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c# this關(guān)鍵字用法代碼詳解

 更新時(shí)間：2020年02月13日 16:34:14   作者：inner_cosmos  
在本篇文章里小編給大家整理的是關(guān)于c# this關(guān)鍵字用法以及相關(guān)實(shí)例代碼，有興趣的朋友們可以學(xué)習(xí)下。

用法1 為原始類型擴(kuò)展方法

先說一下，this 后面跟的類型，就是要拓展方法的類型。注意要寫在靜態(tài)類中的靜態(tài)方法，不然有些情況下訪問

/// <summary>
  /// 擴(kuò)展類 用于為原始類擴(kuò)展方法 
  /// </summary>
  public static class AM_Extends
  {
    /// <summary>
    /// 為string類擴(kuò)展了一個(gè)child方法，實(shí)現(xiàn)某功能
    /// </summary>
    /// <param name="str"></param>
    /// <param name="new_str"></param>
    public static void Child( this string str,string new_str)
    {
      object obj = str;
      str=new_str;
    }
  }

定義擴(kuò)展方法
private void Form1_Load(object sender, EventArgs e)
    {
      string st1 = "123";
      string st2 = "";
      string st3 = "";
      st3 = st2.Child(st1);//st3的值為“123”
    }

調(diào)用實(shí)例

用法2 this代表當(dāng)前類的實(shí)例對(duì)象

用法3 用this串聯(lián)構(gòu)造函數(shù)

public class Test
  {
    public Test()
    {
      Console.WriteLine("無參構(gòu)造函數(shù)");
    }
    // this()對(duì)應(yīng)無參構(gòu)造方法Test()
　　　　 // 先執(zhí)行Test()，后執(zhí)行Test(string text)
    public Test(string text) : this()
    {
      Console.WriteLine(text);
      Console.WriteLine("有參構(gòu)造函數(shù)");
    }
  }

用法4 索引器（基于索引器封裝EPList，用于優(yōu)化大數(shù)據(jù)下頻發(fā)的Linq查詢引發(fā)的程序性能問題，通過索引從list集合中查詢數(shù)據(jù)）

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Text;

namespace MyDemo.Web
{
  /// <summary>
  /// EPList 支持為L(zhǎng)ist創(chuàng)建索引
  /// </summary>
  /// <typeparam name="T">類型</typeparam>
  public class EPList<T>
  {
    #region 成員變量

    /// <summary>
    /// 索引
    /// </summary>
    private List<string[]> m_Index = new List<string[]>();

    /// <summary>
    /// 緩存數(shù)據(jù)
    /// </summary>
    private Dictionary<string, List<T>> m_CachedData = new Dictionary<string, List<T>>();

    /// <summary>
    /// List數(shù)據(jù)源
    /// </summary>
    private List<T> m_ListData = new List<T>();

    /// <summary>
    /// 通過索引值取數(shù)據(jù)
    /// </summary>
    /// <param name="indexFields">索引字段</param>
    /// <param name="fieldValues">字段值</param>
    /// <returns></returns>
    public List<T> this[string[] indexFields]
    {
      get
      {
        string key = string.Join(",", indexFields);
        if (m_CachedData.ContainsKey(key)) return m_CachedData[key];
        return new List<T>();
      }
    }

    #endregion

    #region 公共方法

    /// <summary>
    /// 創(chuàng)建索引
    /// </summary>
    /// <param name="indexFields">索引字段</param>
    public void CreateIndex(string[] indexFields)
    {
      if (m_Index.Contains(indexFields)) return;
      m_Index.Add(indexFields);
    }

    /// <summary>
    /// 添加
    /// </summary>
    /// <param name="record">記錄</param>
    public void Add(T record)
    {
      m_ListData.Add(record);
      m_Index.ForEach(indexFields =>
      {
        string key = getKey(record, indexFields);
        if (m_CachedData.ContainsKey(key))
        {
          m_CachedData[key].Add(record);
        }
        else
        {
          List<T> list = new List<T> { record };
          m_CachedData.Add(key, list);
        }
      });
    }

