C# this关键字的四种用法(转)

时间：2023-01-29 12:51:03

用法一  this代表当前类的实例对象

namespace Demo
{
public class Test
    {
private string scope = "全局变量";
public string getResult()
        {
string scope = "局部变量";
　　　　　　　// this代表Test的实例对象
　　　　　　　// 所以this.scope对应的是全局变量
　　　　　　  // scope对应的是getResult方法内的局部变量
return this.scope + "-" + scope;
        }
    }

class Program
    {
static void Main(string[] args)
        {
try
            {
                Test test = new Test();
                Console.WriteLine(test.getResult());
            }
catch (Exception ex)
            {
                Console.WriteLine(ex);
            }
finally
            {
                Console.ReadLine();
            }

        }
    }
}

用法二  用this串联构造函数

namespace Demo
{
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");
        }
    }

class Program
    {
static void Main(string[] args)
        {
try
            {
                Test test = new Test("张三");
            }
catch (Exception ex)
            {
                Console.WriteLine(ex);
            }
finally
            {
                Console.ReadLine();
            }
        }
    }
}

用法三  为原始类型扩展方法

namespace Demo
{
public static class Extends
    {
　　　　 // string类型扩展ToJson方法
public static object ToJson(this string Json)
        {
return Json == null ? null : JsonConvert.DeserializeObject(Json);
        }
// object类型扩展ToJson方法
public static string ToJson(this object obj)
        {
var timeConverter = new IsoDateTimeConverter { DateTimeFormat = "yyyy-MM-dd HH:mm:ss" };
return JsonConvert.SerializeObject(obj, timeConverter);
        }
public static string ToJson(this object obj, string datetimeformats)
        {
var timeConverter = new IsoDateTimeConverter { DateTimeFormat = datetimeformats };
return JsonConvert.SerializeObject(obj, timeConverter);
        }
public static T ToObject<T>(this string Json)
        {
return Json == null ? default(T) : JsonConvert.DeserializeObject<T>(Json);
        }
public static List<T> ToList<T>(this string Json)
        {
return Json == null ? null : JsonConvert.DeserializeObject<List<T>>(Json);
        }
public static DataTable ToTable(this string Json)
        {
return Json == null ? null : JsonConvert.DeserializeObject<DataTable>(Json);
        }
public static JObject ToJObject(this string Json)
        {
return Json == null ? JObject.Parse("{}") : JObject.Parse(Json.Replace("&nbsp;", ""));
        }
    }

class Program
    {
static void Main(string[] args)
        {
try
            {
                List<User> users = new List<User>{
new User{ID="1",Code="zs",Name="张三"},
new User{ID="2",Code="ls",Name="李四"}
                };

// list转化json字符串
string json = users.ToJson();
　　　　　　　　　 // string转化List
                users = json.ToList<User>();

// string转化DataTable
                DataTable dt = json.ToTable();
            }
catch (Exception ex)
            {
                Console.WriteLine(ex);
            }
finally
            {
                Console.ReadLine();
            }
        }
    }

public class User
    {
public string ID { get; set; }
public string Code { get; set; }
public string Name { get; set; }
    }
}

用法四  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
    }
}

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;
}

通过索引高效查询数据

/// <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);
}

 

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