Linq 中按照多個值進行分組(GroupBy)

/// <summary>要查詢的對象</summary>
class Employee {
   public int ID { get;set; }
   public string FName { get; set; }
   public int Age { get; set; }
   public char Sex { get; set; }
}

如果對這個類的Age和Sex的連個字段進行分組，方法如下：

// 先造一些數據
List<Employee> empList = new List<Employee>();
empList.Add(new Employee() {
   ID = 1, FName = "John", Age = 23, Sex = 'M'
});
empList.Add(new Employee() {
   ID = 2, FName = "Mary", Age = 25, Sex = 'F'
});

empList.Add(new Employee() {
   ID = 3, FName = "Amber", Age = 23, Sex = 'M'
});
empList.Add(new Employee() {
   ID = 4, FName = "Kathy", Age = 25, Sex = 'M'
});
empList.Add(new Employee() {
   ID = 5, FName = "Lena", Age = 27, Sex = 'F'
});

empList.Add(new Employee() {
   ID = 6, FName = "Bill", Age = 28, Sex = 'M'
});

empList.Add(new Employee() {
   ID = 7, FName = "Celina", Age = 27, Sex = 'F'
});
empList.Add(new Employee() {
   ID = 8, FName = "John", Age = 28, Sex = 'M'
});

接下來的做法是：

// 實現多key分組的擴展函數版本
var sums = empList
         .GroupBy(x => new { x.Age, x.Sex })
         .Select(group => new {
            Peo = group.Key, Count = group.Count()
         });
foreach (var employee in sums) {
   Console.WriteLine(employee.Count + ": " + employee.Peo);
}

// 實現多key分組的lambda版本
var sums2 = from emp in empList
            group emp by new { emp.Age, emp.Sex } into g
            select new { Peo = g.Key, Count = g.Count() };
foreach (var employee in sums) {
   Console.WriteLine(employee.Count + ": " + employee.Peo);
}

這個例子中就充分利用了匿名類型。