为什么我要在Equals覆盖中执行object.ReferenceEquals（null，this）？

Question

Consider the following code that I was reviewing:

public override bool Equals(object other)
{
    return !object.ReferenceEquals(null, this)
           && (object.ReferenceEquals(this, other)
           || ((other is MyType) && this.InternalEquals((MyType)other)));
}

The first line in this code triggered my curiosity. Whenever this is null, the method should return false. Now I am pretty sure the programmer meant to write !object.ReferenceEquals(other, null) , to shortcut situations with null , but he's insistent that this can be null. I'm insistent that it cannot (unless someone uses direct memory manipulation). Should we leave it in?

Answer 1

While I certainly wouldn't normally check this for nullity, it's possible, without any actual memory nastiness - just a bit of reflection:

using System;

public class Test
{
    public void CheckThisForNullity()
    {
        Console.WriteLine("Is this null? {0}", this == null);
    }

    static void Main(string[] args)
    {
        var method = typeof(Test).GetMethod("CheckThisForNullity");
        var openDelegate = (Action<Test>) Delegate.CreateDelegate(
               typeof(Action<Test>), method);
        openDelegate(null);
    }
}

Alternatively, generate IL which uses call instead of callvirt to call an instance method on a null target. Entirely legit, just not something the C# compiler would normally do.

This has nothing to do with finalization, which is hairy in its own right but in different ways. It's possible for a finalizer to run while an instance method is executing if the CLR can prove that you're not going to use any fields in the instance (which I would strongly expect to include the this reference).

As for the code presented - nope, that looks like it's just a mistake. I would rewrite it as:

public override bool Equals(object other)
{
    return Equals(other as MyType);
}

public bool Equals(MyType other)
{
    if (ReferenceEquals(other, null))
    {
        return false;
    }
    // Now perform the equality check
}

... assuming that MyType is a class, not a struct. Note how I'm using another public method with the right parameter type - I'd implement IEquatable<MyType> at the same time.

Answer 2

C# doesn't normally allow methods to be called on null . I think that the programmer who wrote that is either coming from a C++ background (where I think methods can be called on null , as long as they don't access a data member of this ) or writing defensively for special scenarios (such as invocation by reflection, as has already been said).