Is, IsNot. We use these 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 isn't reached.
If value IsNot Nothing Then
Console.WriteLine(3)
End If
End Sub
End Module1
2
Is, IsNot notes. These operators can only be used with reference types. The Nothing constant is a special instance of a reference type. We cannot use the Is-operator to perform casting.
Note These are most commonly used with the Nothing constant. But any two references can be compared.
And The result depends on the memory locations—not the object data the references point to.
A review. Reference types are common in VB.NET programs—we create them based on classes. Things like Strings and StringBuilders are reference types.
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 Jun 14, 2023 (simplify).