ToString on null string

Question

Why does the second one of these produce an exception while the first one doesn't?

string s = null; MessageBox.Show(s); MessageBox.Show(s.ToString()); 

Updated - the exception I can understand, the puzzling bit (to me) is why the first part doesn't show an exception. This isn't anything to do with the Messagebox, as illustrated below.

Eg :

string s = null, msg; msg = "Message is " + s; //no error msg = "Message is " + s.ToString(); //error 

The first part appears to be implicitly converting a null to a blank string.

Answer 1

because you cannot call instance method ToString() on a null reference.

And MessageBox.Show() is probably implemented to ignore null and print out empty message box.

Answer 2

It is because MessageBox.Show() is implemented with pinvoke, it calls the native Windows MessageBox() function. Which doesn't mind getting a NULL for the lpText argument. The C# language has much stricter rules for pure .NET instance methods (like ToString), it always emits code to verify that the object isn't null. There's some background info on that in this blog post.