I must be going out of my mind, because my unit tests are failing because the following code is throwing a null ref exception:

int pid = 0;
if (parentCategory != null)
{
    Console.WriteLine(parentCategory.Id);
    pid = parentCategory.Id;
}

The line that's throwing it is:

pid = parentCategory.Id;

The console.writeline is just to debug in the NUnit GUI, but that outputs a valid int.

Edit: It's single-threaded so it can't be assigned to null from some other thread, and the fact the the Console.WriteLine successfully prints out the value shows that it shouldn't throw.

Edit: Relevant snippets of the Category class:

public class Category
{
    private readonly int id;

    public Category(Category parent, int id)
    {
        Parent = parent;
        parent.PerformIfNotNull(() => parent.subcategories.AddIfNew(this));
        Name = string.Empty;
        this.id = id;
    }
    public int Id
    {
        get { return id; }
    }
}

Well if anyone wants to look at the full code, it's on Google Code at http://code.google.com/p/chefbook/source/checkout

I think I'm going to try rebooting the computer... I've seen pretty weird things fixed by a reboot. Will update after reboot.

Update: Mystery solved. Looks like NUnit shows the error line as the last successfully executed statement... Copy/pasted test into new console app and ran in VS showed that it was the line after the if statement block (not shown) that contained a null ref. Thanks for all the ideas everyone. +1 to everyone that answered.