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

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