Question

这个错误是什么意思？

我的错误是：

Cannot implicitly convert type 'System.Linq.IQueryable<System.Linq.IGrouping<decimal,Project1.Domain.Models.DepartmentBreakdownReport>>' to 'System.Linq.IQueryable<Project1.Domain.Models.DepartmentBreakdownReport>'. An explicit conversion exists (are you missing a cast?)

我想知道它是什么，以便我能够解决它。它只发生在我的结果上添加LINQ GroupBy时。

 public IQueryable<DepartmentBreakdownReport> GetDepartmentBreakdownBySupplierIDAndReviewID(int ClientID, int? SupplierID, int? ReviewID) {
            return (from d in Db.Details
                    join h in Db.Headers
                    on new { d.ClientID, d.ClaimID }
                    equals new { h.ClientID, h.ClaimID }
                    where d.ClientID == ClientID && h.SupplierID == SupplierID

                    join sd in Db.SuppDepts
                    on new { a = d.ClientID, b = d.CategoryID ?? 0 }
                    equals new { a = sd.ClientID, b = sd.CategoryID }

                    join r in Db.Reviews
                    on new { h.ClientID, h.ReviewID }
                    equals new { r.ClientID, r.ReviewID }

                    join rp in Db.ReviewPeriods
                    on new { a = r.ClientID, b = r.ReviewPeriodID ?? 0 }
                    equals new { a = rp.ClientID, b = rp.ReviewPeriodID }
                    where r.ReviewID == ReviewID

                    join su in Db.Suppliers
                   on new { h.ClientID, h.SupplierID }
               equals new { su.ClientID, su.SupplierID }

                    select new DepartmentBreakdownReport {
                        DepartmentName = sd.DepartmentName,
                        SumOfAmount = d.Amount,
                        SupplierID = h.SupplierID,
                        ReviewID = h.ReviewID,
                        ReviewName = rp.ReviewPeriodName,
                        SupplierName = su.SupplierName,
                        ClientID = d.ClientID
                    }).GroupBy(r=>r.Amount);
        }

Answer 1

这个错误是什么意思？

您的返回类型为IQueryable<DepartmentBreakdownReport>，但您尝试返回IGrouping<decimal,Project1.Domain.Models.DepartmentBreakdownReport>

当您致电.GroupBy时，您将返回类型更改为IGrouping而非IQueryable，它还会将小数作为IGrouping表达式的一部分引入（这是您要分组的金额变量通过）。

要解决此问题，您只需将方法签名更改为：

即可

public IGrouping<decimal,Project1.Domain.Models.DepartmentBreakdownReport> GetDepartmentBreakdownBySupplierIDAndReviewID(int ClientID, int? SupplierID, int? ReviewID)

Answer 2

对我而言，方法名称GetDepartmentBreakdownBySupplierIDAndReviewID意味着：

首先，方法返回类型是这样的，假设SupplierId和ReviewId的int类型：

IGrouping<Tuple<int, int>, IQueryable<DepartmentBreakdownReport>> 
                                       GetDepartmentBreakdownBySupplierIDAndReviewID(...)

其次，返回应该是最终投影，例如：

select new DepartmentBreakdownReport { ... })
    .GroupBy(r => new Tuple<int, int>(r.SupplierID, r.ReviewID));

无法从IQueryable <decimal，project1，model>转换为IQueryable <project1，model>

2 个答案: