Delegate &&Lambda

匿名函数及委托的使用：

public delegate void NoReturnNoParaOutClass();//delegate can be defined in class or out of class
    public sealed class LambdaShow
    {
        public delegate void NoReturnNoPara();//1 委托的声明   委托是一种类型
        public delegate int WithReturnNoPara();
        public delegate void NoReturnWithPara(int id, string name);
        public delegate LambdaShow WithReturnWithPara(DateTime time);
        public delegate LambdaShow WithReturnWithParaRefOut(ref DateTime time, out int i);

        public void Show()
        {
            Student student = new Student()
            {
                Id = 123,
                Name = "Smith"
            };
            student.Study();

            {
                NoReturnNoPara method = new NoReturnNoPara(this.DoNothing1);//2委托的实例化
                method.Invoke();//3 委托实例的调用
                method();
                this.DoNothing1();
            }
            {
                NoReturnWithPara method = new NoReturnWithPara(this.DoNothing);
                //NoReturnWithPara method = this.DoNothing;
                method.Invoke(123, "珍惜");
            }
            {
                NoReturnWithPara method = new NoReturnWithPara(
                    delegate (int id, string name)//匿名方法
                    {
                        Console.WriteLine($"{id} {name} DoNothing out");
                    }
                    );
                method.Invoke(123, "绚烂的夏");
            }
            {
                NoReturnWithPara method = new NoReturnWithPara(
                     (int id, string name) =>//goes to  lambda表达式：匿名方法--方法
                    {
                        Console.WriteLine($"{id} {name} DoNothing out");
                    }
                    );
                method.Invoke(123, "追梦");
            }
            {
                NoReturnWithPara method = new NoReturnWithPara(
                     (id, name) =>//委托约束，去掉参数类型，自动推算
                     {
                         Console.WriteLine($"{id} {name} DoNothing out");
                     }
                    );
                method.Invoke(123, "装逼的岁月");
            }
            {
                NoReturnWithPara method = new NoReturnWithPara(
                     (id, name) => Console.WriteLine($"{id} {name} DoNothing out")
                     //方法体只有一行，就可以把大括号和分号去掉
                    );
                method.Invoke(123, "美羡");
            }
            {
                NoReturnWithPara method = (id, name) => Console.WriteLine($"{id} {name} DoNothing out");
                //委托实例化的时候可以不要new NoReturnWithPara
                method.Invoke(123, "晴月");
            }  
        }

        private void DoNothing(int id, string name)
        {
            //Console.WriteLine("{0} {1} DoNothing out",id, name);
            Console.WriteLine($"{id} {name} DoNothing out");
        }

        private void DoNothing1()
        {
            Console.WriteLine("DoNothing1");
        }

        private void DoNothing2()
        {
            Console.WriteLine("DoNothing2");
        }
    }
}

 系统自带delegate...(来自System)

Action 无返回值

#region 程序集 mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
// C:Program Files (x86)Reference AssembliesMicrosoftFramework.NETFrameworkv4.5mscorlib.dll
#endregion

namespace System
{
    //
    // 摘要:
    //     封装一个方法，该方法只有一个参数并且不返回值。 若要浏览此类型的.NET Framework 源代码，请参阅Reference Source。
    //
    // 参数:
    //   obj:
    //     此委托封装的方法的参数。
    //
    // 类型参数:
    //   T:
    //     此委托封装的方法的参数类型。
    public delegate void Action<in T>(T obj);
}

Func: 带返回值

#region 程序集 mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
// C:Program Files (x86)Reference AssembliesMicrosoftFramework.NETFrameworkv4.5mscorlib.dll
#endregion

using System.Runtime.CompilerServices;

namespace System
{
    //
    // 摘要:
    //     封装一个方法，该方法不具有参数，且返回由 TResult 参数指定的类型的值。
    //
    // 类型参数:
    //   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")]
    public delegate TResult Func<out TResult>();
}

匿名类型 Var:

                    //1 只能声明局部变量，不能是字段、也不能是静态属性
                    //2 声明的时候必须被初始化;               
                    var model = new
                    {
                        Id = 1,
                        Name = "Alpha Go",
                        Age = 21,
                        ClassId = 2
                    };

                    Console.WriteLine(model.Id);
                    Console.WriteLine(model.Name);
                    //model.Name = "张伟东";//只读,不能更改。

扩展方法：

/// <summary>
    /// 扩展方法：就是在不修改类型封装前提下，给类型额外的扩展一个方法
    /// 
    /// 不能滥用,尤其是一些基类型-->object
    /// </summary>
    public static class ExtendShow
    {
        /// <summary>
        /// 扩展方法：静态类里的静态方法，第一个参数类型前面加上this
        /// </summary>
        /// <param name="iParameter"></param>
        /// <returns></returns>
        public static int ToInt(this int? iParameter) //int? 表示类型可null可int,常用于数据库。
        {
            return iParameter ?? -1;//if null return -1; else return itself.
        }
        
