使用 Linq 进行分组答案

【问题标题】：Using Linq for grouping使用 Linq 进行分组
【发布时间】：2013-01-24 16:47:11
【问题描述】：

class Program
{
    static void Main(string[] args)
    {
        List<EmployeeLocalRegister> lclEmployees = new List<EmployeeLocalRegister>() { 
            new EmployeeLocalRegister(){Name = "A", Phone= "A1"}, 
            new EmployeeLocalRegister(){Name = "B", Phone= "B1"},
            new EmployeeLocalRegister(){Name = "A", Phone= "A2"}, 
            new EmployeeLocalRegister(){Name = "B", Phone= "B2"},
            new EmployeeLocalRegister(){Name = "B", Phone= "B3"},
            new EmployeeLocalRegister(){Name = "C", Phone= "C1"}};

        List<EmployeeTelDir> telDir = new List<EmployeeTelDir>();

        var queryEmployeeLocalRegisterByName =
        from empl in lclEmployees
        group empl by empl.Name;

        foreach (var employeeGroup in queryEmployeeLocalRegisterByName)
        {
            Console.WriteLine(employeeGroup.Key);
            List<string> phone = new List<string>();
            foreach (EmployeeLocalRegister employee in employeeGroup)
            {

                Console.WriteLine("    {0}", employee.Phone);
                phone.Add(employee.Phone);
            }

            telDir.Add(new EmployeeTelDir() { Name = employeeGroup.Key, Phone = phone });
        }

        Console.ReadKey();
    }
}

public class EmployeeLocalRegister
{
    public string Name;
    public string Phone;
}

public class EmployeeTelDir
{
    public string Name;
    public List<string> Phone;
}

}

我正在使用上面的代码将List<EmployeeLocalRegister> 转换为List<EmployeeTelDir>。这是唯一优化的方式吗？

我可以编写更简单的代码来使用 Linq 查询从 List<EmployeeLocalRegister> 到 List<EmployeeTelDir> 来回转换吗？

【问题讨论】：

标签： linq c#-4.0 group-by

【解决方案1】：

如果您不需要Console.WriteLine(...)，您的代码可以用 LINQ 进行总结：

List<EmployeeTelDir> telDir = (from empl in lclEmployees
                               group empl by empl.Name into employeeGroup
                               select new EmployeeTelDir
                               {
                                   Name = employeeGroup.Key,
                                   Phone = (from employee in employeeGroup
                                           select employee.Phone).ToList()  // The ToList() is the Holy Grail of the LINQ queries
                               }).ToList();

对于倒置操作：

List<EmployeeLocalRegister> inverse = (from employeeTelDir in telDir
                                       from phone in employeeTelDir.Phone  // Doing 2 from ... in ... successively corresponds to the SelectMany() LINQ method
                                       select new EmployeeLocalRegister
                                       {
                                           Name = employeeTelDir.Name,
                                           Phone = phone
                                       }).ToList();

【讨论】：

【解决方案2】：

lambda 方式：

var telDir = lclEmployees.GroupBy(e => e.Name, 
                                  (n, p) => new EmployeeTelDir 
                                            { 
                                              Name = n, 
                                              Phone = p.Select(k => k.Phone).ToList() 
                                            }).ToList();

【讨论】：