    #endregion

    #region 私有方法

    /// <summary>
    /// 獲取值
    /// </summary>
    /// <param name="record">記錄</param>
    /// <param name="fieldName">字段名</param>
    /// <returns></returns>
    private object getValue(T record, string fieldName)
    {
      Type type = typeof(T);
      PropertyInfo propertyInfo = type.GetProperty(fieldName);
      return propertyInfo.GetValue(record, null);
    }

    /// <summary>
    /// 獲取Key
    /// </summary>
    /// <param name="record">記錄</param>
    /// <param name="indexFields">索引字段</param>
    private string getKey(T record, string[] indexFields)
    {
      List<string> values = new List<string>();
      foreach (var field in indexFields)
      {
        string value = Convert.ToString(getValue(record, field));
        values.Add(field + ":" + value);
      }
      return string.Join(",", values);
    }

    /// <summary>
    /// 獲取Key
    /// </summary>
    /// <param name="indexFields">索引字段</param>
    /// <param name="fieldValues">字段值</param>
    /// <returns></returns>
    private string getKey(string[] indexFields, object[] fieldValues)
    {
      if (indexFields.Length != fieldValues.Length) return string.Empty;
      for (int i = 0; i < indexFields.Length; i++)
      {
        fieldValues[i] = indexFields[i] + ":" + fieldValues[i];
      }
      string key = string.Join(",", fieldValues);
      return key;
    }

    #endregion
  }
}

 創(chuàng)建EPList

給EPList創(chuàng)建索引，并添加數(shù)據(jù)

private EPList<SysDepartInfo> GetEPListData()
{
  EPList<SysDepartInfo> eplist = new EPList<SysDepartInfo>();
  eplist.CreateIndex(new string[] { "ParentId" });
  string sql = "select Id,ParentId,Code,Name from SysDepart";
  SqlHelper.ExecuteReader(sql, null, (reader) =>
  {
    SysDepartInfo record = new SysDepartInfo();
    record.Id = Convert.ToString(reader["Id"]);
    record.ParentId = Convert.ToString(reader["ParentId"]);
    record.Code = Convert.ToString(reader["Code"]);
    record.Name = Convert.ToString(reader["Name"]);
    eplist.Add(record);
  });
  return eplist;
}

給EPList創(chuàng)建索引，并添加數(shù)據(jù)
private EPList<SysDepartInfo> GetEPListData()
{
EPList<SysDepartInfo> eplist = new EPList<SysDepartInfo>();
eplist.CreateIndex(new string[] { "ParentId" });
string sql = "select Id,ParentId,Code,Name from SysDepart";
SqlHelper.ExecuteReader(sql, null, (reader) =>
{
SysDepartInfo record = new SysDepartInfo();
record.Id = Convert.ToString(reader["Id"]);
record.ParentId = Convert.ToString(reader["ParentId"]);
record.Code = Convert.ToString(reader["Code"]);
record.Name = Convert.ToString(reader["Name"]);
eplist.Add(record);
});
return eplist;
}

給EPList創(chuàng)建索引，并添加數(shù)據(jù)

/// <summary>
/// 獲取子節(jié)點(diǎn)
/// </summary>
/// <param name="data"></param>
/// <param name="parentId"></param>
private IEnumerable<TreeInfo> CreateChildren(EPList<SysDepartInfo> data, TreeInfo node)
{
  string id = node == null ? "0" : node.id;
  List<TreeInfo> childNodes = new List<TreeInfo>();
  // ParentId字段上創(chuàng)建了索引，所以這里就可以通過索引值直接取出下一層子節(jié)點(diǎn)數(shù)據(jù)，避免Linq查詢引發(fā)的效率問題
  var indexValues = new string[] { "ParentId:" + id };
  var childData = data[indexValues];
  childData.ForEach(record =>
  {
    var childNode = new TreeInfo
    {
      id = record.Id,
      text = record.Code + " " + record.Name
    };
    childNodes.Add(childNode);
    childNode.children = CreateChildren(data, childNode);
  });
  return childNodes.OrderBy(record => record.text);
}

通過索引高效查詢數(shù)據(jù)

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