        

        public static void ExtendLambdaShow(this LambdaShow lambda, int iParameter, string sParameter)
        {
            Console.WriteLine($"LambdaShow=={iParameter}==={sParameter}");
        }
        /// <summary>
        /// 如果跟实例方法相同，优先实例方法
        /// </summary>
        /// <param name="lambda"></param>
        public static void Show(this LambdaShow lambda)
        {
            Console.WriteLine($"LambdaShow");
        }


        //不能滥用,尤其是一些基类型-->object
        public static void DoNothing(this object oParameter)
        { }

Linq:(利用扩展方法，借助lambda表达式筛选)

/// <summary>
        /// 准备数据
        /// </summary>
        /// <returns></returns>
        private List<Student> GetStudentList()
        {
            #region 初始化数据
            List<Student> studentList = new List<Student>()
            {
                new Student()
                {
                    Id=1,
                    Name="打兔子的猎人",
                    ClassId=2,
                    Age=35
                },
                new Student()
                {
                    Id=1,
                    Name="Alpha Go",
                    ClassId=2,
                    Age=23
                },
                 new Student()
                {
                    Id=1,
                    Name="白开水",
                    ClassId=2,
                    Age=27
                },
                 new Student()
                {
                    Id=1,
                    Name="狼牙道",
                    ClassId=2,
                    Age=26
                },
                new Student()
                {
                    Id=1,
                    Name="Nine",
                    ClassId=2,
                    Age=25
                },
                new Student()
                {
                    Id=1,
                    Name="Y",
                    ClassId=2,
                    Age=24
                },
                new Student()
                {
                    Id=1,
                    Name="小昶",
                    ClassId=2,
                    Age=21
                },
                 new Student()
                {
                    Id=1,
                    Name="yoyo",
                    ClassId=2,
                    Age=22
                },
                 new Student()
                {
                    Id=1,
                    Name="冰亮",
                    ClassId=2,
                    Age=34
                },
                 new Student()
                {
                    Id=1,
                    Name="瀚",
                    ClassId=2,
                    Age=30
                },
                new Student()
                {
                    Id=1,
                    Name="毕帆",
                    ClassId=2,
                    Age=30
                },
                new Student()
                {
                    Id=1,
                    Name="一点半",
                    ClassId=2,
                    Age=30
                },
                new Student()
                {
                    Id=1,
                    Name="小石头",
                    ClassId=2,
                    Age=28
                },
                new Student()
                {
                    Id=1,
                    Name="大海",
                    ClassId=2,
                    Age=30
                },
                 new Student()
                {
                    Id=3,
                    Name="yoyo",
                    ClassId=3,
                    Age=30
                },
                  new Student()
                {
                    Id=4,
                    Name="unknown",
                    ClassId=4,
                    Age=30
                }
            };
            #endregion
            return studentList;
        }

 //
        // 摘要:
        //     基于谓词筛选值序列。
        //
        // 参数:
        //   source:
        //     System.Collections.Generic.IEnumerable`1 进行筛选。
        //
        //   predicate:
        //     用于测试每个元素是否满足条件的函数。
        //
        // 类型参数:
        //   TSource:
        //     中的元素的类型 source。
        //
        // 返回结果:
        //     System.Collections.Generic.IEnumerable`1 ，其中包含输入序列中满足条件的元素。
        //
        // 异常:
        //   T:System.ArgumentNullException:
        //     source 或 predicate 为 null。
        public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate);

//在Func<TSource, bool> predicate中：TSource是输入参数类型，bool是输出参数类型. 

//
    // 摘要:
    //     封装一个方法，该方法具有一个参数，且返回由 TResult 参数指定的类型的值。 若要浏览此类型的.NET Framework 源代码，请参阅Reference
    //     Source。
    //
    // 参数:
    //   arg:
    //     此委托封装的方法的参数。
    //
    // 类型参数:
    //   T:
    //     此委托封装的方法的参数类型。
    //
    //   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")]
    public delegate TResult Func<in T, out TResult>(T arg);

借助反编译工具查看源码（迭代器没搞清楚，以后补充）

// System.Linq.Enumerable
/// <summary>Filters a sequence of values based on a predicate.</summary>
/// <param name="source">An <see cref="T:System.Collections.Generic.IEnumerable`1" /> to filter.</param>
/// <param name="predicate">A function to test each element for a condition.</param>
/// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
/// <returns>An <see cref="T:System.Collections.Generic.IEnumerable`1" /> that contains elements from the input sequence that satisfy the condition.</returns>
/// <exception cref="T:System.ArgumentNullException">
///         <paramref name="source" /> or <paramref name="predicate" /> is <see langword="null" />.</exception>
[__DynamicallyInvokable]
public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    if (predicate == null)
    {
        throw Error.ArgumentNull("predicate");
    }
    if (source is Enumerable.Iterator<TSource>)
    {
        return ((Enumerable.Iterator<TSource>)source).Where(predicate);
    }
    if (source is TSource[])
    {
        return new Enumerable.WhereArrayIterator<TSource>((TSource[])source, predicate);
    }
    if (source is List<TSource>)
    {
        return new Enumerable.WhereListIterator<TSource>((List<TSource>)source, predicate);
    }
    return new Enumerable.WhereEnumerableIterator<TSource>(source, predicate);
}

