The below excerpt is from the book: Applied Microsoft .Net Framework Programming by Jeffrey Richter. If you are a C# programmer and you are not aware of is and as operators in C#, then reading the following extract would be very useful.
C# offers another way to cast using the is operator. The is operator checks whether an object is compatible with a given type, and the result of the evaluation is a Boolean: true or false. The is operator will never throw an exception. The following code demonstrates:
System.Object o = new System.Object();
System.Boolean b1 = (o is System.Object); // b1 is true.
System.Boolean b2 = (o is Employee); // b2 is false.
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If the object reference is null, the is operator always returns false because there is no object available to check its type. The is operator is typically used as follows:
if (o is Employee)
{
Employee e = (Employee) o;
// Use e within the ‘if’ statement.
}
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In this code, the CLR is actually checking the object’s type twice: the is operator first checks to see if o is compatible with the Employee type. If it is, then inside the if statement, the CLR again verifies that o refers to an Employee when performing the cast. Because this programming paradigm is quite common, C# offers a way to simplify this code and improve its performance by providing an as operator:
Employee e = o as Employee;
if (e != null)
{
// Use e within the ‘if’ statement.
}
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In this code, the CLR checks if o is compatible with the Employee type, and if it is, as returns a non−null pointer to the same object. If o is not compatible with the Employee type, then the as operator returns null. Notice that the as operator causes the CLR to verify an object’s type just once. The if statement simply checks whether or not e is null—this check can be performed much more efficiently than verifying an object’s type.
The as operator works just like casting except the as operator will never throw an exception. Instead, if the object can’t be cast, the result is null. You’ll want to check to see whether the resulting reference is null, or attempting to use the resulting reference will cause a System.NullReferenceException exception to be thrown. The following code demonstrates:
System.Object o = new System.Object(); // Creates a new Object object
Employee e = o as Employee; // Casts o to an Employee
// The cast above fails: no exception is thrown, but e is set to null.
e.ToString(); // Accessing e throws a NullReferenceException.
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