Is, IsNot

We use these VB.NET operators to check reference types. With these, we can check reference types against special value such as Nothing.

Some notes

We compare references to Nothing. The "Is" and "IsNot" operators are most often used with the Nothing constant. We can (for example) detect a null string.

Example

We see how "IsNot Nothing" and "Is Nothing" are evaluated with a local variable. This pattern of code is useful. It helps if you are not sure the variable is set to something.

Module Module1
    Sub Main()
        Dim value As String = "cat"

        ' Check if it is NOT Nothing.
        If value IsNot Nothing Then
            Console.WriteLine(1)
        End If

        ' Change to Nothing.
        value = Nothing

        ' Check if it IS Nothing.
        If value Is Nothing Then
            Console.WriteLine(2)
        End If

        ' This is not reached.
        If value IsNot Nothing Then
            Console.WriteLine(3)
        End If
    End Sub
End Module
1
2

TypeOf

We can use the VB.NET Is-operator with TypeOf. This compares the type of a variable to an existing type. This does not perform casting, but does check the types for equivalence.

Module Module1
    Sub Main()
        Dim value As String = "abc"

        ' Use Is-operator with TypeOf.
        If TypeOf value Is String
            Console.WriteLine("Is String")
        End If
    End Sub
End Module
Is String

IsNothing

We can use the IsNothing Function instead of using the Is-operator alongside Nothing. This performs the same test, and is slightly shorter.

Module Module1
    Sub Main()
        Dim value As String = Nothing
        ' Use IsNothing function instead of 2 separate keywords.
        If IsNothing(value) Then
            Console.WriteLine("IsNothing")
        End If
    End Sub
End Module
IsNothing

A review

Reference types are common in VB.NET programs—we create them based on classes. Things like Strings and StringBuilders are reference types.