String IsNullOrEmpty

A String sometimes has no valid characters. The reference itself may be Nothing. And if a String does exist, it may be empty or have only whitespace characters.

These situations are detected with String.IsNullOrEmpty and IsNullOrWhiteSpace. We often use these functions in an If-Then statement.

IsNullOrEmpty

Here we use the String IsNullOrEmpty Function. This Function returns a Boolean. It checks the String argument against the Nothing constant. And it checks its Length as well.

Module Module1
    Sub Main()
        ' Test a Nothing String.
        Dim test1 As String = Nothing
        Dim result1 As Boolean = String.IsNullOrEmpty(test1)
        Console.WriteLine(result1)

        ' Test an empty String.
        Dim test2 As String = ""
        If (String.IsNullOrEmpty(test2)) Then
            Console.WriteLine("Is empty")
        End If
    End Sub
End Module
True
Is empty

IsNullOrWhiteSpace

This program uses String.IsNullOrWhiteSpace. This Function checks also for the Nothing reference. But it checks instead for whitespace, not just an empty String.

Module Module1
    Sub Main()
        ' Test a Nothing String.
        Dim test1 As String = Nothing
        Dim test2 As String = "  "

        If String.IsNullOrWhiteSpace(test1) Then
            Console.WriteLine(1)
        End If

        If String.IsNullOrWhiteSpace(test2) Then
            Console.WriteLine(2)
        End If

        Dim test3 As String = "Sam"
        If String.IsNullOrWhiteSpace(test3) Then
            Console.WriteLine(3) ' Not reached.
        End If
    End Sub
End Module
1
2

Notes, IsNullOrEmpty

Internally, we find String IsNullOrEmpty is implemented in a straightforward way. It checks the String against null (Nothing) and checks the Length against zero.

Notes, IsNullOrWhiteSpace

String IsNullOrWhiteSpace does more work when String data does exist. It scans over every character in the String to see if every character is whitespace.

IsNullOrEmpty is similar to IsNullOrWhiteSpace. IsNullOrWhiteSpace checks the entire String for whitespace. If the string contains only whitespace, the Function returns True.