索引器 (编程)

索引器（Indexer）在面向对象程序设计中允许一个类或者结构可以如同数组一样被索引（indexed）访问。为类定义一个索引器后，该类的行为就会像数组一样用数组访问运算符（[ ]）来访问该类的实例。这是一种运算符重载。

C#语言中通过重定义operator[]实现了索引器。Python语言中，类如果定义了__getitem__(self, key)与__setitem__(self, key, value)方法就能实现indexer。

例子
以下为 C#中在类中使用indexer:

class OurFamily
{
	public OurFamily(params string[] pMembers)
	{
	    familyMembers = new List<string>();
	    familyMembers.AddRange(pMembers);
	}
	
	private List<string> familyMembers;
	
	public string this[int index] //索引器定义的时候不带有名称，但带有 this 关键字，它指向对象实例。
	{
		// The get accessor
		get
		{
		    return familyMembers[index];
		}

		// The set accessor with 
		set
		{
		    familyMembers[index] = value;
		}
	}

	public int this[string val] //索引器（Indexer）可被重载。索引器声明的时候也可带有多个参数，参数不必须是整型。也可以是其他类型，例如字符串类型
	{
		// Getting index by value (first element found)
		get
		{
		    return familyMembers.FindIndex(m => m == val);
		}
	}

	public int Length => familyMembers.Count;
	
}

使用:

void Main()
{
    var doeFamily = new OurFamily("John", "Jane");
    for (int i = 0; i < doeFamily.Length; i++)
    {
        var member = doeFamily[i];
        var index = doeFamily[member]; // same as i in this case, but it demonstrates indexer overloading allowing to search doeFamily by value.
        Console.WriteLine($"{member} is the member number {index} of the {nameof(doeFamily)}");
    }
}

代码输出为:
   John is the member number 0 of the doeFamily
   Jane is the member number 1 of the doeFamily

参见 
 变异子与访问子

参考文献

面向对象的程序设计
运算符_(编程)