VB.NET Language in a Nutshell, Second Edition

Name

Is Operator

Syntax

                  object1 Is object2

object1 (required; Object or any reference type)

object2 (required; Object or any reference type)

Return Value

Boolean

Description

Compares two object variables or reference variables to determine whether they reference the same object

Rules at a Glance

• Both object1 and object2 must be reference-type variables. This includes string variables, object variables, and array variables, for instance.

• The operation returns a result of True if the references are identical and False if they are not.

• It is also possible to determine whether an object contains a valid reference by replacing object2 with the special Nothing keyword. For example:

  If oDrive Is Nothing Then

  returns True if oDrive does not refer to an object and False if it does. This is the only method that should be used to test for an uninitialized object reference.

Programming Tips and Gotchas

• You can call the IsReference function to ensure that both object1 and object2 are reference types.

• You may wonder why there is a special Is operator for reference types. When you perform a comparison of scalar variables, you want to know if their values are the same. But in the case of objects, you want to know if two references point to a single object. (Since many objects have identical property values, a test for equal values is meaningless.) Hence, the Is operator is used for this purpose.

• Typically, the Is operator is used in an If...Then...Else construct to take some action if two reference-type ...