Why does casting a null to a primitive(ie: int) in

问题:

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?

回答1:

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.

回答2:

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.

回答3:

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

回答4:

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