如何保持干爽，同时使用LINQ到实体和辅助方法？ [英] How to stay DRY whilst using LINQ to Entities and helper methods?

问题描述

比方说，我有决定是否某些字符串匹配，像这样的一个特殊的方式：

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));
}

我想拉匹配了使用LINQ to实体和这个数据库帮手。然而，当我试试这个：

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);

我得到LINQ到实体无法识别方法......

I get "LINQ to Entities does not recognize the method..."

如果我重新写代码：

IQueryable<Blah> blahs = query.Where(b =>
      string.IsNullOrEmpty(searchText) ||
      (b.Name != null &&
        (b.Name.Trim().ToLower().StartsWith(searchText.Trim().ToLower()) ||
         b.Name.Contains(" " + searchText)));

这在逻辑上等同，那么事情做工精细的问题是，该代码并不像。可读了，我必须重新写为每个不同的实体，我想匹配

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.

据我可以从喜欢的this 之一，我想什么这样做是不可能的时刻，但我希望我失去了一些东西，我是谁？

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?

推荐答案

使用自由可用库调用 LINQKit （如@Eranga提到的）这个任务就变得合理。使用LINQKit我现在有代码如下：

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)));
}

和需要这样调用（注意AsExpandable（）调用）

And needs to be called like this (note the AsExpandable() call)

IQueryable<Blah> blahs = query().AsExpandable().Where(StringsMatch(searchText, b => b.Name));

神奇的部分是searchIn.Invoke（E）的调用和使用AsExpandable（中），这增加了一层包装，使他们的工作。

The magic parts are the searchIn.Invoke(e) calls and the use of AsExpandable() which adds a wrapper layer that allows them to work.

AsExpandable（）位进行了详细原作者的这里。

The AsExpandable() bit is explained in detail by the original author here.

请注意，我在一些细节上还是有点朦胧表达式，所以请添加评论/编辑这个答案，如果它可以变得更好/更短/清晰。

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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