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C#内置泛型委托：Func委托
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1、什么是Func委托
Func委托代表有返回类型的委托
2、Func委托定义
查看Func的定义：
using
System.Runtime.CompilerServices;
namespace
System
{
//
//
摘要:
//
封装一个方法，该方法具有两个参数，并返回由 TResult 参数指定的类型的值。
//
//
参数:
//
arg1:
//
此委托封装的方法的第一个参数。
//
//
arg2:
//
此委托封装的方法的第二个参数。
//
//
类型参数:
//
T1:
//
此委托封装的方法的第一个参数的类型。
//
//
T2:
//
此委托封装的方法的第二个参数的类型。
//
//
TResult:
//
此委托封装的方法的返回值类型。
//
//
返回结果:
//
此委托封装的方法的返回值。
[TypeForwardedFrom("System.Core, Version=3.5.0.0, Culture=Neutral, PublicKeyToken=b77a5c561934e089")]
publicdelegate TResult Func<in T1, in T2, out TResult>(T1 arg1, T2 arg2);
}
你会发现，Func其实就是有多个输出参数并且有返回值的delegate。
3、示例
Func至少0个输入参数，至多16个输入参数，根据返回值泛型返回。必须有返回值，不可void。
Func<int> 表示没有输入参参，返回值为int类型的委托。
Func<object,string,int> 表示传入参数为object, string ，返回值为int类型的委托。
Func<object,string,int> 表示传入参数为object, string， 返回值为int类型的委托。
Func<T1,T2,,T3,int> 表示传入参数为T1,T2,,T3(泛型)，返回值为int类型的委托。
代码示例如下：
using
System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Text;
using
System.Threading.Tasks;
namespace
FunDemo
{
class
Program
{
static
void Main(string[] args)
{
// 无参数，只要返回值
Func<int> fun1 = new Func<int>(FunWithNoPara);
int result1= fun1();
Console.WriteLine(result1);
Console.WriteLine("----------------------------");
Func<int> fun2 = delegate { return19; };
int result2 = fun2();
Console.WriteLine(result2);
Console.WriteLine("----------------------------");
Func<int> fun3 = () => { return3; };
int result3 = fun3();
Console.WriteLine(result3);
Console.WriteLine("----------------------------");
//有一个参数，一个返回值
Func<int, int> fun4 = new Func<int, int>(FunWithPara);
int result4 = fun4(4);
Console.WriteLine($"这里是一个参数一个返回值的方法，返回值是：{result4}");
Console.WriteLine("----------------------------");
// 使用委托
Func<int, string> fun5 = delegate (int i) { return i.ToString(); };
string result5 = fun5(5);
Console.WriteLine($"这里是一个参数一个返回值的委托，返回值是：{result5}");
Console.WriteLine("----------------------------");
// 使用匿名委托
Func<int, string> fun6 = (int i) =>
{
return i.ToString();
};
string result6 = fun6(6);
Console.WriteLine($"这里是一个参数一个返回值的匿名委托，返回值是：{result6}");
Console.WriteLine("----------------------------");
// 多个输入参数
Func<int, string, bool> fun7 = new Func<int, string, bool>(FunWithMultiPara);
bool result7 = fun7(2, "2");
Console.WriteLine($"这里是有多个输入参数的方法，返回值是：{result7}");
Console.WriteLine("----------------------------");
// 使用委托
Func<int, string, bool> fun8 = delegate (int i, string s)
{
return i.ToString().Equals(s) ? true : false;
};
bool result8 = fun8(2, "abc");
Console.WriteLine($"这里是有多个输入参数的委托，返回值是：{result8}");
Console.WriteLine("----------------------------");
// 使用匿名委托
Func<int, string, bool> fun9 = (int i, string s) =>
{
return i.ToString().Equals(s) ? true : false;
};
bool result9 = fun9(45, "ert");
Console.WriteLine($"这里是有多个输入参数的匿名委托，返回值是：{result9}");
Console.ReadKey();
}
staticint FunWithNoPara()
{
return10;
}
staticint FunWithPara(int i)
{
return i;
}
staticbool FunWithMultiPara(int i,string s)
{
return i.ToString().Equals(s) ? true : false;
}
}
}
运行结果：
4、真实示例
在下面的示例中，利用Func委托封装数据库通用访问类。
1、定义BaseModel基类
using
System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Text;
using
System.Threading.Tasks;
namespace
FunApplication.Model
{
public
class
BaseModel
{
public
int Id { get; set; }
}
}
2、定义Student类继承自BaseModel基类
using
System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Text;
using
System.Threading.Tasks;
namespace
FunApplication.Model
{
public
class
Student : BaseModel
{
public
string Name { get; set; }
publicint Age { get; set; }
publicint Sex { get; set; }
publicstring Email { get; set; }
}
}
3、定义数据库访问方法接口
using
