VB.NET - TypeOf Use (Is, IsNot)

TypeOf Use (Is, IsNot)

This page was last reviewed on Sep 5, 2024.

TypeOf. How can we test the type of an Object instance in VB.NET? With the TypeOf operator, we can access and test the type of a variable.

Object

With the Is keyword, we can test for a specific type (like String or a class). And IsNot tests that the type is not the specified one.

Class

CType

Example. To begin, this VB.NET example program introduces a simple class called Example. The Example class derives from Object (as do all classes).

Part 1 We create an instance of the Example class, and refer to it through a Object reference variable.

Part 2 We use the TypeOf operator along with the Is-keyword to test the the local variable is an instance of the Example class.

Part 3 The IsNot operator performs the opposite check—it is a clear way to combine the "Not" and "Is" operators.

IsNot, Is

Part 4 With TryCast, we cast our Object local variable back into an Example reference.

TryCast

Part 5 With TypeOf, all base classes of a variable are also checked, and this means the Is-keyword will return True for Object.

Class Example Public Dim _name As String = "bird" End Class Module Module1 Sub Main() ' Part 1: refer to a new Example instance as an Object. Dim t As Object = New Example() ' Part 2: use Is-operator with TypeOf to test type. If TypeOf t Is Example Console.WriteLine("Is Example") End If ' Part 3: use IsNot operator with TypeOf. If TypeOf t IsNot String Console.WriteLine("IsNot String") End If ' Part 4: use TryCast to get back to Example type. Dim e = TryCast(t, Example) If Not IsNothing(e) Console.WriteLine($"Name = {e._name}") End If ' Part 5: TypeOf Is can detect base classes like Object. If TypeOf e Is Object Console.WriteLine("Is Object") End If End Sub End Module

Is Example IsNot String Name = bird Is Object

Summary. Though we cannot use the Is-keyword to directly test the type of a variable, we can use it as part of a TypeOf expression. This allows us to test types (and base types) in VB.NET.

Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.

Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.

This page was last updated on Sep 5, 2024 (edit link).

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