What is the equivalent of C#'s `default` in VB.NET?

Question

I\'m normally at home in C#, and I\'m looking at a performance issue in some VB.NET code -- I want to be able to compare something to the default value for a type (kind of l

Answer 1

This is not a complete solution, as your original C# code doesn't compile. You can use Nothing via a local variable:

Public Class GenericThing(Of T)
    Public Sub Foo(id As T)
        Dim defaultValue As T = Nothing
        If id <> defaultValue Then
            Console.WriteLine("Not default")
        Else
            Console.WriteLine("Default")
        End If
    End Function
End Class

That doesn't compile, in the same way that the C# version doesn't compile - you can't compare values of unconstrained type parameters like that.

You can use EqualityComparer(Of T) though - and then you don't even need the local variable:

If Not EqualityComparer(Of T).Default.Equals(id, Nothing) Then

Answer 2

Unlike C#, VB.NET doesn't require a local variable to be initialized with an expression. It gets initialized to its default value by the runtime. Just what you need as a substitute for the default keyword:

    Dim def As T2    '' Get the default value for T2
    If id.Equals(def) Then
       '' etc...
    End If

Don't forget the comment, it is going to make somebody go 'Huh?' a year from now.

Answer 3

The problem in your code is the IsNot operator, not the Nothing keyword. From the docs:

The IsNot operator determines if two object references refer to different objects. However, it does not perform value comparisons.

You're trying to do a value comparison with a reference operator. Once you realize this, either Jon Skeet's or Hans Passant's answers become obvious solutions.