2

is there any way to do this on a class and still retain the capability to detect whether a variable of the Type is null ? I can do this for structs, (cause struct can't really be null, with a struct, instead of comparing x with null, you use a separate method caleld IsNull or HasValue... but you can't use that technique with a class cause when the class variable is null, of course, you can't call such an instance method on it !

public class myClass: IEquatable<myClass> { public int IntValue { get; set; } public static bool operator ==(myClass cA, myClass cB) { return cA is myClass && cB is myClass && cB.Equals(cA); } public static bool operator !=(myClass cA, myClass cB) { return !(cA == cB); } public override bool Equals(object o) { return o is myClass && this == (myClass)o; } public override bool Equals(myClass o) { return IntValue == o.IntValue; } }

but when I go:

myClass x; if (x != null) // this returns false when it should be true { //code to execute with x here }

For what it's worth, the only reason I want this to be a class is because it participates in a simple inheritance relationship. This is an immutable class, and in effect what I am trying to do here is code it so it can behave like an immutable and nullable class, exactly like an immutable, nullable struct behaves (such as Nullable<int> or Nullable<float> etc.

Improve this question
2
  • Just thinking aloud here, but what would be the disadvantage of always returning true in your overload when cA or cB is null? Commented Feb 25, 2011 at 14:45
  • The is operator calls in public static bool operator ==(myClass cA, myClass cB) { return cA is myClass && cB is myClass && cB.Equals(cA); } are not needed. Commented Feb 25, 2011 at 14:46

6 Answers 6

Reset to default
5

That's why IsNull should be a static method taking a parameter. string.IsNullOrEmpty is a good example. After that, nothing prevents you from making it an extension method.

public class MyClass { public static bool IsNull(MyClass other) { return ReferenceEquals(other, null); } public static bool HasValue(MyClass other) { return !IsNull(other); } // other code } public static class MyClassExtension { public static bool IsNull(this MyClass myClass) { return MyClass.IsNull(myClass); } }

This will let you do the following without throwing:

MyClass myClass = null; if(myClass.IsNull()) { //... }
Improve this answer
Sign up to request clarification or add additional context in comments.

2 Comments

Combine with @jjrdk's ReferenceEquals suggestion from below, I think this will work.. Thanks!
Oh! Thank you. I was hesitant in doing it myself so I +1 him instead. :) Thanks for the accept.
3

If you're not initializing x so it is null, you need to do this

 myClass x = new myClass(); if (x != null) {/* is true */}
Improve this answer

4 Comments

Whether or not x is null, writing (x != null) calls the code in my operator override....
added more of the code to make this explicit... but the != is NOT the standard .net !=.... It is running my override of !=. So whether the variable is or is not null has nothing to do with it. It is the code in my override that controls what the != method will return.
yes but the is in your == comparisons fails because the second argument is null, I don't see why you expect a null check to return false when the object is null
yes, this was exactly the problem... How to check for null when every call to == or to != was being rerouted to my overrides...
3

You can always use:

ReferenceEquals(x, null)

this returns a boolean value showing whether x is null.

Improve this answer

1 Comment

Wish I could check two answers as solutions, your answer, combined with Coincoins from below ended up being the solution... Thanks
2
  1. You should not do this at all.
  2. You should only do this for an immutable class, if you really must. And then,
  3. You should follow the guidelines
Improve this answer

1 Comment

1) Thanks for your help. 2) I need to do this. 3) This type is immutable. 4) Stay in your box with your guidelines.
2 
public class myClass: IEquatable<myClass> { public static bool operator ==(myClass cA, myClass cB) { return (cB == null && cA = null) || (cA is myClass && cB is myClass && cB.Equals(cA)); } }
Improve this answer

Comments

1

Why not just override GetHashCode and Equals?

when you override them both it allows you do do both == and != very easily..

Improve this answer

Comments

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.