Nullable Values and Nothing

Nothing means "no value", but it behaves differently for reference types and value types.

Nothing for reference types

For classes, strings, and arrays, Nothing means "no object":

Module Program
    Function FindUser(id As Integer) As String
        If id = 1 Then Return "Ada"
        Return Nothing
    End Function

    Sub Main()
        Dim user = FindUser(2)
        Console.WriteLine(user Is Nothing)
        Console.WriteLine(If(user, "(guest)"))
        Console.WriteLine(If(user?.ToUpper(), "(no name)"))
        Console.WriteLine(String.IsNullOrEmpty(user))
    End Sub
End Module

True
(guest)
(no name)
True

• Test with Is Nothing / IsNot Nothing.
• If(value, fallback) supplies a default.
• ?. stops and returns Nothing if the left side is Nothing.

Nothing for value types: the default value

Assigning Nothing to an Integer, Boolean, or Date doesn't mean "empty" — it sets the type's default value:

Module Program
    Sub Main()
        Dim n As Integer = Nothing
        Dim b As Boolean = Nothing
        Dim d As Date = Nothing
        Console.WriteLine(n)
        Console.WriteLine(b)
        Console.WriteLine(d.Year)
    End Sub
End Module

0
False
1

This surprises developers coming from C#, where int x = null doesn't compile.

Nullable value types

To let a value type be truly "missing", make it nullable with ? (short for Nullable(Of T)):

Module Program
    Sub Main()
        Dim age As Integer? = Nothing
        Console.WriteLine(age.HasValue)
        Console.WriteLine(age.GetValueOrDefault(-1))
        Console.WriteLine(If(age, 0))

        age = 36
        Console.WriteLine(age.HasValue & " " & age.Value)

        Dim total As Integer? = age + 1
        Console.WriteLine(total)

        Dim missing As Integer? = Nothing
        Dim sum = missing + 5
        Console.WriteLine(sum.HasValue)
    End Sub
End Module

False
-1
0
True 36
37
False

Arithmetic with a missing value gives a missing result. Nullable types are common for optional database columns and optional form fields.

Comparing with Nothing

Module Program
    Sub Main()
        Dim score As Integer? = Nothing
        Console.WriteLine(score Is Nothing)
        If score > 50 Then
            Console.WriteLine("passed")
        Else
            Console.WriteLine("not passed (or unknown)")
        End If
    End Sub
End Module

True
not passed (or unknown)

Use Is Nothing to test a nullable value — score = Nothing compares with a missing value and is never True. A comparison involving a missing value is treated as False in an If.

Common mistakes

• Calling .Value on an empty nullable — it throws InvalidOperationException. Check HasValue or use GetValueOrDefault.
• Forgetting that a String field starts as Nothing, not "".