索引

索引器允许类或结构的实例就像数组一样进行索引。 索引器类似于 属性，不同之处在于它们的访问器采用参数。

在下面的示例中，定义了一个泛型类，并为其提供了简单的 get 和 set 访问器方法（作为分配和检索值的方法）。 Program 类为存储字符串创建了此类的一个实例。

 

 

 
class SampleCollection<T>
{
    // Declare an array to store the data elements.
    private T[] arr = new T[100];
// Define the indexer, which will allow client code
    // to use [] notation on the class instance itself.
    // (See line 2 of code in Main below.)        
    public T this[int i]
    {
        get
        {
            // This indexer is very simple, and just returns or sets
            // the corresponding element from the internal array.
            return arr[i];
        }
        set
        {
            arr[i] = value;
        }
    }
}
// This class shows how client code uses the indexer.
class Program
{
    static void Main(string[] args)
    {
        // Declare an instance of the SampleCollection type.
        SampleCollection<string> stringCollection = new SampleCollection<string>();
// Use [] notation on the type.
        stringCollection[0] = "Hello, World";
        System.Console.WriteLine(stringCollection[0]);
    }
}
// Output:
// Hello, World.

 

 

 

 

 

 

 

 

 

 

说明

有关更多示例，请参见 相关章节。

 

索引器概述

 

• 使用索引器可以用类似于数组的方式为对象建立索引。
• get 访问器返回值。 set 访问器分配值。
• this 关键字用于定义索引器。
• value 关键字用于定义由 set 索引器分配的值。
• 索引器不必根据整数值进行索引，由您决定如何定义特定的查找机制。
• 索引器可被重载。
• 索引器可以有多个形参，例如当访问二维数组时。

 

class TempRecord
{
    // Array of temperature values
    private float[] temps = new float[10] { 56.2F, 56.7F, 56.5F, 56.9F, 58.8F, 
                                            61.3F, 65.9F, 62.1F, 59.2F, 57.5F };
// To enable client code to validate input 
    // when accessing your indexer.
    public int Length
    {
        get { return temps.Length; }
    }
    // Indexer declaration.
    // If index is out of range, the temps array will throw the exception.
    public float this[int index]
    {
        get
        {
            return temps[index];
        }
set
        {
            temps[index] = value;
        }
    }
}
class MainClass
{
    static void Main()
    {
        TempRecord tempRecord = new TempRecord();
        // Use the indexer's set accessor
        tempRecord[3] = 58.3F;
        tempRecord[5] = 60.1F;
// Use the indexer's get accessor
        for (int i = 0; i < 10; i++)
        {
            System.Console.WriteLine("Element #{0} = {1}", i, tempRecord[i]);
        }
// Keep the console window open in debug mode.
        System.Console.WriteLine("Press any key to exit.");
        System.Console.ReadKey();
}
}
/* Output:
        Element #0 = 56.2
        Element #1 = 56.7
        Element #2 = 56.5
        Element #3 = 58.3
        Element #4 = 58.8
        Element #5 = 60.1
        Element #6 = 65.9
        Element #7 = 62.1
        Element #8 = 59.2
        Element #9 = 57.5
    */
 
******************************************************************************************
 
 
// Using a string as an indexer value
class DayCollection
{
    string[] days = { "Sun", "Mon", "Tues", "Wed", "Thurs", "Fri", "Sat" };
// This method finds the day or returns -1
    private int GetDay(string testDay)
    {
for (int j = 0; j < days.Length; j++)
        {
            if (days[j] == testDay)
            {
                return j;
            }
        }
throw new System.ArgumentOutOfRangeException(testDay, "testDay must be in the form \"Sun\", \"Mon\", etc");
    }
// The get accessor returns an integer for a given string
    public int this[string day]
    {
        get
        {
            return (GetDay(day));
        }
    }
}
class Program
{
    static void Main(string[] args)
    {
        DayCollection week = new DayCollection();
        System.Console.WriteLine(week["Fri"]);
// Raises ArgumentOutOfRangeException
        System.Console.WriteLine(week["Made-up Day"]);
// Keep the console window open in debug mode.
        System.Console.WriteLine("Press any key to exit.");
        System.Console.ReadKey();
    }
}
// Output: 5