c# this關鍵字用法程式碼詳解
阿新 • • 發佈：2020-02-15
用法1 為原始型別擴充套件方法
先說一下，this 後面跟的型別，就是要拓展方法的型別。注意要寫在靜態類中的靜態方法，不然有些情況下訪問
/// <summary>
/// 擴充套件類 用於為原始類擴充套件方法
/// </summary>
public static class AM_Extends
{
/// <summary>
/// 為string類擴充套件了一個child方法，實現某功能
/// </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;
}
}
定義擴充套件方法
private void Form1_Load(object sender,EventArgs e)
{
string st1 = "123";
string st2 = "";
string st3 = "";
st3 = st2.Child(st1);//st3的值為“123”
}
呼叫例項
用法2 this代表當前類的例項物件
用法3 用this串聯建構函式
public class Test
{
public Test()
{
Console.WriteLine("無參建構函式");
}
// this()對應無參構造方法Test()
// 先執行Test()，後執行Test(string text)
public Test(string text) : this()
{
Console.WriteLine(text);
Console.WriteLine("有參建構函式");
}
}用法4 索引器（基於索引器封裝EPList，用於優化大資料下頻發的Linq查詢引發的程式效能問題，通過索引從list集合中查詢資料）
using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Text;
namespace MyDemo.Web
{
/// <summary>
/// EPList 支援為List建立索引
/// </summary>
/// <typeparam name="T">型別</typeparam>
public class EPList<T>
{
#region 成員變數
/// <summary>
/// 索引
/// </summary>
private List<string[]> m_Index = new List<string[]>();
/// <summary>
/// 快取資料
/// </summary>
private Dictionary<string,List<T>> m_CachedData = new Dictionary<string,List<T>>();
/// <summary>
/// List資料來源
/// </summary>
private List<T> m_ListData = new List<T>();
/// <summary>
/// 通過索引值取資料
/// </summary>
/// <param name="indexFields">索引欄位</param>
/// <param name="fieldValues">欄位值</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>
/// 建立索引
/// </summary>
/// <param name="indexFields">索引欄位</param>
public void CreateIndex(string[] indexFields)
{
if (m_Index.Contains(indexFields)) return;
m_Index.Add(indexFields);
}
/// <summary>
/// 新增
/// </summary>
/// <param name="record">記錄</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>
/// 獲取值
/// </summary>
/// <param name="record">記錄</param>
/// <param name="fieldName">欄位名</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>
/// 獲取Key
/// </summary>
/// <param name="record">記錄</param>
/// <param name="indexFields">索引欄位</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>
/// 獲取Key
/// </summary>
/// <param name="indexFields">索引欄位</param>
/// <param name="fieldValues">欄位值</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
}
}
建立EPList給EPList建立索引，並新增資料
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;
}
給EPList建立索引，並新增資料
private EPList<SysDepartInfo> GetEPListData()
{
EPList<SysDepartInfo> eplist = new EPList<SysDepartInfo>();
eplist.CreateIndex(new string[] { "ParentId" });
string sql = "select Id,Name from SysDepart";
SqlHelper.ExecuteReader(sql,(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;
}
給EPList建立索引，並新增資料
/// <summary>
/// 獲取子節點
/// </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欄位上建立了索引，所以這裡就可以通過索引值直接取出下一層子節點資料，避免Linq查詢引發的效率問題
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);
}
通過索引高效查詢資料
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