要在C＃中的lambda表达式中应用多个标准 [英] To apply mulitple criteria in lambda expression in c#

问题描述

我有两个主表列表和放置。在列表表中有一个字段 PlaceId ，它引用一个Place实体/行/对象。我想查询这两个表，这样我得到了他们这样的。

I have two main tables Listings and Place . In listing table there is a field PlaceId which referes to a Place entity/row/object . I want to query on both tables so that i get both of them like this .

 var query = context.Listings
       .Include("Place")
       .Where(l => l.Place.TypeId == Type.Ro)
       .OrderBy(l => l.Id).ToList();

现在我想对这个查询添加一些过滤器，这里是条件。

after this now i want to put some filter on this query , here is the condition .

我只有一个这样的字符串 var filter =1,2,4; 。现在我想过滤列表gett所有这些列表，其中卧室等于1或2或4。

i got only a string like this var filter = "1,2,4"; . Now i want to filter on listing to gett all these listing where bedroom is equal to 1 OR 2 OR 4 .

我做了什么

 string minBeds = "1,2,4";

 foreach (var item in minBeds.Split(','))
 {
      int minBed = int.Parse(item);
      query = query.Where(l=>l.Place.Bedroom == minBed).ToList();
 }

但这样做是给我零结果。

But doing this is giving me Zero result.

推荐答案

筛选方式的问题。第一遍之后，除了 Bedroom == 1 ，过滤掉第二遍的所有东西，除了卧室== 2 ，但由于列表中只有 Bedroom == 1 ，所以在结果集中不会有任何内容。

The problem with the way you're filtering it. After the first pass, you're filtering out everything except where Bedroom == 1, on the second pass you're filtering out everything except where Bedroom == 2, but since the only items in the list have Bedroom == 1, you won't have anything in the result set.

解决方案是使用常规的C＃ || 运算符：

The solution is to use the conventional C# || operator:

query = query.Where(l => l.Place.Bedroom == "1" || 
                         l.Place.Bedroom == "2" || 
                         l.Place.Bedroom == "4");

或者如果要更灵活，请使用 包含 方法：

Or if you want to be more flexible, use the Contains method:

string[] minBeds = "1,2,4".Split(',');
query = query.Where(l => minBeds.Contains(l.Place.Bedroom));

请注意，如果 Bedroom 需要首先将输入转换为适当的类型：

Note if Bedroom is an integer, you'll need to convert the input to an appropriate type first:

var minBeds = "1,2,4".Split(',').Select(int.Parse);
query = query.Where(l => minBeds.Contains(l.Place.Bedroom));

另外请注意，我已经删除了 ToList 这里。除非您需要通过索引访问项目并从结果集合中添加/删除项目，这很可能只是浪费资源。你通常可以依靠Linq的本地惰性来延迟处理，直到你真正需要结果。

Also note, I've eliminated the ToList here. Unless you need to access items by index and add / remove items from the result collection, it's most likely just a waste of resources. You can usually rely on Linq's native laziness to delay processing to query until you really need the result.

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