C#中如何生成一个18位的随机数,

public class TestClass { public String get(int len) { try { String str = ""; while (str.Length < len) { raStr += ra.Next(); } return raStr.Substring(0, len); } catch (Exception e) { return ""; } } } public class APPClass { static void Main() { TestClass test=new TestClass(); string str=test.get(18); Console.WriteLine(str); } }