Why does casting a null to a primitive(ie: int) in .net 2.0 throw a null ref exception and not a invalid cast exception?

I was going through some code and came across a scenario where my combobox has not been initialized yet. This is in .NET 2.0 and in the following code, this.cbRegion.SelectedValue is null.

int id = (int)this.cbRegion.SelectedValue;

This code threw a null reference exception instead of an invalid cast exception. I was wondering if anyone knew why it would throw a null reference exception instead of a invalid cast?

From stackoverflow

• It's attempting to read the object before it casts it. Hence you're getting the null exception instead of a cast exception.

• The exception is on the Selected Value which is null. It's never even getting to the cast.

  liggett78 : This is not quite correct. It throws while trying to cast or, specifically, to unbox a null reference.

• It has to do with Boxing and unboxing. It is trying to pull an int out of the box (unbox), but the object is null, so you get a null reference exception before it ever gets the change to cast.

  Richard R : After reading the article it makes sense

• If you compile

  object o = null;
  int a = (int)o;
  

  and look at the MSIL code, you'll see something like

  ldnull
  ...
  unbox.any int32
  

  Now the behavior for unbox.any is specified as follows:

  InvalidCastException is thrown if obj is not a boxed type.

  NullReferenceException is thrown if obj is a null reference.

  This is what you see in your code.

  Richard R : COOL, I never even thought about decompiling it and checking out the instructions used. thanks