C# coalesce operator Throws

Question

I have a class with a string property. I use the coalesce operator when reading from it as it might be null, but it still throws me an NullRefrenceExeption.

string name = user.Section.ParentSection.Name ?? string.Empty;

To be more specific, its the ".ParentSection" that's null so is it because it don't even have ".name" ? If that's the case should i test ".ParentSection" first with an if block?

I assume there are something about the Coalesce operator i dont understand, hope someone can shed some light on whats going wrong here.

Answer 1

To be more specific, its the ".ParentSection" that's null so is it because it don't even have ".name" ?

Yes.

If that's the case should i test ".ParentSection" first with an if block?

Yes.

Answer 2

You'll need to check if Section and ParentSection are null. You could use an if-statement for this or write an extension method like this:

public static class MaybeMonad
{
    public static TOut With<TIn, TOut>(this TIn input, Func<TIn, TOut> evaluator)
        where TIn : class
        where TOut : class
    {
        if (input == null)
        {
            return null;
        }
        else
        {
            return evaluator(input);
        }
    }
}

You would use this method like so:

string name = user.With(u => u.Section)
                  .With(s => s.ParentSection)
                  .With(p => p.Name) ?? string.Empty;

I think it's a lot cleaner than an if-statement with a lot of &&.

Some further reading: http://www.codeproject.com/Articles/109026/Chained-null-checks-and-the-Maybe-monad

Answer 3

You need to check if user, user.Section, or user.Section.ParentSection are null before you can use the null coalescing operator on a property of user.Section.ParentSection.

Answer 4

Nested property access is not safe if any of the objects accessed are null this will throw a NullReferenceException. You will have to explicitly test for the outer objects to be not null.

E.g.:

string name = string.Empty;
if(user!=null && user.Section!=null && user.Section.ParentSection !=null)
   name = user.Section.ParentSection.Name ?? string.Empty;

In general I would try to avoid nested access to properties, you are violating the Law of Demeter. Some refactoring might make this unnecessary in the first place.

Answer 5

The ?? operator checks if the left side is null and if so returns the right one, if not the left one. In your case the left-side is the "Name" property in the object user.Section.ParentSection and this is null.

In those cases either think on what might be null or do something like this:

string name = user == null 
              || user.Section == null 
              || user.ParentSection == null 
              || user.Section.ParentSection.Name == null 
                 ? string.Empty 
                 : user.Section.ParentSection.Name;

(yeah it's ugly I know)

Answer 6

Chances are user or user.Section or user.Section.ParentSection is a null value.

The ?? operator doesn't prevent checks like:

if (user != null && user.Section != null && user.Section.ParentSection != null){

Make sure that everything up to the string property is valid and exists, then you can use ??. You can't call (null).Name, no matter how many times you try.

Answer 7

Yes you need to check if Section or ParentSection are null before you check Name

Answer 8

It is probably best to do something like this:

if(user!=null && user.Section!=null && user.Section.ParentSection!=null)
{
     string name = user.Section.ParentSection.Name ?? string.Empty;
}

Answer 9

The null coalescing operator takes a statement like:

a = b ?? c;  

What this says is "evaluate b; if it has a non-null value then assign that to a. Otherwise assign the value of c to a".

However within your b you're using a user object which may be null that has a section object that may be null that has a parent section property that may be null that had a name property that may be null. If you wanted to check all of these (and typically you should) then you can do something like:

string name = string.Empty;  
if (user != null &&  
     user.Section != null &&  
     user.Section.ParentSection != null)  
{  
  name = user.Section.ParentSection.Name ?? string.Empty;  
}

As soon as the IF check fails it will not check further and therefore you don't get a NullReferenceException when you assume an object is present and then try to access one of its properties.