ArrayList.GetEnumerator(Int32,Int32)方法用于获取ArrayList中一系列元素的枚举数。
句法:
public virtual System.Collections.IEnumerator GetEnumerator (int index, int count);
参数:
index: It is the zero-based starting index of type Int32 of the ArrayList section that the enumerator should refer to.
count: It is the number of elements of the type Int32 in the ArrayList section that the enumerator should refer to.
返回值:该方法为ArrayList中指定范围的元素返回IEnumerator。
例外情况:
- ArgumentOutOfRangeException:如果索引或计数小于零。
- ArgumentException:如果索引和计数未在ArrayList中指定有效范围。
下面的程序说明了上面讨论的方法的使用:
范例1:
// C# code to get an enumerator for a
// range of elements in the ArrayList
using System;
using System.Collections;
class GFG {
// Driver code
public static void Main()
{
// Creating an ArrayList
ArrayList myList = new ArrayList();
// adding elements in myList
myList.Add(14);
myList.Add(45);
myList.Add(78);
myList.Add(48);
myList.Add(49);
myList.Add(51);
myList.Add(77);
// To get an Enumerator
// for the ArrayList
IEnumerator enumerator = myList.GetEnumerator();
// If MoveNext passes the end of the
// collection, the enumerator is positioned
// after the last element in the ArrayList
// and MoveNext returns false.
while (enumerator.MoveNext()) {
Console.WriteLine(enumerator.Current);
}
Console.WriteLine("After GetEnumerator(Int32, Int32) Method: ");
// To get an Enumerator for a range
// of elements of the ArrayList
// here index is 3 and count is 2
IEnumerator e = myList.GetEnumerator(3, 2);
// If MoveNext passes the end of the
// collection, the enumerator is positioned
// after the last element in the ArrayList
// and MoveNext returns false.
while (e.MoveNext()) {
Object obj1 = e.Current;
Console.WriteLine(obj1);
}
}
}
输出:
14
45
78
48
49
51
77
After GetEnumerator(Int32, Int32) Method:
48
49
范例2:
// C# code to get an enumerator for a
// range of elements in the ArrayList
using System;
using System.Collections;
class GFG {
// Driver code
public static void Main()
{
// Creating an ArrayList
ArrayList myList = new ArrayList();
// adding elements in myList
myList.Add("C");
myList.Add("C++");
myList.Add("Java");
myList.Add("Python");
myList.Add("C#");
myList.Add("HTML");
myList.Add("CSS");
Console.WriteLine("After GetEnumerator(Int32, Int32) Method: ");
// To get an Enumerator for a range
// of elements of the ArrayList
// this will give error as index is
// less than zero
IEnumerator e = myList.GetEnumerator(-1, 2);
// If MoveNext passes the end of the
// collection, the enumerator is positioned
// after the last element in the ArrayList
// and MoveNext returns false.
while (e.MoveNext()) {
Object obj1 = e.Current;
Console.WriteLine(obj1);
}
}
}
运行时错误:
Unhandled Exception:
System.ArgumentOutOfRangeException: Non-negative number required.
Parameter name: index
笔记:
- C#语言的foreach语句隐藏了枚举器的复杂性。因此,建议使用foreach ,而不是直接操作枚举器。
- 枚举数可用于读取集合中的数据,但不能用于修改基础集合。
- 在调用MoveNext或Reset之前,Current返回相同的对象。 MoveNext将Current设置为下一个元素。
- 只要集合保持不变,枚举数将保持有效。如果对集合进行了更改(例如添加,修改或删除元素),则枚举数将无法恢复,并且其行为是不确定的。
- 此方法是O(1)操作。
参考:
- https://docs.microsoft.com/zh-cn/dotnet/api/system.collections.arraylist.getenumerator?view=netframework-4.7.2