linq结合案例条件 [英] linq join with case condition

问题描述

我可以知道在使用linq时如何选择条件"条件吗? 注释掉的代码是我的问题.我该如何放置条件? 我的代码:

Hi may i know how to do a select "case" condition in using linq? The commented out code are my question. how do i put the condition there? my code:

var r = from u in Users
    join p in Payments on u.Id equals p.UserId
    join soi in SaleOrderItems on p.ReferenceId equals soi.Id
           //if soi.InventoryTypeId == 1
              //then join i in Inventories on soi.InventoryOrCourseId equals i.Id
           //elseif soi.InventorytypeId ==2
              //then join c in Courses on soi.InventoryOrCourseId equals c.Id
    where u.Id == 5
    select new{ u, p, soi, either i or c};

推荐答案

您必须使用一些外部连接技巧来完成此操作，一种简单的方法是通过DefaultIfEmpty().本质上，您创建一个内部联接，然后使用缺少的行将其展开:

You have to use some outer join trick to accomplish this, one straightforward method is via DefaultIfEmpty(). Essentially you create an inner join then expand it with missing rows:

var r = from u in Users
    join p in Payments on u.Id equals p.UserId
    join soi in SaleOrderItems on p.ReferenceId equals soi.Id
    join i in Inventories on new {a = soi.InventoryTypeId, b = soi.InventoryOrCourseId } equals new {a = 1, b = i.Id} into g1
    from oi in g1.DefaultIfEmpty()
    join c in Courses on new {a = soi.InventoryTypeId, b = soi.InventoryOrCourseId } equals new {a = 2, b = c.Id} into g2
    from oc in g2.DefaultIfEmpty()
    where u.Id == 5
    select new{ u, p, soi, ic = oi ?? oc};

请注意最后一条语句ic = oi ?? oc，因为匿名类型不同，匿名类型将使用System.Object声明，因此它可以容纳两种类型，如果您要使用强类型支持，也许更好的选择是返回两种类型oc和ic，然后进行测试.您最好根据您后期使用查询的方式来决定.

Be careful about this last statement ic = oi ?? oc, since the types differ the anonymous type will use System.Object declaration so it can accommodate both types, if you want to use strong typed support maybe a better option would be to return both oc and ic and then test. You should best decide that based on how you use this query late ron.

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