VB.NET - TypeOf Use (Is, IsNot)

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.

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

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