FunApplication.Model;
using
System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Text;
using
System.Threading.Tasks;
namespace
FunApplication.IDAL
{
public
interface
IBaseDAL
{
T Query<T>(int id) where T : BaseModel;
List<T> QueryAll<T>() where T : BaseModel;
int Insert<T>(T t) where T : BaseModel;
int Update<T>(T t) where T : BaseModel;
int Delete<T>(int id) where T : BaseModel;
}
}
4、定义属性帮助类
using
System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Reflection;
using
System.Text;
using
System.Threading.Tasks;
namespace
FunApplication.AttributeExtend
{
public
static
class
AttributeHelper
{
public
static
string GetColumnName(this PropertyInfo prop)
{
if (prop.IsDefined(typeof(ColumnAttribute), true))
{
ColumnAttribute attribute = (ColumnAttribute)prop.GetCustomAttribute(typeof(ColumnAttribute), true);
return attribute.GetColumnName();
}
else
{
return prop.Name;
}
}
}
}
5、定义ColumnAttribute类
using
System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Text;
using
System.Threading.Tasks;
namespace
FunApplication.AttributeExtend
{
[AttributeUsage(AttributeTargets.Property)]
public
class
ColumnAttribute : Attribute
{
public ColumnAttribute(string name)
{
this._Name = name;
}
privatestring _Name = null;
publicstring GetColumnName()
{
returnthis._Name;
}
}
}
6、定义数据库方法接口实现类
using
FunApplication.IDAL;
using
FunApplication.Model;
using
System;
using
System.Collections.Generic;
using
System.Configuration;
using
System.Data.SqlClient;
using
System.Linq;
using
System.Text;
using
System.Threading.Tasks;
using
System.Reflection;
using
FunApplication.AttributeExtend;
namespace
FunApplication.DAL
{
public
class
BaseDAL : IBaseDAL
{
//
数据库链接字符串
private
static
string strConn = ConfigurationManager.ConnectionStrings["DbConnection"].ConnectionString;
publicint Delete<T>(int id) where T : BaseModel
{
int result = 0;
using (SqlConnection conn = new SqlConnection(strConn))
{
string strSQL = "delete from Student where Id=@Id";
SqlParameter para = new SqlParameter("Id", id);
SqlCommand command = new SqlCommand(strSQL, conn);
command.Parameters.Add(para);
conn.Open();
result = command.ExecuteNonQuery();
}
return result;
}
publicint Insert<T>(T t) where T : BaseModel
{
int result = 0;
using (SqlConnection conn = new SqlConnection(strConn))
{
Type type = typeof(T);
var propArray = type.GetProperties().Where(p => p.Name != "Id");
string strSQL = "insert into Student Values (@Name,@Age,@Sex,@Email) ";
SqlCommand command = new SqlCommand(strSQL, conn);
var parameters = propArray.Select(p => new SqlParameter($"@{p.GetColumnName()}", p.GetValue(t) ?? DBNull.Value)).ToArray();
command.Parameters.AddRange(parameters);
conn.Open();
result = command.ExecuteNonQuery();
}
return result;
}
public T Query<T>(int id) where T : BaseModel
{
Type type = typeof(T);
string columnString = string.Join(",", type.GetProperties().Select(p => $"[{p.GetColumnName()}]"));
string sql = $"SELECT {columnString} FROM [{type.Name}] WHERE Id={id}";
T t = null;// (T)Activator.CreateInstance(type);using (SqlConnection conn = new SqlConnection(strConn))
{
SqlCommand command = new SqlCommand(sql, conn);
conn.Open();
SqlDataReader reader = command.ExecuteReader();
List<T> list = this.ReaderToList<T>(reader);
t = list.FirstOrDefault();
}
return t;
}
public List<T> QueryAll<T>() where T : BaseModel
{
Type type = typeof(T);
string columnString = string.Join(",", type.GetProperties().Select(p => $"[{p.GetColumnName()}]"));
string sql = $"SELECT {columnString} FROM [{type.Name}] ";
List<T> list = new List<T>();
using (SqlConnection conn = new SqlConnection(strConn))
{
SqlCommand command = new SqlCommand(sql, conn);
conn.Open();
SqlDataReader reader = command.ExecuteReader();
list = this.ReaderToList<T>(reader);
}
return list;
}
publicint Update<T>(T t) where T : BaseModel
{
int result = 0;
using (SqlConnection conn = new SqlConnection(strConn))
{
Type type = typeof(T);
var propArray = type.GetProperties().Where(p => p.Name != "Id");
string columnString = string.Join(",", propArray.Select(p => $"[{p.GetColumnName()}]=@{p.GetColumnName()}"));
var parameters = propArray.Select(p => new SqlParameter($"@{p.GetColumnName()}", p.GetValue(t) ?? DBNull.Value)).ToArray();
//必须参数化 否则引号？ 或者值里面还有引号string strSQL = $"UPDATE [{type.Name}] SET {columnString} WHERE Id={t.Id}";
SqlCommand command = new SqlCommand(strSQL, conn);
command.Parameters.AddRange(parameters);
conn.Open();
result = command.ExecuteNonQuery();
}
return result;
}
private List<T> ReaderToList<T>(SqlDataReader reader) where T : BaseModel
{
Type type = typeof(T);
List<T> list = new List<T>();
while (reader.Read())//表示有数据 开始读 {
T t = (T)Activator.CreateInstance(type);
foreach (var prop in type.GetProperties())
{
object oValue = reader[prop.GetColumnName()];
if (oValue is DBNull)
oValue = null;
prop.SetValue(t, oValue);//除了guid和枚举 }
list.Add(t);
}
return list;
}
}
}
7、在Main()方法中调用
using
FunApplication.DAL;
using
FunApplication.Model;
using
System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Text;
using
System.Threading.Tasks;
namespace
FunApplication
{
class
Program
{
static
void Main(string[] args)
{
#region MyRegion
BaseDAL dal = new BaseDAL();
// 查询
Student student = dal.Query<Student>(2);
Console.WriteLine($"姓名：{student.Name},年龄：{student.Age},Email地址：{student.Email}");
Console.WriteLine("----------------------------");
// 查询所有
List<Student> list = dal.QueryAll<Student>();
Console.WriteLine($"集合个数：{list.Count}");
Console.WriteLine("----------------------------");
// 插入
Student studentIns = new Student()
{
Name = "小明",
Age = 20,
Sex = 2,
Email = "xiaoming@qq.com"
};
bool resultIns = dal.Insert<Student>(studentIns) > 0 ? true : false;
Console.WriteLine($"插入执行结果：{resultIns}");
Console.WriteLine("----------------------------");
// 更新
Student studentUpd = new Student()
{
Id = 1,
Name = "zhangsan1234",
Age = 20,
Sex = 2,
Email = "zhangsan1234@qq.com"
};
bool resultUpd = dal.Update<Student>(studentUpd) > 0 ? true : false;
Console.WriteLine($"更新执行结果：{resultUpd}");
Console.WriteLine("----------------------------");
// 删除bool resultDel = dal.Delete<Student>(3) > 0 ? true : false;
Console.WriteLine($"删除执行结果：{resultDel}");
#endregion
Console.ReadKey();
}
}
}
8、结果
9、优化
仔细观察上面步骤7中的代码，你会发现在每个方法中都有重复的代码，打开链接，执行SqlCommand命令，那么这些重复的代码能不能提取到一个公共的方法中进行调用呢？答案是可以的，那就是利用Func委托，看下面优化后的代码：
using
FunApplication.AttributeExtend;
using
FunApplication.IDAL;
using
FunApplication.Model;
using
System;
using
System.Collections.Generic;
using
System.Configuration;
using
System.Data;
using
System.Data.SqlClient;
using
System.Linq;
using
System.Reflection;
using
System.Text;
using
System.Threading.Tasks;
namespace
FunApplication.DAL
{
public
class
FunBaseDAL : IBaseDAL
{
//
数据库链接字符串
private
static
string strConn = ConfigurationManager.ConnectionStrings["DbConnection"].ConnectionString;
publicint Delete<T>(int id) where T : BaseModel
{
Type type = typeof(T);
string sql = $"delete from {type.Name} where Id=@Id";
Func<SqlCommand, int> func = (SqlCommand command) =>
{
SqlParameter para = new SqlParameter("Id", id);
command.Parameters.Add(para);
return command.ExecuteNonQuery();
};
return ExcuteSql<int>(sql, func);
}
publicint Insert<T>(T t) where T : BaseModel
{
int result = 0;
Type type = typeof(T);
var propArray = type.GetProperties().Where(p => p.Name != "Id");
string strSQL = "insert into Student Values (@Name,@Age,@Sex,@Email) ";
