String.Contains() with ignoring case and culture

Question

I had found only StringComparison, but that's not working for Contains. Another necessary thing is ignoring case, and "ä" should equials to "a".

Answer 1

public static bool CustomContains(this string source, string toCheck)
{
        CompareInfo ci = new CultureInfo("en-US").CompareInfo;
        CompareOptions co = CompareOptions.IgnoreCase | CompareOptions.IgnoreNonSpace;
        return ci.IndexOf(source, toCheck, co) != -1;
}