C#索引属性用法实例分析

// indexedproperty.cs
using System;

public class Document
{
  // 以下类型允许文档的查看方式与字的数组一样：
  public class WordCollection
  {
    readonly Document document; // 包含文档
    internal WordCollection(Document d)
    {
      document = d;
    }
    // Helper 函数 -- 从字符"begin"开始在字符数组"text"中搜索
    // 字数"wordCount"。如果字数小于 wordCount，
    // 则返回 false。将"start"和
    // "length"设置为单词在文本中的位置和长度:
    private bool GetWord(char[] text, int begin, int wordCount, out int start, out int length) 
    { 
      int end = text.Length;
      int count = 0;
      int inWord = -1;
      start = length = 0; 
      for (int i = begin; i <= end; ++i) 
      {
        bool isLetter = i < end && Char.IsLetterOrDigit(text[i]);
        if (inWord >= 0) 
        {
          if (!isLetter) 
          {
            if (count++ == wordCount) 
            {
              start = inWord;
              length = i - inWord;
              return true;
            }
            inWord = -1;
          }
        }
        else 
        {
          if (isLetter)
            inWord = i;
        }
      }
      return false;
    }
    // 获取和设置包含文档中的字的索引器：
    public string this[int index] 
    {
      get 
      { 
        int start, length;
        if (GetWord(document.TextArray, 0, index, out start, out length))
          return new string(document.TextArray, start, length);
        else
          throw new IndexOutOfRangeException();
      }
      set 
      {
        int start, length;
        if (GetWord(document.TextArray, 0, index, out start, out length)) 
        {
          // 用字符串"value"替换位于 start/length 处的
          // 字：
          if (length == value.Length) 
          {
            Array.Copy(value.ToCharArray(), 0, document.TextArray, start, length);
          }
          else 
          {
            char[] newText = 
              new char[document.TextArray.Length + value.Length - length];
            Array.Copy(document.TextArray, 0, newText, 0, start);
            Array.Copy(value.ToCharArray(), 0, newText, start, value.Length);
            Array.Copy(document.TextArray, start + length, newText, start + value.Length, document.TextArray.Length - start - length);
            document.TextArray = newText;
          }
        }          
        else
          throw new IndexOutOfRangeException();
      }
    }
    // 获取包含文档中字的计数：
    public int Count 
    {
      get 
      { 
        int count = 0, start = 0, length = 0;
        while (GetWord(document.TextArray, start + length, 0, out start, out length))
          ++count;
        return count; 
      }
    }
  }
  // 以下类型允许文档的查看方式像字符的"数组"
  // 一样：
  public class CharacterCollection
  {
    readonly Document document; // 包含文档
    internal CharacterCollection(Document d)
    {
     document = d; 
    }
    // 获取和设置包含文档中的字符的索引器：
    public char this[int index] 
    {
      get 
      { 
        return document.TextArray[index]; 
      }
      set 
      { 
        document.TextArray[index] = value; 
      }
    }
    // 获取包含文档中字符的计数：
    public int Count 
    {
      get 
      { 
        return document.TextArray.Length; 
      }
    }
  }
  // 由于字段的类型具有索引器，
  // 因此这些字段显示为"索引属性"：
  public WordCollection Words;
  public CharacterCollection Characters;
  private char[] TextArray; // 文档的文本。
  public Document(string initialText)
  {
    TextArray = initialText.ToCharArray();
    Words = new WordCollection(this);
    Characters = new CharacterCollection(this);
  }
  public string Text 
  {
    get 
    { 
      return new string(TextArray); 
    }
  }
}

class Test
{
  static void Main()
  {
    Document d = new Document(
      "peter piper picked a peck of pickled peppers. How many pickled peppers did peter piper pick?"
    );
    // 将字"peter"更改为"penelope"：
    for (int i = 0; i < d.Words.Count; ++i) 
    {
      if (d.Words[i] == "peter") 
        d.Words[i] = "penelope";
    }
    // 将字符"p"更改为"P"
    for (int i = 0; i < d.Characters.Count; ++i) 
    {
      if (d.Characters[i] == 'p')
        d.Characters[i] = 'P';
    }
    Console.WriteLine(d.Text);
  }
}