String.Contains and String.LastIndexOf C# return different result?

Question

I have this problem where String.Contains returns true and String.LastIndexOf returns -1. Could someone explain to me what happened? I am using .NET 4.5.

    static void Main(string[] args)
    {
        String wikiPageUrl = @"http://it.wikipedia.org/wiki/ʿAbd_Allāh_al-Sallāl";

        if (wikiPageUrl.Contains("wikipedia.org/wiki/"))
        {

            int i = wikiPageUrl.LastIndexOf("wikipedia.org/wiki/");

            Console.WriteLine(i);

        }
    }

Answer 1

While @sa_ddam213's answer definitely fixes the problem, it might help to understand exactly what's going on with this particular string.

If you try the example with other "special characters," the problem isn't exhibited. For example, the following strings work as expected:

string url1 = @"http://it.wikipedia.org/wiki/»Abd_Allāh_al-Sallāl";
Console.WriteLine(url1.LastIndexOf("it.wikipedia.org/wiki/")); // 7

string url2 = @"http://it.wikipedia.org/wiki/~Abd_Allāh_al-Sallāl";
Console.WriteLine(url2.LastIndexOf("it.wikipedia.org/wiki/")); // 7

The character in question, "ʿ", is called a spacing modifier letter¹. A spacing modifier letter doesn't stand on its own, but modifies the previous character in the string, this case a "/". Another way to put this is that it doesn't take up its own space when rendered.

LastIndexOf, when called with no StringComparison argument, compares strings using the current culture.

When strings are compared in a culture-sensitive manner, the "/" and "ʿ" characters are not seen as two distinct characters--they're processed into one character, which does not match the parameter passed in to LastIndexOf.

When you pass in StringComparison.Ordinal to LastIndexOf, the characters are treated as distinct, due to the nature of Ordinal comparison.

Another way to make this work would be to use CompareInfo.LastIndexOf and supply the CompareOptions.IgnoreNonSpace option:

Console.WriteLine(
    CultureInfo.CurrentCulture.CompareInfo.LastIndexOf(
        wikiPageUrl, @"it.wikipedia.org/wiki/", CompareOptions.IgnoreNonSpace));
// 7

Here we're saying that we don't want combining characters included in our string comparison.

As a sidenote, this means that @Partha's answer and @Noctis' answer only work because the character is being applied to a character that doesn't appear in the search string that's passed to LastIndexOf.

Contrast this with the Contains method, which by default performs an Ordinal (case sensitive and culture insensitive) comparison. This explains why Contains returns true and LastIndexOf returns false.

For a fantastic overview of how strings should be manipulated in the .NET framework, check out this article.

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¹: Is this different than a combining character or is it a type of combining character? would appreciate if someone would clear that up for me.