c# this关键字用法

• 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;
        }
    }

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

• 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
    }
}

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;
}

/// <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);
}

参考文献：   https://www.cnblogs.com/yellowcool/p/7908607.html