c# - 如何在 LINQ to Entities Join 中使用 Convert.Int32() 方法？

Question

我的 LINQ to Entities 查询有问题，我正在连接四个表，但是下面的连接列的两个表没有相同的数据类型是查询。

var qry = (from i in dc.INTERNETDATAs
           join c in dc.COVERS on Convert.ToInt32(i.COVERTYPE) equals c.COV_TYPE
           join b in dc.BENEFITS on c.COVERID equals b.COVERID
           join p in dc.POLICies on i.REFERENCENO equals p.BACS_POLNO into grp
           where i.DATELOADED >= startDate
             && i.DATELOADED < endDate
             && i.CAMPAIGNTYPE == campaignType
             && c.COVERTYPEID == coverTypeId
           select i).DefaultIfEmpty();

在上面的代码中第一次加入

join c in dc.COVERS on Convert.ToInt32(i.COVERTYPE) equals c.COV_TYPE

给出以下错误

InnerException: System.NotSupportedException Message=LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression. Source=System.Data.Entity

我怎样才能做到这一点？

Answer 1

假设 i.COVERTYPE 是一个字符串，避免使用 Convert.ToInt32()。使用 SqlFunctions.StringConvert而是在 *c.COV_TYPE*​​ 上。它来自 System.Data.Objects.SqlClient ，并且可以像这样调用:

var qry = (from i in dc.INTERNETDATAs
           join c in dc.COVERS on i.COVERTYPE equals SqlFunctions.StringConvert(c.COV_TYPE)
           join b in dc.BENEFITS on c.COVERID equals b.COVERID
           join p in dc.POLICies on i.REFERENCENO equals p.BACS_POLNO into grp
           where i.DATELOADED >= startDate
             && i.DATELOADED < endDate
             && i.CAMPAIGNTYPE == campaignType
             && c.COVERTYPEID == coverTypeId
           select i).DefaultIfEmpty();