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
/【本文由：http://www.yidunidc.com/mgzq.html复制请保留原URL】// </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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