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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