动态构建 LINQ-To-Entities Where 子句答案

【问题标题】：Dynamically Building LINQ-To-Entities Where Clause动态构建 LINQ-To-Entities Where 子句
【发布时间】：2014-12-28 19:38:49
【问题描述】：

如何动态构建 where 子句，有时 OwnerID 将为零，只有 itemID 和 LocationID 将作为搜索条件提供，在这种情况下，LINQ 应该是

(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，那么将仅提供LocationID 作为搜索条件，在这种情况下，LINQ 应该是

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

有时会提供整个OwnerID、ItemID 和LocationID 作为搜索条件，那么LINQ 将是这样的

(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子句在改变，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OwnerDetails.LocationId）。

【问题讨论】：

标签： c# linq entity-framework linq-to-sql linq-to-entities

【解决方案1】：

您可以使用基于方法的查询轻松完成。您可以一次添加一个条件，并在末尾调用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();

【讨论】：

这是在这种情况下最易读和最正确的方法

【解决方案2】：

最简单的就是检查 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();

【讨论】：