动态构建LINQ-To-Entities Where子句

Question

(from s in repository.ItemOwners.Include("OwnerDetails")
 where s.ItemId == searchCriteria.ItemID && 
       s.OwnerDetails.LocationId == searchCriteria.LocationID
 select new { s.OwnerDetails.OwnerName, s.OwnerDetails.MobNumber }).ToList();

有时OwnerID和ItemId将为零，那么只会提供LocationID作为搜索条件，在这种情况下，LINQ应为

(from s in repository.ItemOwners.Include("OwnerDetails")
 where s.OwnerDetails.LocationId == searchCriteria.LocationID
 select new { s.OwnerDetails.OwnerName, s.OwnerDetails.MobNumber }).ToList();

有时会提供完整的OwnerID、ItemID和LocationID作为搜索条件，然后LINQ将如下所示

(from s in repository.ItemOwners.Include("OwnerDetails")
 where s.OwnerId == searchCriteria.OwnerID && 
       s.ItemId == searchCriteria.ItemID && 
       s.OwnerDetails.LocationId == searchCriteria.LocationID
 select new { s.OwnerDetails.OwnerName, s.OwnerDetails.MobNumber }).ToList();

这里只有where子句在变化，请帮我解决。如何动态构建where子句(请注意，这里我有一个导航属性，它是OwnerDetails.LocationId)。

Answer 1

通过使用基于方法的查询，您可以轻松地完成此操作。您可以一次添加一个条件，最后调用Select和ToList：

// Where(x => true) might not be necessary, you can try skipping it.
var query = repository.ItemOwners.Include("OwnerDetails").Where(x => true);

if (searchCriteria.OwnerID != null)
    query = query.Where(s => s.OwnerID == searchCriteria.OwnerID);
if (searchCriteria.ItemID != null)
    query = query.Where(s => s.ItemID == searchCriteria.ItemID);
if (searchCriteria.OwnerID != null)
    query = query.Where(s => s..OwnerDetails.LocationId == searchCriteria.LocationID);

var results = query.Select(s => new { s.OwnerDetails.OwnerName, s.OwnerDetails.MobNumber }).ToList();

Answer 2

最简单的方法就是检查Where子句中的零条件：

(from s in repository.ItemOwners.Include("OwnerDetails")
where (searchCriteria.OwnerID == 0 || s.OwnerId == searchCriteria.OwnerID) && 
(searchCriteria.ItemID == 0 || s.ItemId == searchCriteria.ItemID) && 
s.OwnerDetails.LocationId == searchCriteria.LocationID
select new { s.OwnerDetails.OwnerName, s.OwnerDetails.MobNumber }).ToList();