1 cái Extension Method để extend cái class String, vẫn chép từ SO

StringExtension.cs

// You can use an extension method:

public static class StringExtension
{
    public static string GetLast(this string source, int tail_length)
    {
       if(tail_length >= source.Length)
          return source;
       return source.Substring(source.Length - tail_length);
    }
}

// And then call:

string mystring = "34234234d124";
string res = mystring.GetLast(4);