使用Entity Framework连接多个表

我正在尝试使用EF加入3个表，但它会抛出一个错误说

consider swaping conditions on either side of equals 

可以请一些人帮忙

  var billdata = from billtotal in context.billTotals join billcard in context.billClubcards on billtotal.OrderID equals billcard.OrderID join billtender in context.billTenders on billtender.OrderID equals billtotal.OrderID select billtotal; 

Answer 1

编译错误非常正确： –

The name 'billtender' is not in scope on the left side of 'equals'. Consider swapping the expressions on either side of 'equals'.

您加入的桌子需要在左侧，您加入的桌子需要在右侧。 因此： –

 var billData = from billtotal in context.billTotals join billcard in context.billClubcards on billtotal.OrderId equals billcard.OrderId join billtender in context.billTenders on billtotal.OrderId equals billtender.OrderId select billtotal; 

如果您想知道原因，那是因为查询语法只是底层扩展方法的语法糖： –

 context.billTotals .Join( context.billClubcards, billtotal => billtotal.OrderId, billclubcard => billclubcard.OrderId, (billtotal, billclubcard) => billtotal) .Join( context.billTenders, billtotal => billtotal.OrderId, billtender => billtender.OrderId, (billtotal, billtender) => billtotal); 

您的原始实施将扩展为： –

 context.billTotals .Join( context.billClubcards, billtotal => billtotal.OrderId, billclubcard => billclubcard.OrderId, (billtotal, billclubcard) => billtotal) .Join( context.billTenders, billtotal => billtender.OrderId, // billtender isn't in scope! billtender => billtotal.OrderId, // billtotal isn't in scope! (billtotal, billtender) => billtotal); 

Answer 2

您加入的表的属性需要在equals的右侧

开关

from： on billtender.OrderID equals billtotal.OrderID

to： on billtotal.OrderID equals billtender.OrderID