var parameters = propArray.Select(p => new SqlParameter($"@{p.GetColumnName()}", p.GetValue(t) ?? DBNull.Value)).ToArray();
Func<SqlCommand, int> func = (SqlCommand command) =>
{
command.Parameters.AddRange(parameters);
return command.ExecuteNonQuery();
};
result = ExcuteSql<int>(strSQL, func);
return result;
}
public T Query<T>(int id) where T : BaseModel
{
Type type = typeof(T);
string columnString = string.Join(",", type.GetProperties().Select(p => $"[{p.GetColumnName()}]"));
string sql = $"SELECT {columnString} FROM [{type.Name}] WHERE Id=@Id";
T t = null;
DataTable dt = new DataTable();
Func<SqlCommand, T> func = (SqlCommand command) =>
{
SqlParameter para = new SqlParameter("@Id", id);
command.Parameters.Add(para);
SqlDataAdapter adapter = new SqlDataAdapter(command);
//SqlDataReader reader = command.ExecuteReader();
//List<T> list = this.ReaderToList<T>(reader); adapter.Fill(dt);
List<T> list = ConvertToList<T>(dt);
T tResult = list.FirstOrDefault();
return tResult;
};
t = ExcuteSql<T>(sql, func);
return t;
}
public List<T> QueryAll<T>() where T : BaseModel
{
Type type = typeof(T);
string columnString = string.Join(",", type.GetProperties().Select(p => $"[{p.GetColumnName()}]"));
string sql = $"SELECT {columnString} FROM [{type.Name}] ";
T t = null;
Func<SqlCommand, List<T>> func = (SqlCommand command) =>
{
SqlDataReader reader = command.ExecuteReader();
List<T> list = this.ReaderToList<T>(reader);
return list;
};
return ExcuteSql<List<T>>(sql, func);
}
publicint Update<T>(T t) where T : BaseModel
{
int result = 0;
Type type = typeof(T);
var propArray = type.GetProperties().Where(p => p.Name != "Id");
string columnString = string.Join(",", propArray.Select(p => $"[{p.GetColumnName()}]=@{p.GetColumnName()}"));
var parameters = propArray.Select(p => new SqlParameter($"@{p.GetColumnName()}", p.GetValue(t) ?? DBNull.Value)).ToArray();
//必须参数化 否则引号？ 或者值里面还有引号string strSQL = $"UPDATE [{type.Name}] SET {columnString} WHERE Id={t.Id}";
Func<SqlCommand, int> func = (SqlCommand command) =>
{
command.Parameters.AddRange(parameters);
return command.ExecuteNonQuery();
};
result = ExcuteSql<int>(strSQL, func);
return result;
}
//多个方法里面重复对数据库的访问 想通过委托解耦，去掉重复代码private T ExcuteSql<T>(string sql, Func<SqlCommand, T> func)
{
using (SqlConnection conn = new SqlConnection(strConn))
{
using (SqlCommand command = new SqlCommand(sql, conn))
{
conn.Open();
SqlTransaction sqlTransaction = conn.BeginTransaction();
try
{
command.Transaction = sqlTransaction;
T tResult = func.Invoke(command);
sqlTransaction.Commit();
return tResult;
}
catch (Exception ex)
{
sqlTransaction.Rollback();
throw;
}
}
}
}
private List<T> ReaderToList<T>(SqlDataReader reader) where T : BaseModel
{
Type type = typeof(T);
List<T> list = new List<T>();
while (reader.Read())//表示有数据 开始读 {
T t = (T)Activator.CreateInstance(type);
foreach (var prop in type.GetProperties())
{
object oValue = reader[prop.GetColumnName()];
if (oValue is DBNull)
oValue = null;
prop.SetValue(t, oValue);//除了guid和枚举 }
list.Add(t);
}
reader.Close();
return list;
}
}
}
10、在Main()方法中调用
using
FunApplication.DAL;
using
FunApplication.Model;
using
System;
using
System.Collections.Generic;
using
System.Linq;
using
System.Text;
using
System.Threading.Tasks;
namespace
FunApplication
{
class
Program
{
static
void Main(string[] args)
{
#region 传统实现
//BaseDAL dal = new BaseDAL();//// 查询//Student student = dal.Query<Student>(2);
//Console.WriteLine($"姓名：{student.Name},年龄：{student.Age},Email地址：{student.Email}");
//Console.WriteLine("----------------------------");//// 查询所有//List<Student> list = dal.QueryAll<Student>();
//Console.WriteLine($"集合个数：{list.Count}");
