Question

我已在线搜索此代码以合并多个列表并返回一个。如果所有属性都是字符串，则代码有效，但如果某些属性为double，则会出现错误。这个错误是“运营商'??'不能应用于'double'和'double'类型的操作数。任何帮助是极大的赞赏。谢谢

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication11
{
    class Program
    {
        static void Main(string[] args)
        {

            List<Project> lst1; List<Project> lst2; List<Project> lst3;
            lst1 = new List<Project> 
            {
                new Project { ProjectId = 1, ProjectName = "P1" },
                new Project { ProjectId = 2, ProjectName = "P2" },
                new Project { ProjectId = 3, ProjectName = "P3" }
            };
                    lst2 = new List<Project>
            {
                new Project { ProjectId = 1, CustomerNum = 1},
                new Project { ProjectId = 2, CustomerNum = 2},
                new Project { ProjectId = 3, CustomerNum = 3}
            };
                    lst3 = new List<Project>
            {
                new Project { ProjectId = 1, Address = 10},
                new Project { ProjectId = 2, Address = 20},
                new Project { ProjectId = 3, Address = 30}
            };

                    List<Project> lst = lst1.Union(lst2).Union(lst3).ToLookup(x => x.ProjectId).Select(x => new Project()
                    {
                        ProjectId = x.Key,
                        ProjectName = x.Select(y => y.ProjectName).Aggregate((z1, z2) => z1 ?? z2),
                        CustomerNum = x.Select(y => y.CustomerNum).Aggregate((z1, z2) => z1 ?? z2),
                        Address = x.Select(y => y.Address).Aggregate((z1, z2) => z1 ?? z2)
                    }).ToList();


            foreach (var item in lst)
            {
                Console.WriteLine("{0}, {1}, {2}, {3}", item.ProjectId, item.ProjectName, item.CustomerNum, item.Address);

            }     
        }
    }

    public class Project
    {
        public int ProjectId { get; set; }
        public string ProjectName { get; set; }
        public double CustomerNum { get; set; }
        public double Address { get; set; }
    }
}

Answer 1

Null-coalescing对左侧的非可空类型（double）不起作用

来自this MSDN article

?? operator被称为null-coalescing运算符，用于为可空值类型或引用类型定义默认值

所以，您可以使用double?（Nullable<double>），这样可行。但是，正如所写，左手边总会有一个值，因此你的问题。 Alternatively, you could check if the type is a value type？

Answer 2

如果您希望代码执行与其他类型相同的操作，则可以执行以下操作

CustomerNum = x.Select(y => y.CustomerNum).Aggregate((z1, z2) => z1)

Answer 3

??用于说“如果左侧为空，则使用右侧的值”。由于Double是值类型，因此它永远不能为null。因此，您将始终只是到达酒店的左侧。编译器告诉你你的代码是可疑的。

使用Double？如果你想保留完全相同的代码。

Answer 4

我提出了一个有效的简单（长）解决方案。这是合并列表的代码

        List<Project> lst = new List<Project>();

        for (int i = 0; i < lst1.Count; i++)
        {
            double cusnum = 0;
            var lst2q = (from s in lst2
                            where s.ProjectId == lst1[i].ProjectId
                            select s).ToArray();

            if (lst2q.Length > 0)
            {
                cusnum = lst2q[0].CustomerNum;
            }


            var lst3q = (from s in lst3
                         where s.ProjectId == lst1[i].ProjectId
                         select s).ToArray();

            double cusadd = 0;
            if (lst3q.Length > 0)
            {
                cusadd = lst3q[0].Address;
            }

            lst.Add(new Project
                {
                    ProjectId = lst1[i].ProjectId,
                    ProjectName = lst1[i].ProjectName,
                    CustomerNum = cusnum,
                    Address = cusadd,
                });
        }

c＃合并具有双重属性的列表

4 个答案: