C# 3.0 Cookbook, 3rd Edition

4.6. Creating a Value Type That Can Be Initialized to Null

Problem

You have a variable that is a numeric type, which will hold a numeric value obtained from a database. The database may return this value as a null. You need a simple, clean way to store this numeric value, even if it is returned as a null.

Solution

Use a nullable value type. There are two ways of creating a nullable value type. The first way is to use the ? type modifier:

	int? myDBInt = null;

The second way is to use the Nullable<T> generic type:

	Nullable<int> myDBInt = new Nullable<int>();

Discussion

Both of the following statements are equivalent:

	int? myDBInt = null;
	Nullable<int> myDBInt = new Nullable<int>();

In both cases, myDBInt is a nullable type and is initialized to null.

A nullable type implements the INullableValue interface, which has two read-only property members, HasValue and Value. The HasValue property returns false if the nullable value is set to null; otherwise, it returns true. If HasValue returns true, you can access the Value property, which contains the currently stored value. If HasValue returns false and you attempt to read the Value property, you will get an InvalidOperationException thrown. This is because the Value property is undefined at this point. Below is an example of a test of nullable value using the HasValue property value:

	if (myDBInt.HasValue)
	    Console.WriteLine("Has a value: " + myDBInt.Value);
	else
	    Console.WriteLine("Does not have a value (NULL)");

In addition, one can simply compare ...