.NET StringBuilder - check if ends with string

Question

What is the best (shortest and fastest) way to check if StringBuilder ends with specific string?

If I want to check just one char, that's not a problem sb[sb.Length-1] == 'c', but how to check if it's ends with longer string?

I can think about something like looping from "some string".Length and read characters one by one, but maybe there exists something more simple? :)

At the end I want to have extension method like this:

StringBuilder sb = new StringBuilder("Hello world");
bool hasString = sb.EndsWith("world");

Answer 1

To avoid the performance overhead of generating the full string, you can use the ToString(int,int) overload that takes the index range.

public static bool EndsWith(this StringBuilder sb, string test)
{
    if (sb.Length < test.Length)
        return false;

    string end = sb.ToString(sb.Length - test.Length, test.Length);
    return end.Equals(test);
}

Edit: It would probably be desirable to define an overload that takes a StringComparison argument:

public static bool EndsWith(this StringBuilder sb, string test)
{
    return EndsWith(sb, test, StringComparison.CurrentCulture);
}

public static bool EndsWith(this StringBuilder sb, string test, 
    StringComparison comparison)
{
    if (sb.Length < test.Length)
        return false;

    string end = sb.ToString(sb.Length - test.Length, test.Length);
    return end.Equals(test, comparison);
}

Edit₂: As pointed out by Tim S in the comments, there is a flaw in my answer (and all other answers that assume character-based equality) that affects certain Unicode comparisons. Unicode does not require two (sub)strings to have the same sequence of characters to be considered equal. For example, the precomposed character é should be treated as equal to the character e followed by the combining mark U+0301.

Thread.CurrentThread.CurrentCulture = new CultureInfo("en-US");

string s = "We met at the cafe\u0301";
Console.WriteLine(s.EndsWith("café"));    // True 

StringBuilder sb = new StringBuilder(s);
Console.WriteLine(sb.EndsWith("café"));   // False

If you want to handle these cases correctly, it might be easiest to just call StringBuilder.ToString(), and then use the built-in String.EndsWith.