C# null coalescing operator returning null

Question

Recently my co-worker showed me a block of code that was not working correctly:

public class SomeClass
{
    private IList _categories;

    publ         


        

Answer 1

(1) DoSomethingElse() could be setting the _categories field to null, before the error appears. A way to test this is to make the _categories field readonly. If that is the error, then you will get a compiler error that a readonly field cannot be used as an assignment target.
(2) Your _categories field is being set via some other function in a different thread. Either way, the following should fix your problem, or at least make it clear where it is.

public class SomeClass
{
    private static readonly object CategoryListLock = new object();
    private readonly List _categories = new List();
    private bool _loaded = false;

    public void SetCategories()
    {
        if(!_loaded)
        {
            lock(CategoryListLock)
            {
                if(!_loaded)
                {
                    _categories.AddRange(GetCategories());
                    _loaded = true;
                }
            }
        }
        DoSomethingElse();
    }

    public IList GetCategories()
    {
        return RetrieveCategories().Select(Something).ToList();
    }
}

**After seeing your edit, it looks like you have two different fields that are IList _categories. It doesn't make sense that the _categories field in the CategoryListControl is null, but the static _categories in the CategoryRepository class looks like it should be null based on what you posted. Perhaps you are getting confused about which field is throwing the error. I understand that the line is called in the CategoryListControl, so your error will say it is in the CategoryListControl class, but the actual exception could be from the GetChildren() method, which attempts to make a children list from a null list). Since the fields are named the same thing, it is easy to see how they could get confused. Test this by making the _categories field in CategoryRepository a readonly initialized field.

Even if the _categories field in the CategoryRepository is not always null, it could be subject to any of the threading concerns that I explained how to fix for the Control class**

ItTo make sure you are debugging the correct _categories field, try this.

    _categories = GetCategories() ?? new List();
    if(_categories == null){
          throw new Exception("WTF???");
     }
    DoSomethingElse();

If you don't get the Exception with "WTF???" then you know the source of the error is elsewhere.

And, regarding the Linq extensions: Neither Where() nor ToList() can return null. Both methods will throw an ArgumentNullException if any parameters are null. I checked this with reflector.

Please let us know what results you get with this. I'm curious now too.