Question

我有以下代码：

var allCountryRates = (from c in allCountryCombinations
                        join r in Db.PaymentRates_VisaImmigrationPermit 
                            on new { c.HomeCountryId, c.HostCountryId } 
                            equals new { r.HomeCountryId, r.HostCountryId }
                        select r);

基本上，如果在r中找到c，则根据联接的两个条件，我想选择r。如果找不到r的{{1}}，那么我想生成一个c的空记录并选择该记录。

这可能吗？我觉得我非常接近，但不知道如何进一步接受它。

我上面的代码在Guid.NewGuid()上给出了一个错误，它读取了“，join子句中某个表达式的类型不正确。在'Join'调用中类型推断失败。”

参考。 How to do joins in LINQ on multiple fields in single join

编辑最新版本。

join

Answer 1

由于您不想要内部联接，而是需要左联接，因此您希望使用GroupJoin运算符而不是Join运算符。

var allCountryRates = (from c in allCountryCombinations
                        join r in Db.PaymentRates_VisaImmigrationPermit 
                            on new { c.HomeCountryId, c.HostCountryId } 
                            equals new { r.HomeCountryId, r.HostCountryId }
                        into matches
                        let match = matches.Any() ? matches.First() : emptyMatch
                        select match);

Answer 2

您可以尝试这样的事情： var allCountryRates =（来自于Db.PaymentRates_VisaImmigrationPermit中的r，其中allCountryCombinations.Any（c =＆gt; c.HomeCountryId == r.HomeCountryId＆amp;＆amp; c.HostCountryId == r.HostCountryId）选择r）.FirstOrDefault（）;

我创建了一个ConsoleApp来测试你的问题并且它有效。

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

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

            List<PaymentRates_VisaImmigrationPermit> PaymentRates_VisaImmigrationPermits = new List<PaymentRates_VisaImmigrationPermit>() { 
            new PaymentRates_VisaImmigrationPermit(){HomeCountryId= new Guid("5fb7097c-335c-4d07-b4fd-000004e2d28a"),HostCountryId=new Guid("5fb7097c-335c-4d07-b4fd-000004e2d28a")},
            new PaymentRates_VisaImmigrationPermit(){HomeCountryId=new Guid("5fb7097c-335c-4d07-b4fd-000004e2d28b"),HostCountryId=new Guid("5fb7097c-335c-4d07-b4fd-000004e2d28b")}
            };
            List<allCountryCombination> allCountryCombinations = new List<allCountryCombination>() {             
            new allCountryCombination(){HomeCountryId=new Guid("5fb7097c-335c-4d07-b4fd-000004e2d28b"),HostCountryId=new Guid("5fb7097c-335c-4d07-b4fd-000004e2d28b")},
            new allCountryCombination(){HomeCountryId=new Guid("5fb7097c-335c-4d07-b4fd-000004e2d28c"),HostCountryId=new Guid("5fb7097c-335c-4d07-b4fd-000004e2d28c")}
            };

            var allCountryRates = (from r in PaymentRates_VisaImmigrationPermits where allCountryCombinations.Any(c => c.HomeCountryId == r.HomeCountryId && c.HostCountryId == r.HostCountryId) select r).FirstOrDefault();

            int sa = 0;
        }

        class PaymentRates_VisaImmigrationPermit
        {
            public Guid? HomeCountryId { get; set; }

            public Guid? HostCountryId { get; set; }
        }

        class allCountryCombination
        {
            public Guid HomeCountryId { get; set; }

            public Guid HostCountryId { get; set; }
        }

    }
}

如何在LINQ中对两个参数执行连接

2 个答案: