`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.

Another Function

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.

Argument 1 If the String passed to LastIndexOf is not found, you will receive the value -1. This should be tested with an If-expression.

Argument 2 Use the argument StringComparison.OrdinalIgnoreCase. Lowercase and uppercase letters will be considered equal.

Result The substring "PERLS" is found in the input string in the substring "Perls".

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 Module9
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.

Here We provide an array argument to LastIndexOfAny. The array must contain the set of characters you are trying to find.

Then The function scans from the final character backwards. It checks for those characters one-by-one.

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.

Also If you use LastIndexOfAny in a tight loop, you can allocate the Char() array outside the loop for better speed.

Note If you only need to locate one string, it is a better idea to use the LastIndexOf Function.

Summary

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

LastIndexOf