/// <summary>
        /// IEnumerable是集合的初始接口
        /// </summary>
        /// <typeparam name="TSource"></typeparam>
        /// <param name="source"></param>
        /// <param name="predicate"></param>
        /// <returns></returns>
        public static IEnumerable<TSource> ElevenWhere<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
        {
            if (source == null)
            {
                throw new Exception("source");
            }
            if (predicate == null)
            {
                throw new Exception("predicate");
            }

            //常规情况下数据过滤
            List<TSource> studentListLessThan30 = new List<TSource>();
            foreach (var student in source)
            {
                //if (item.Age < 30)
                if (predicate(student))
                {
                    studentListLessThan30.Add(student);
                }
            }
            return studentListLessThan30;
        }

            {
                var list = studentList.ElevenWhere<Student>(s =>
                {
                    Console.WriteLine("1234678");
                    return s.Age < 30;
                });//linq
                foreach (var item in list)
                {
                    Console.WriteLine($"Name={item.Name} Age={item.Age}");
                }
                //linq to object
            }

{
                //查询运算符
                var list = studentList.Where<Student>(s => s.Age < 30 && s.Name.Length > 3);//陈述句
                foreach (var item in list)
                {
                    Console.WriteLine("Name={0}  Age={1}", item.Name, item.Age);
                }
            }

            {
                //查询表达式
                Console.WriteLine("********************");
                var list = from s in studentList
                           where s.Age < 30 && s.Name.Length > 3
                           select s;

                foreach (var item in list)
                {
                    Console.WriteLine("Name={0}  Age={1}", item.Name, item.Age);
                }
            }

 （不是Sql）

#region linq to object Show
            {
                Console.WriteLine("********************");
                var list = studentList.Where<Student>(s => s.Age < 30)
                                     .Select(s => new
                                     {
                                         IdName = s.Id + s.Name,
                                         ClassName = s.ClassId == 2 ? "高级班" : "其他班"
                                     });
                foreach (var item in list)
                {
                    Console.WriteLine("Name={0}  Age={1}", item.ClassName, item.IdName);
                }
            }
            {
                Console.WriteLine("********************");
                var list = from s in studentList
                           where s.Age < 30
                           select new
                           {
                               IdName = s.Id + s.Name,
                               ClassName = s.ClassId == 2 ? "高级班" : "其他班"
                           };

                foreach (var item in list)
                {
                    Console.WriteLine("Name={0}  Age={1}", item.ClassName, item.IdName);
                }
            }
            {
                Console.WriteLine("********************");
                var list = studentList.Where<Student>(s => s.Age < 30)
                                     .Select(s => new
                                     {
                                         Id = s.Id,
                                         ClassId = s.ClassId,
                                         IdName = s.Id + s.Name,
                                         ClassName = s.ClassId == 2 ? "高级班" : "其他班"
                                     })
                                     .OrderBy(s => s.Id)
                                     .OrderByDescending(s => s.ClassId)
                                     .ThenBy(s => s.ClassId)//排序
                                     .Skip(2)//跳过  分页
                                     .Take(3)//获取数据
                                     ;
                foreach (var item in list)
                {
                    Console.WriteLine($"Name={item.ClassName}  Age={item.IdName}");
                }
            }
            List<Class> classList = new List<Class>()
                {
                    new Class()
                    {
                        Id=1,
                        ClassName="初级班"
                    },
                    new Class()
                    {
                        Id=2,
                        ClassName="高级班"
                    },
                    new Class()
                    {
                        Id=3,
                        ClassName="微信小程序"
                    },
                };
            {
                //inner join
                var list = from s in studentList
                           join c in classList on s.ClassId equals c.Id
                           select new
                           {
                               Name = s.Name,
                               ClassName = c.ClassName
                           };
                foreach (var item in list)
                {
                    Console.WriteLine($"Name={item.Name},CalssName={item.ClassName}");
                }
            }
            {
                var list = studentList.Join(classList, s => s.ClassId, c => c.Id, (s, c) => new
                {
                    Name = s.Name,
                    ClassName = c.ClassName
                });
                foreach (var item in list)
                {
                    Console.WriteLine($"Name={item.Name},CalssName={item.ClassName}");
                }
            }
            {//左连接   (只有左连接，无右连接)右连接就反过来
                var list = from s in studentList
                           join c in classList on s.ClassId equals c.Id
                           into scList
                           from sc in scList.DefaultIfEmpty()
                           select new
                           {
                               Name = s.Name,
                               ClassName = sc == null ? "无班级" : sc.ClassName//c变sc，为空则用
                           };
                foreach (var item in list)
                {
                    Console.WriteLine($"Name={item.Name},CalssName={item.ClassName}");
                }
                Console.WriteLine(list.Count());
            }
            {
                var list = studentList.Join(classList, s => s.ClassId, c => c.Id, (s, c) => new
                {
                    Name = s.Name,
                    CalssName = c.ClassName
                }).DefaultIfEmpty();//为空就没有了
                foreach (var item in list)
                {
                    Console.WriteLine($"Name={item.Name},CalssName={item.CalssName}");
                }
                Console.WriteLine(list.Count());
            }
            #endregion