using System;

​

public static class Program

{

    public static void Main(string[] args)

    {

        var source = "TaxIdentificationNumber";

        var check = "TaxIdentificationNumb";

        var str = StringComparison.CurrentCultureIgnoreCase;

        Console.WriteLine(Contains(source, check, str));

    }

    public static bool Contains(this string source, string toCheck, StringComparison comp)

    {

        return source.IndexOf(toCheck, comp) >= 0;

    }

}

​

​