14 at 11:18
1

A Linqpad microbenchmark for those interested in replicating the perf problem with `ReferenceEquals`: https://github.com/EamonNerbonne/ValueUtils/blob/master/NullCheckBenchmark.linq. Note that apparently sometimes the CLR does inline it, so if you're lucky it won't matter. – Eamon Nerbonne May 29 '14 at 12:30

score 22 · Answer 2 · answered Dec 29 '10 at 15:55

22

use System.Object.ReferenceEquals(a, null)

answered Dec 29 '10 at 15:55

Ben Voigt

277,958
43
419
720

2

The accepted solution `(object)a == null` is faster. – Eamon Nerbonne May 29 '14 at 11:15

score 2 · Answer 3 · answered Mar 11 '19 at 19:16

2

Starting in C# 7, you can simply use the is keyword to do a direct reference comparison. See Thorkil's answer to What is the difference between “x is null” and “x == null”?

answered Mar 11 '19 at 19:16

Malcolm

510
3
5
19

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

3 Answers3