List扩展方法汇总（仅备注）

不管在c/s还是b/s的c#语言开发程序中，经常会用到List的扩展方法，尤其在json格式的数据和服务端交互越来越流行，很多时候总是在开发使用到的时候才去搜索有些扩展方法或者linq的用法，在这里，我们只是做一个备注

因为linq没有必要系统的学习，我们只要简单的在list的扩展会使用即可，至少我只这么认为的

本文没有任何技术性，只是备注list或者list泛型的扩展方法，不至于在您不熟悉但是又使用到的时候到处搜索

  public sealed class Employee
     {
         public string Name { get; set; }
         public double Salary { get; set; }
         public short Dependents { get; set; }
     }
     public class Test
     {
         public void FunTest()
         {
             var employees = new List<Employee>
              {
                     new Employee { Name = "Bob", Salary = , Dependents =  },
                     new Employee { Name = "Sherry", Salary = , Dependents =  },
                     new Employee { Name = "Kathy", Salary = , Dependents =  },
                     new Employee { Name = "Joe", Salary = , Dependents =  },
                     new Employee { Name = "Bob", Salary = , Dependents =  },
                     new Employee { Name = "Bob", Salary = , Dependents =  }
              };
             //泛型方法的类型参数是可以省略的

             //所有求和
             double sumSalary = employees.Sum<Employee>(e => e.Salary);//21.0
             //【linq方式】所有求和
             var sumSalary2 = (from v in employees select v).Sum(e => e.Salary);//21.0

             //按照某一条件求和
             double sumSalaryFilter = employees.Where<Employee>(e => e.Name == "Bob").ToList().Sum(e => e.Salary);//12.0
             //【linq方式】按照某一条件求和
             var sumSalaryFilter2 = (from v in employees where v.Name == "Bob" select v).Sum(e => e.Salary);//12.0
 
             //选择数据而已，即返回自定义改变后的新的投影的数据列表（无法改变源数据）
             var selectChangedNameList = employees.Select<Employee, Employee>(e => { return new Employee { Name = "Bob" }; }).ToList();
             //【linq方式】选择数据而已，即返回自定义改变后的新的投影的数据列表（无法改变源数据）
             var selectChangedNameList2 = (from v in employees select new { Name = "Bob", Salary = v.Salary, Dependents = v.Dependents }).ToList();
             //选择返回某一列的清单投影的数据列表
             var selectSalaryList = employees.Select<Employee, double>(e => { return e.Salary; }).ToList();
 
             //查找符合条件的第一个元素
             Employee employee1 = employees.Find(e => e.Name == "Bob");
             //【linq方式】查找符合条件的第一个元素
             Employee employee2 = (from v in employees where v.Name == "Bob" select v).First();
             //不用lambda方式筛选符合条件的元素列表
             Predicate<Employee> aaaa = new Predicate<Employee>(fun1);
             List<Employee> employee1List = employees.FindAll(aaaa);
             //lambda方式筛选符合条件的元素列表
             List<Employee> employee2List = employees.FindAll(e => e.Name == "Bob");
             //【linq方式】方式筛选符合条件的元素列表
             List<Employee> employee2List2 = (from v in employees where v.Name == "Bob" select v).ToList();

             //max，min，平均值，筛选满足某条件的max和min已经使用linq方式
             employees.Max(e => e.Salary);
             employees.Min(e => e.Salary);
             (from v in employees select v).Max(e => e.Salary);
             (from v in employees select v).Min(e => e.Salary);
             double maxSalaryFilter = employees.Where<Employee>(e => e.Name == "Bob").ToList().Max(e => e.Salary);
             //简单的运算，比如列表总的相加的值
             var aggregateEmployee = employees.Aggregate<Employee>((result, next) => new Employee { Salary = result.Salary + next.Salary });

             //某一列的不重复的汇总
             var groupEmployees = employees.GroupBy(e => new { e.Name }).Select(g => new { g.Key, Count = g.Count() }).ToList();
             var groupEmployees1 = employees.GroupBy(e => e.Name).Select(e => new { Name = e.Key, sumSalary = e.Sum(d => d.Salary) }).ToList();
             var groupEmployees2 = (from v in employees group v by v.Name into g select new { Name = g.Key, SumSalary = g.Sum(d => d.Salary) }).ToList();

             //查询满足某一条件的所有数据
             IEnumerable<Employee> employee3List = employees.Where<Employee>(e => e.Name == "Bob");
             List<Employee> employee4List = employees.Where<Employee>(e => e.Name == "Bob").ToList();
             employee3List = employee4List as IEnumerable<Employee>;

             Console.WriteLine("adfadsfasdfasdf");
         }

         private bool fun1(Employee e)
         {
             return (e.Name == "Bob");
         }
     }