Question

I was working with ?? operator in C# 4 and found a interesting feature.

This line of code always assigns null to existingObject if it is already null regardless of the searchForObject() return value (searchForObject returns a not null object, in fact it is a linq statement not a function and if the following statement will be replaced with simple if construct then existingObject will be assigned a not null object):

    existingObject = extstingObject ?? searchForObject();

Could someone explain why?

Here is a link to MSDN, it says:

A nullable type can contain a value, or it can be undefined. The ??
operator defines the default value to be returned when a nullable type
is assigned to a non-nullable type. If you try to assign a nullable
value type to a non-nullable value type without using the ?? operator,
you will generate a compile-time error. If you use a cast, and the
nullable value type is currently undefined, an
InvalidOperationException exception will be thrown.

The part about assigning a nullable type to a non-nullable type is not what I expected.

The problem was deferred initialization of local variables in the debugger of Visual Studio 2010.

Answer 1

Apart from what I said in this comment:

There is also a chance you’re not actually looking at the same code
that you’re executing. It happens sometimes in Visual Studio. Rebuild
the project, try clearing the bin folder, put some Debug.WriteLines
in to trace what’s really going on in your app (instead of relying on
the IDE’s Watch), etc.

another question comes to mind – is existingObject really null after the assignment? 🙂 How have you asserted that?

Have a look at my question (and the answers):

Surprising CLR / JIT? behaviour – deferred initialization of a local variable