How to order null objects with compareTo in C#

Question

I have a class called UserView and I implemented his compareTo() method.

I have that restriction: "If a UserView is compared with a null UserView, the null go before the other"

I have that compareTo():

 public int CompareTo(UserView other)
    {
        if (this == null && other != null) return -1;
        else if (this == null && other == null) return 0;
        else return Id.CompareTo(other.Id);
    }

And this test:

 [TestMethod]
    public void TestCompareWithNull()
    {
        UserView uv = new UserView(1, "pepe", "1234", "alumno", true);
        UserView uv2 = null;
        UserView uv3 = null;
        Assert.AreEqual(uv2.CompareTo(uv3), 0);
        Assert.AreEqual(uv2.CompareTo(uv), -1);
        Assert.AreEqual(uv.CompareTo(uv3), 1);
    }

When I call compareTo from uv2, which is null, I have NullReferenceException so... how Can I satisfy the given restriction?

Answer 1

If you want compare arbitraty instances (including both null case) you have to implement a static method, since

  this == null

is an impossible condition within any member function. And so the immediate cause of the exception is calling a member function on null instance:

UserView uv2 = null;
// Since uv2 is null, Null Exception will be thrown
uv2.CompareTo(uv3);

A way out is a static method:

public int CompareTo(UserView other) 
{
    return UserView.Compare(this, other);   
}

// please, note "static" 
public static int Compare(UserView left, UserView right)
{
    if (left == null)
      if (right == null) 
        return 0;
      else 
        return -1;
    else if (right == null) 
      return 1; 
    else // Providing that Id can't be null (e.g. it's int)
      return left.Id.CompareTo(right.Id); 
}

....

UserView uv2 = null;
UserView uv3 = null;

// uv2.CompareTo(uv3) will throw Null Exception and
// ... static UserView.Compare will do
Assert.AreEqual(UserView.Compare(uv2, uv3), -1);