C# LINQ 中的 Where 谓词
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Where Predicates in LINQ
提问by JustAProgrammer
How can I specify conditions in Where predicates in LINQ without getting null reference exceptions. For instance, if qis an IQueryable how can I do like:
如何在 LINQ 中的 Where 谓词中指定条件而不会出现空引用异常。例如，如果q是一个 IQueryable 我该怎么做：
Expression<Func<ProductEntity,bool>> predicate = p => !search.CategoryId.HasValue || (search.CategoryId.HasValue && search.CategoryId == p.CategoryId);
var q2 = q.Where(predicate);
Here searchis an object that holds possible search conditions that may or may not be set like search.CategoryId might not be set but if it is I want to get the products that are set by that condition.
这search是一个保存可能的搜索条件的对象，这些条件可能会或可能不会像 search.CategoryId 那样设置，但如果是这样，我想获取由该条件设置的产品。
When I do this I get null reference exceptions.
当我这样做时，我得到空引用异常。
采纳答案by Johannes Rudolph
You can use the null-coalescing operator??to replace a possible null value with a default value. The following sets tries to match the search.Category if it exists or simply creates an "always true" expression. This will be optimized by any good Linq query provider (e.g. LinqToSql).
您可以使用空合并运算符??将可能的空值替换为默认值。以下集合尝试匹配 search.Category 如果它存在或只是创建一个“始终为真”的表达式。这将由任何好的 Linq 查询提供程序（例如 LinqToSql）进行优化。
Expression<Func<ProductEntity,bool>> predicate = p => (search.CategoryId ?? p.CategoryId) == p.CategoryId);
var q2 = q.Where(predicate);
Another possibility would be to dynamically compose a query predicate using PredicateBuilder. That's the way I do it for searches with a similar pattern as you use:
另一种可能性是使用PredicateBuilder动态组合查询谓词。这就是我使用与您使用的模式类似的搜索方式进行的搜索：
var predicate = PredicateBuilder.True<Order>();
if (search.OrderId))
{
predicate = predicate.And(a => SqlMethods.Like(a.OrderID, search.OderID);
}
// ...
var results = q.Where(predicate);
回答by Marc Gravell
Let's dissect the line:
让我们剖析一下这条线：
Expression<Func<ProductEntity,bool> predicate = p => !search.CategoryId.HasValue
|| (search.CategoryId.HasValue && search.CategoryId == p.CategoryId)
var q2 = q.Where(predicate);
So how many ways can we get nullproblems?
那么我们有多少种方式会遇到null问题呢？
search(your "captured" variable) could benullpcould be null, meaning there is anullin the list- you've handledthe case of
search.CategoryIdbeingnull(Nullable<T>) - but maybe
p.CategoryId(the category on a record in the list) isnull(Nullable<T>) - however, I'm not sure that this would cause aNullReferenceException q(the list / source) could benull
search（您的“捕获”变量）可能是nullp可能为空，意味着null列表中有一个- 你处理的情况下
search.CategoryId是null（Nullable<T>） - 但也许
p.CategoryId（列表中记录的类别）是null(Nullable<T>) - 但是，我不确定这会导致NullReferenceException q（列表/来源）可能是null
So: out of 5 options you've eliminated 1; look at the other 4? There is alsothe definite possibility that the issue is caused by something invisible not shown in the code; for example the getcould be:
所以：在 5 个选项中你已经消除了 1 个；看看其他4个？有也了一定的可能性问题是由代码未显示看不见的东西造成的; 例如get可能是：
public int? CategoryId {
get {return innerObject.CategoryId;}
}
and innerObjectcould be null; if you eliminate the other 4 (pretty easy to do), look at at this one as a last resort.
并且innerObject可能是null；如果您消除其他 4 个（很容易做到），请将此作为最后的手段。
