C# 5.0 Programmer's Reference

Delegates ❘ 93
To determine whether a nullable variable contains a value, either use its HasValue property or simply
compare it to null. The following code determines whether the variable count holds a value in a
couple different ways.
if (count.HasValue) Console.WriteLine("count = " + count);
else Console.WriteLine("count = null");
if (count != null) Console.WriteLine("count = " + count);
if (count == null) Console.WriteLine("count = null");
The first line of code uses the variable’s HasValue property. If count has a value, the code displays it.
The else statement on the next line displays “count = null” if the variable has no value.
The next two lines use != and == to compare the variable to the value null.
The syntax used by nullable variables makes them remarkably easy to use and understand. There’s
only one slightly complex issue: null propagation. When you perform a calculation with a nullable
variable that holds no value, its “nullness” propagates into the result. For example, if a nullable integer
contains no value, it probably doesn’t make sense to add another number to it. (What is null + 11)
If any of the operands in an expression contains a null value, the result is a null value. Any time
you need to use a nullable value, you should first check to see whether it is null or a usable value.
Delegates
A delegate is a special data type that refers to a method. The method can be an instance method
provided by an object or a static method defined by a class. Like other types such as classes and
enumerations, delegates can be defined in namespaces and classes but not inside methods.
A delegate variable acts as a pointer to a method. Delegate variables are sometimes called type-safe
function pointers.
The delegate keyword defines a delegate type and specifies the parameters and return type of the
method to which the delegate refers.
After you have defined a delegate type, you can declare variables that are of that type. You can set
those variables equal to methods that match the delegate’s parameters and return value.
For example, the following code defines a delegate type named ShowMessageType. This delegate
represents methods that take a single string as a parameter and that don’t return any result.
private delegate void ShowMessageType(string msg);
The following defines a ShowInConsoleWindow method, which simply displays a message in the
Console window.
// Display a message in the Console window.
private void ShowInConsoleWindow(string message)
{
Console.WriteLine(message);
}
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