这段代码是实现排序

不过，无法编译
s(2,29): error CS0246: 找不到类型或命名空间名称“IComparer”(是否缺少 using
 指令或程序集引用？)
s(10,24): error CS0246:
 找不到类型或命名空间名称“PersoncompareType”(是否缺少 using
 指令或程序集引用？)

求教

{
public enum PersonCompareType
{
firstname,
lastname
}
{

不让发 少发一段
using System;
这里少句话
{
public enum PersonCompareType
{
firstname,
lastname
}
private PersonCompareType compareType;
public PersonComparer(PersoncompareType compareType)
{
this.compareType=compareType;
}
public int Compare(object x,object y)
{
Person p1= x as Person;
Person p1=y as Person;
Switch(compareType)
{
case PersonCompareType.firstname:
return p1.firstname.CompareTo(p2.firstname);
case PersonCompareType.lastname:
 return p1.lastname.CompareTo(p2.lastname);
default:
 throw new ArgumentException("unexpexted compare type");
}
}
}
class a
{
static void Main()
{
Person[] persons={
 new Person("Emerson","Fittipaldi"),
 new Person("Niki","Lauda"),
 new Person("Ayrton","Senna"),
 new Person("Michael","Schumacher"),
};
Array.Sort(persons ,new PersonComparer(PersonComparer.PersonCompareType.firstname));
foreach(Person p in persons)
{
Console.WriteLine(p);
}
}
}
public class Person
{
private string firstname,lastname;
public Person()
{
}
public Person (string firstName,string lastName)
{
this.firstname=firstName;
this.lastname=lastName;
}
public string Firstname
{
get
{
return firstname;
}
}
public string Lastname
{
get
{
return lastname;
}
}
public override string ToString()
{
return string.Format("{0} {1}",firstname,lastname);
}
}

public class PersonComparer：

IComparer

上两楼为少的那段话