Question

我都是C＃，Linq和.Netcore的新手，我想做的就是将sql中的以下查询转换为linq

SELECT
p.policyId,
p.insuranceId,
p.policyType,
p.coverageType,
p.price,
us.FirebaseUID
FROM Policies p
JOIN Users us ON p.userId = us.FirebaseUID

我的策略是父类ExampleRes内的嵌套类，我遵循此question答案以填充策略列表，但我收到此错误：

Error CS1061: 'ExampleRes.PoliciesInfo' does not contain a definition for 'ToList' and no extension method 'ToList' accepting a first argument of type 'ExampleRes.PoliciesInfo' could be found (are you missing a using directive or an assembly reference?)

这是我正在使用的查询

public List<ExampleRes> GetUserInfo(string fuid)
    {

        var policies = (from pol in _context.Policy
                        join us in _context.User
                        on pol.userId equals us.FirebaseUID
                        select new ExampleRes
                        {
                            userId = us.userId,
                            fuid = us.FirebaseUID,
                            policy = (new ExampleRes.PoliciesInfo
                            {
                                policyId = pol.policyId,
                                policyType = pol.policyType,
                                policyPrice = pol.price,
                                coverageType = pol.coverageType
                            }).toList()
                        }).toList();

        return policies;
    }

这是 ExampleRes类

public class ExampleRes
{
    public ExampleRes()
    {
    }

    // User info
    public int userId { get; set;}
    public string fuid { get; set; }
    public List<PoliciesInfo> policy { get; set; }

    // Policies info
    public class PoliciesInfo{

        public int policyId { get; set; }
        public int policyType { get; set; }
        public int coverageType { get; set; }
        public int policyPrice { get; set; }

    }

}

Answer 1

您的查询应如下所示：

var policies = (from pol in _context.Policy
                join us in _context.User
                on pol.userId equals us.FirebaseUID
                select new ExampleRes
                {
                    userId = us.userId,
                    fuid = us.FirebaseUID,
                    policy = new List<ExampleRes.PoliciesInfo> {
                        new ExampleRes.PoliciesInfo
                        {
                             policyId = pol.policyId,
                             policyType = pol.policyType,
                             policyPrice = pol.price,
                             coverageType = pol.coverageType
                        }
                    }
                }).ToList();

已更新：

我误解了这个问题，应该是这样的。但是我相信应该有一个更好的解决方案。

var policies = (from us in _context.User
                select new ExampleRes
                {
                    userId = us.userId,
                    fuid = us.FirebaseUID,
                    policy = (
                        from pol in _context.Policy
                        where pol.userId == us.FirebaseUID
                        select new ExampleRes.PoliciesInfo
                        {
                            policyId = pol.policyId,
                            policyType = pol.policyType,
                            policyPrice = pol.price,
                            coverageType = pol.coverageType
                        }
                    ).ToList()
                }).Where(u => u.policy.Count > 0).ToList();

再次更新

@Ilan Keshet感谢将linq函数查询与非函数查询结合在一起是一个坏习惯。这是我修改后的解决方案：

var policies = (from pol in _context.Policy
                group pol by pol.userId into uid
                where uid.Count() > 0
                join us in _context.User
                on uid.Key equals us.FirebaseUID
                select new ExampleRes
                {
                    userId = us.userId,
                    fuid = us.FirebaseUID,
                    policy = (from pol in _context.Policy
                              where pol.userId == uid.Key
                              select new ExampleRes.PoliciesInfo
                              {
                                  policyId = pol.policyId,
                                  policyType = pol.policyType,
                                  policyPrice = pol.price,
                                  coverageType = pol.coverageType
                              }).ToList()
                }).ToList();

Answer 2

我认为您正在寻找Nested query

尝试这样的事情：

var policies = (from pol in _context.Policy
                        join us in _context.User
                        on pol.userId equals us.FirebaseUID into policies
                        select new ExampleRes
                        {
                            userId = us.userId,
                            fuid = us.FirebaseUID,
                            policy = (from p in policies
                                      select new ExampleRes.PoliciesInfo
                            {
                                policyId = p.policyId,
                                policyType = p.policyType,
                                policyPrice = p.price,
                                coverageType = p.coverageType
                            }).ToList()
                        }).ToList();

或

var policies = (from us in _context.User
                        select new ExampleRes
                        {
                            userId = us.userId,
                            fuid = us.FirebaseUID,
                            policy = _context.Policy
                                             .Where(pol=>pol.userId == us.FirebaseUID)
                                             .Select(new ExampleRes.PoliciesInfo
                                             {
                                                 policyId = pol.policyId,
                                                 policyType = pol.policyType,
                                                 policyPrice = pol.price,
                                                 coverageType = pol.coverageType
                                             }).ToList()
                        }).ToList();

Answer 3

您不能在ToList()上呼叫ExampleRes.PoliciesInfo，而不是任何类型的Enumerable

policy = (new ExampleRes.PoliciesInfo
            {
                policyId = pol.policyId,
                policyType = pol.policyType,
                policyPrice = pol.price,
                coverageType = pol.coverageType
             }).toList()  // <== doesn't make sense

简而言之，您需要使用实例化的类作为元素创建一个新的列表

new List<blah> { new blah() {...} }

Enumerable.ToList Method (IEnumerable)

从IEnumerable创建列表。

Answer 4

首先toList()（小t）将导致编译时错误。将其更改为ToList()（大写T）。

第二，policy属性是List<PoliciesInfo>（policysinfo的列表），您设置了错误的内容。

您应该做的是这样：

policy = new List<Examples.PoliciesInfo> {                 
             new Examples.PoliciesInfo {
                   policyId = pol.policyId,
                   policyType = pol.policyType,
                   policyPrice = pol.price,
                   coverageType = pol.coverageType
             }
             // you can create more Examples.PoliciesInfo here, just seperate them with comma
         }.ToList()

这将解决。

Linq查询以填充收到错误的嵌套类中的列表

4 个答案: