Not much to say, please look at the code

/// <summary>
///  Determine whether the string is a number
/// </summary>
public static bool IsNumber(string s)
{
 if (string.IsNullOrWhiteSpace(s)) return false;
 const string pattern = "^[0-9]*$";
 Regex rx = new Regex(pattern);
 return rx.IsMatch(s);
}