使用LINQ to Entities和帮助方法时如何保持DRY? [英] How to stay DRY whilst using LINQ to Entities and helper methods?
问题描述
public bool stringsMatch (string searchFor,string searchIn)
{
if(string.IsNullOrEmpty(searchFor))
{
return true;
}
return searchIn!= null&&
(searchIn.Trim()。ToLower()。StartsWith(searchFor.Trim()。ToLower())||
searchIn.Contains(+ searchFor));
}
我想使用Linq To Entities从数据库中拉出匹配项,帮手。但是,当我尝试这样做:
IQueryable&Blan& blahs = query.Where(b => stringsMatch(searchText,b.Name);
我得到LINQ to Entities不认识方法...
如果我重写代码如下:
IQueryable< Blah> blahs = query.Where(b =>
string.IsNullOrEmpty(searchText)||
(b.Name!= null& &
(b.Name.Trim()。ToLower()。StartsWith(searchText.Trim()。ToLower())||
b.Name.Contains(+ searchText))) ;
这在逻辑上是等效的,那么事情就行了,问题是代码不是可以读取,我必须为每个要匹配的不同实体重新编写它。
从我可以从这是一个,我想要现在是不可能的,但我希望我错过了一些东西I?
使用一个免费的库,名为 LINQKit (由@Eranga提到)这个任务变得合理。使用LINQKit我现在的代码如下:
protected Expression< Func< T,bool>> stringMatch(string searchFor,Expression< Func< T,string>> searchIn)
{
if(string.IsNullOrEmpty(searchFor))
{
return e =>真正;
}
return
e =>
(searchIn.Invoke(e)!= null&&
(searchIn.Invoke(e).Trim()。ToLower()。StartsWith(searchFor.Trim()。ToLower() ||
searchIn.Invoke(e).Contains(+ searchFor)));
}
需要像这样调用(注意AsExpandable()调用) / p>
IQueryable< Blah> blahs = query()。AsExpandable()。Where(StringsMatch(searchText,b => b.Name));
魔法部分是searchIn.Invoke(e)调用和使用AsExpandable()一个允许他们工作的包装层。
AsExpandable()位由原作者详细解释这里。
请注意,我还有一些朦胧的一些细节表达式,所以请添加评论/编辑这个答案,如果它可以做得更好/更短/更清晰。
Lets say that I have a particular way of deciding whether some strings "match", like this:
public bool stringsMatch(string searchFor, string searchIn)
{
if (string.IsNullOrEmpty(searchFor))
{
return true;
}
return searchIn != null &&
(searchIn.Trim().ToLower().StartsWith(searchFor.Trim().ToLower()) ||
searchIn.Contains(" " + searchFor));
}
I would like to pull matches out of a database using Linq To Entities and this helper. However, when I try this:
IQueryable<Blah> blahs = query.Where(b => stringsMatch(searchText, b.Name);
I get "LINQ to Entities does not recognize the method..."
If I re-write the code as:
IQueryable<Blah> blahs = query.Where(b =>
string.IsNullOrEmpty(searchText) ||
(b.Name != null &&
(b.Name.Trim().ToLower().StartsWith(searchText.Trim().ToLower()) ||
b.Name.Contains(" " + searchText)));
Which is logically equivalent, then things work fine. The problem is that the code isn't as readable, and I have to re-write it for each different entity I want to match.
As far as I can tell from questions like this one, what I want to do is impossible at the moment, but I'm hoping that I'm missing something, am I?
Using a freely available library called LINQKit (as mentioned by @Eranga) this task becomes reasonable. Using LINQKit the code I have now looks like:
protected Expression<Func<T, bool>> stringsMatch(string searchFor, Expression<Func<T, string>> searchIn)
{
if (string.IsNullOrEmpty(searchFor))
{
return e => true;
}
return
e =>
(searchIn.Invoke(e) != null &&
(searchIn.Invoke(e).Trim().ToLower().StartsWith(searchFor.Trim().ToLower()) ||
searchIn.Invoke(e).Contains(" " + searchFor)));
}
And needs to be called like this (note the AsExpandable() call)
IQueryable<Blah> blahs = query().AsExpandable().Where(StringsMatch(searchText, b => b.Name));
The magic parts are the searchIn.Invoke(e) calls and the use of AsExpandable() which adds a wrapper layer that allows them to work.
The AsExpandable() bit is explained in detail by the original author here.
Note that I'm still a bit hazy on some of the details of expressions, so please add a comment/edit this answer if it can be made better/shorter/clearer.
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