//Console.WriteLine("----------------------------");//// 插入//Student studentIns = new Student()
//{
// Name = "小明",
// Age = 20,
// Sex = 2,
// Email = "xiaoming@qq.com"
//};
//bool resultIns = dal.Insert<Student>(studentIns) > 0 ? true : false;
//Console.WriteLine($"插入执行结果：{resultIns}");
//Console.WriteLine("----------------------------");//// 更新//Student studentUpd = new Student()
//{
// Id = 1,
// Name = "zhangsan1234",
// Age = 20,
// Sex = 2,
// Email = "zhangsan1234@qq.com"
//};
//bool resultUpd = dal.Update<Student>(studentUpd) > 1 ? true : false;
//Console.WriteLine($"更新执行结果：{resultUpd}");
//Console.WriteLine("----------------------------");//// 删除//bool resultDel = dal.Delete<Student>(5) > 1 ? true : false;
//Console.WriteLine($"删除执行结果：{resultDel}");#endregion#region 利用委托
// 查询
FunBaseDAL dal = new FunBaseDAL();
Student student = dal.Query<Student>(1);
Console.WriteLine($"姓名：{student.Name},年龄：{student.Age},Email地址：{student.Email}");
Console.WriteLine("----------------------------");
// 查询所有
List<Student> list = dal.QueryAll<Student>();
Console.WriteLine($"集合个数：{list.Count}");
Console.WriteLine("----------------------------");
// 插入
Student studentIns = new Student()
{
Name = "tom",
Age = 19,
Sex = 1,
Email = "tom@163.com"
};
bool resultIns = dal.Insert<Student>(studentIns) > 0 ? true : false;
Console.WriteLine($"插入执行结果：{resultIns}");
Console.WriteLine("----------------------------");
List<Student> list1 = dal.QueryAll<Student>();
Console.WriteLine($"插入后集合个数：{list1.Count}");
Console.WriteLine("----------------------------");
// 更新
Student studentUpd = new Student()
{
Id = 2,
Name = "马六123",
Age = 20,
Sex = 2,
Email = "maliu1234@qq.com"
};
bool resultUpd = dal.Update<Student>(studentUpd) > 0 ? true : false;
Console.WriteLine($"更新执行结果：{resultUpd}");
Console.WriteLine("----------------------------");
// 删除bool resultDel = dal.Delete<Student>(8) > 0 ? true : false;
Console.WriteLine($"删除执行结果：{resultDel}");
List<Student> list2 = dal.QueryAll<Student>();
Console.WriteLine($"删除后集合个数：{list2.Count}");
Console.WriteLine("----------------------------");
#endregion
Console.ReadKey();
}
}
}
11、结果
注意
在使用SqlDataReader的时候有时会报错：“已有打开的与此Command相关联的DataReader，必须先将它关闭”。
同时打开两个或循环多个sqldatareader会出现以上错误。因为用的是sqldatareader做数据库的数据读取，sqlconnection开启没有关闭。
一个SqlConnection只能执行一次事务，没用一次必须关闭然后再开启。上面我只用了一次没有关闭，直接开启所以会报错。解决方案有如下两种：
1、其实不用多次打开在开启，那样实现起来很麻烦。直接在连接字符串的后面加上MultipleActiveResultSets=true即可。 配置文件定义如下：
<?xml version="1.0" encoding="utf-8" ?> <configuration> <connectionStrings> <!--<add name="DbConnection" connectionString="Server=.;Initial Catalog=MyDb;User ID=sa;Password=123456;MultipleActiveResultSets=True"/>--> <!--配置文件里面添加MultipleActiveResultSets=True--> <add name="DbConnection" connectionString="Server=.;Initial Catalog=MyDb;User ID=sa;Password=123456;MultipleActiveResultSets=True"/> </connectionStrings> <startup> <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" /> </startup> </configuration>
2、使用DataTable
在上面是使用的SqlDataReader读取数据，然后转换成List<T>，可以用DataTable代替SqlDataReader，这样就不会报错了，代码如下：
///
<summary>
///
将DataTable转换成List
///
</summary>
///
<typeparam name="T"></typeparam>
///
<param name="dt"></param>
///
<returns></returns>
private List<T> ConvertToList<T>(DataTable dt) where T:BaseModel
{
Type type = typeof(T);
List<T> list = new List<T>();
foreach(DataRow dr in dt.Rows)
{
T t = (T)Activator.CreateInstance(type);
foreach(PropertyInfo prop in type.GetProperties())
{
object value = dr[prop.GetColumnName()];
if(value is DBNull)
{
value = null;
}
prop.SetValue(t, value);
}
list.Add(t);
}
return list;
}
原文：https://www.cnblogs.com/dotnet261010/p/10109837.html
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