LINQ PredicateBuilder有条件的AND，OR和NOT过滤器 [英] Linq PredicateBuilder with conditional AND, OR and NOT filters
问题描述
我们必须使用LINQ to SQL的一个项目，为此我需要重写几个搜索页面，让客户选择他们是否愿意执行的和的或的或的搜索。
We have a project using LINQ to SQL, for which I need to rewrite a couple of search pages to allow the client to select whether they wish to perform an and or an or search.
不过，我觉得有关重做的LINQ查询使用的 PredicateBuilder 并得到了这方面的工作非常好，我认为。我有效地有一个类包含我的谓词，例如：
I though about redoing the LINQ queries using PredicateBuilder and have got this working pretty well I think. I effectively have a class containing my predicates, e.g.:
internal static Expression<Func<Job, bool>> Description(string term)
{
return p => p.Description.Contains(term);
}
要执行搜索我这样做（为简洁起见省略了一些代码）
To perform the search i'm doing this (some code omitted for brevity):
public Expression<Func<Job, bool>> ToLinqExpression()
{
var predicates = new List<Expression<Func<Job, bool>>>();
// build up predicates here
if (SearchType == SearchType.And)
{
query = PredicateBuilder.True<Job>();
}
else
{
query = PredicateBuilder.False<Job>();
}
foreach (var predicate in predicates)
{
if (SearchType == SearchType.And)
{
query = query.And(predicate);
}
else
{
query = query.Or(predicate);
}
}
return query;
}
而我与此相当快乐，我有两个问题：
While i'm reasonably happy with this, I have two concerns:
- 的如果评价一个检索类别属性/ else块感觉他们可能是一个潜在的代码味道。
- 客户端现在坚持能够执行'而不是'/'或不'搜索。
要解决2点，我想我可以通过简单地重写我的表情，这样做如：
To address point 2, I think I could do this by simply rewriting my expressions, e.g.:
internal static Expression<Func<Job, bool>> Description(string term, bool invert)
{
if (invert)
{
return p => !p.Description.Contains(term);
}
else
{
return p => p.Description.Contains(term);
}
}
不过这种感觉就像有点杂牌的，这通常意味着有一个更好的解决方案在那里。谁能推荐如何可以改善？我知道动态LINQ，但我真的不希望失去LINQ的强类型。
However this feels like a bit of a kludge, which usually means there's a better solution out there. Can anyone recommend how this could be improved? I'm aware of dynamic LINQ, but I don't really want to lose LINQ's strong typing.
推荐答案
如果您正在寻找少线路可以代替的if / else三元运算符：
If you are looking for less lines you can replace the if/else with ternary operator:
query = SearchType == SearchType.And ? PredicateBuilder.True<Job>() : PredicateBuilder.False<Job>();
foreach (var predicate in predicates)
{
query = SearchType == SearchType.And ? query.And(predicate) : query.Or(predicate);
}
的'而不是'/'或不部分 运营商应该做的伎俩
for the 'and not' / 'or not' part the ! operator should do the trick.
PD！： 你测试的foreach 部分正确设置谓词？据我记得你正在建设将在稍后的时间点执行的表情，所以你可能有一个文字引用只是在最后一次迭代的最后一组断言，这就是为什么你必须使用一个临时变量保存每个迭代的值。
PD: Did you test the foreach part is correctly setting the predicates?, as far as i remember you are building the expression that will be executed at later point in time, so you may have a literal reference just to the last set predicate in the final iteration, and that's why you must use a temp variable to save the value of each iteration.
修改
如果你想否定一个表达式程序，这是一个很微妙的，你可以尝试这样的：
If you want to negate a expression programmatic, that's a tricky one, you can try something like:
internal static Expression<Func<Job, bool>> Description(string term, bool invert)
{
return NegateExp<Func<Job, bool>>(p => p.Description.Contains(term), invert);
}
而NegateExp方法是这样的：
And the NegateExp method will be something like:
public static Expression<TDelegate> NegateExp<TDelegate>(Expression<TDelegate> expression, bool inverse)
{
if (inverse)
{
return Expression.Lambda<TDelegate>(Expression.Not(expression.Body), expression.Parameters);
}
return expression;
}
您可以看看这个问题，更多的例子的http://stackoverflow.com/questions/2166591/is-there-any-way-to -negate-A-谓词
You can take a look at this question for more examples http://stackoverflow.com/questions/2166591/is-there-any-way-to-negate-a-predicate
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