C#从foreach语句中枚举元素看数组详解

color: #ff0000">前言

在foreach语句中使用枚举，可以迭代数组或集合中的元素，且无须知道集合中的元素的个数。如图显示了调用foreach方法的客户端和集合之间的关系。数组或集合实现带GetEnumerator()方法的IEnumerable接口。GetEnumerator()方法返回一个实现lEnumerable接口的枚举，接着foreach语句就可以使用IEnumerable接口迭代集合了。

GetEnumerator()方法用IEnumerable接口定义，foreach语句并不真的需要在集合类中实现这个接口。有一个名为GetEnumerator()的方法它返回实现了IEnumerator接口的对象就足够了。

先定义一个Person类，这个类有自动实现的属性Firstname和Lastname,以及从Object类重写ToString方法和继承泛型接口IEquatable以比较两个对象是否相等，实现泛型接口IComparer以比较两个对象用来排序。

public class Person : IEquatable<Person>,IComparable<Person>
 {
  public int Id { get; private set; }
  public string FirstName { get; set; }
  public string LastName { get; set; }

  public override string ToString()
  {
   return String.Format("{0}, {1} {2}", Id, FirstName, LastName);
  } 

  public bool Equals(Person other)
  {
   if (other == null)
    return base.Equals(other);

   return this.FirstName == other.FirstName && this.LastName == other.LastName;
  }

  public int CompareTo(Person other)
  {
   if (other == null) throw new ArgumentNullException("other");

   int result = this.LastName.CompareTo(other.LastName);
   if (result == 0)
   {
    result = this.FirstName.CompareTo(other.FirstName);
   }

   return result;
  }

 }

 Person[] persons = {
    new Person { FirstName = "Simen03", LastName = "Go" },
    new Person { FirstName = "Simen02", LastName = "Go" },
    new Person { FirstName = "Simen01", LastName = "Go" }
   };
   Array.Sort(persons);
   foreach (var person in persons)
    Console.WriteLine(person);

// loop start (head: IL_009b)
   IL_008a: ldloc.2
   IL_008b: ldloc.3
   IL_008c: ldelem.ref
   IL_008d: stloc.s person
   IL_008f: ldloc.s person
   IL_0091: call void [mscorlib]System.Console::WriteLine(object)
   IL_0096: nop
   IL_0097: ldloc.3
   IL_0098: ldc.i4.1
   IL_0099: add
   IL_009a: stloc.3

   IL_009b: ldloc.3
   IL_009c: ldloc.2
   IL_009d: ldlen
   IL_009e: conv.i4
   IL_009f: blt.s IL_008a
  // end loop								 
 1/3    1 2 3 下一页 尾页