LastIndexOf

This Function searches a String in reverse. It checks a string from the last part of the string first, going backwards. With it we locate a pattern.

LastIndexOfAny, meanwhile, searches for a string from an array argument. We can combine multiple LastIndexOf calls into 1 LastIndexOfAny call.

LastIndexOf example

The first example (index1) shows how to find the last instance of a Char—the last "e" is located. A String argument can also be passed to LastIndexOf.

Module Module1
    Sub Main()
        Dim value As String = "Dot Net Perls"

        ' Find a character.
        Dim index1 As Integer = value.LastIndexOf("e"c)
        Console.WriteLine(index1)

        ' Find a string.
        Dim index2 As Integer = value.LastIndexOf("Perls")
        Console.WriteLine("{0}, {1}", index2, value.Substring(index2))

        ' Nonexistent.
        Dim index3 As Integer = value.LastIndexOf("Nope")
        Console.WriteLine(index3)

        ' Search case-insensitively.
        Dim index4 As Integer =
            value.LastIndexOf("PERLS", StringComparison.OrdinalIgnoreCase)
        Console.WriteLine("{0}, {1}", index4, value.Substring(index4))
    End Sub
End Module
9
8, Perls
-1
8, Perls

LastIndexOfAny

This finds the last position of an acceptable character. We pass it an array of Char(). It searches for any of the contained elements.

Module Module1
    Sub Main()
        ' Input string.
        Dim value As String = "aaBBccBB"
        ' Search for any of these Chars.
        Dim index As Integer = value.LastIndexOfAny(New Char() {"c", "a"})
        Console.WriteLine(index)
    End Sub
End Module
5

Performance

The LastIndexOfAny function is not ideal in its performance. You must allocate an array. With a custom For loop, you could avoid this allocation.

The IndexOf family of functions is often useful. With LastIndexOf, you can reverse the search order, giving precedence to the final instances of characters.