c# this关键字用法代码详解

用法1 为原始类型扩展方法

先说一下，this 后面跟的类型，就是要拓展方法的类型。注意要写在静态类中的静态方法，不然有些情况下访问

/// <summary>
 /// 扩展类 用于为原始类扩展方法
 /// </summary>
 public static class AM_Extends
 {
   /// <summary>
   /// 为string类扩展了一个child方法，实现某功能
   /// </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;
   }
 }

定义扩展方法

private void Form1_Load(object sender, EventArgs e)
   {
     string st1 = "123";
     string st2 = "";
     string st3 = "";
     st3 = st2.Child(st1);//st3的值为“123”
   }

调用实例

用法2 this代表当前类的实例对象

用法3 用this串联构造函数

public class Test
 {
   public Test()
   {
     Console.WriteLine("无参构造函数");
   }
   // this()对应无参构造方法Test()
// 先执行Test()，后执行Test(string text)
   public Test(string text) : this()
   {
     Console.WriteLine(text);
     Console.WriteLine("有参构造函数");
   }
 }

用法4 索引器（基于索引器封装EPList，用于优化大数据下频发的Linq查询引发的程序性能问题，通过索引从list集合中查询数据）

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

namespace MyDemo.Web
{
 /// <summary>
 /// EPList 支持为List创建索引
 /// </summary>
 /// <typeparam name="T">类型</typeparam>
 public class EPList<T>
 {
   #region 成员变量

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

/// <summary>
   /// 缓存数据
   /// </summary>
   private Dictionary<string, List<T>> m_CachedData = new Dictionary<string, List<T>>();

/// <summary>
   /// List数据源
   /// </summary>
   private List<T> m_ListData = new List<T>();

/// <summary>
   /// 通过索引值取数据
   /// </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>
   /// 创建索引
   /// </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">记录</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>
   /// 获取值
   /// </summary>
   /// <param name="record">记录</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>
   /// 获取Key
   /// </summary>
   /// <param name="record">记录</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>
   /// 获取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
 }
}

创建EPList

给EPList创建索引，并添加数据

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;
}

给EPList创建索引，并添加数据

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;
}

给EPList创建索引，并添加数据

/// <summary>
/// 获取子节点
/// </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字段上创建了索引，所以这里就可以通过索引值直接取出下一层子节点数据，避免Linq查询引发的效率问题
 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);
}

通过索引高效查询数据

来源：https://www.cnblogs.com/MatureMan/p/12303347.html