c# - When can a null check throw a NullReferenceException

Question

I know this might seem impossible at first and it seemed that way to me at first as well, but recently I have seen exactly this kind of code throw a NullReferenceException, so it is definitely possible.

Unfortunately, there are pretty much no results on Google that explain when code like foo == null can throw a NRE, which can make it difficult to debug and understand why it happened. So in the interest of documenting the possible ways this seemingly bizzare occurance could happen:

In what ways can this code foo == null throw a NullReferenceException?

Answer 1

in C# you can overload operators to add custom logic on some comparison like this. For example:

class Test
{
    public string SomeProp { get; set; }
    
    public static bool operator ==(Test test1, Test test2)
    {
        return test1.SomeProp == test2.SomeProp;
    }

    public static bool operator !=(Test test1, Test test2)
    {
        return !(test1 == test2);
    }
}

then this would produce a null reference exception:

Test test1 = null;
bool x = test1 == null;