Why does .ToString() on a null string cause a null error, when .ToString() works fine on a nullable int with null value?

Question

selectedItem has two fields:

• int? _cost
• string _serialNumber

In this example, _cost and _serialNumber of selectedItem are BOTH null. I am reading through the fields of selectedItem via their properties, and filling in textboxes with their values, when...

TextBox1.Text = selectedItem.Cost.ToString(); //no error TextBox2.Text = selectedItem.SerialNumber.ToString(); //error 

I understand that SerialNumber.ToString() is redundant (because it is already a string), but I don't understand why this causes this exception:

Nullable object must have a value.

• int? _cost is nullable, and does not have a value, yet it does not give me the exception.
• string _serialNumber is nullable, and does not have a value, yet it does give me the exception.

This question touches on it, the guy is essentially asking the same thing, but there is no designated answer, and it also doesn't explain why a nullable int? For example, can I use .ToString() on a nullable int but not on a null string?

Answer 1

Because string type's null really points to nothing, there isn't any object in memory.
But int? type(nullable) even with value set to null still points to some object.
If you read Jeffrey Richter's "CLR via C#" you'll find out that nullable type are just facade classes for common types with some incapsulated logics in order to make work with DB null more convenient.

Check msdn to learn about nullable types.