PredicateBuilder< True>之间的差异和PredicateBuilder< False&gt ;？ [英] Difference between PredicateBuilder<True> and PredicateBuilder<False>?

问题描述

我有代码：

   var predicate = PredicateBuilder.True<Value>();

predicate = predicate.And(x => x.value1 == "1");
predicate = predicate.And(x => x.value2 == "2");

var vals = Value.AsExpandable().Where(predicate).ToList();

如果我有 PredicateBuilder.True< Value>（），它会带回我期望的值，但是如果我有 PredicateBuilder.False< Value>（），它会带回0条记录。有人可以解释一下区别是什么，为什么在一种情况下我得到0条记录，而在另一种情况下我得到了我期望的结果。我已经阅读了 PredicateBuilder 的文档，但这有点令人困惑。我觉得这与我 Anding 谓词在一起？

If I have PredicateBuilder.True<Value>(), it brings back what I expect, but if I have PredicateBuilder.False<Value>(), it brings back 0 records. Can someone explain what the the difference is and why in one scenario I get back 0 records an in the other I get what I expect. I already read the PredicateBuilder documenation, but it was a bit confusing. I have a feeling it has to do with the fact that I am Anding predicates together?

推荐答案

使用 PredicateBuilder 递增地构建0个或更多条件的谓词时，从可以添加到中性谓词开始的位置很方便，因为您可以随后遍历条件，然后将和或或与最后一个条件一起使用。例如，

When using PredicateBuilder to incrementally build a predicate of 0 or more conditions, it is convenient to start off with a "neutral" predicate that can be added to, since you can then just iterate through the conditions and either and or or them together wth the last one. E.g.

var predicate = PredicateBuilder.True<Value>();

foreach (var item in itemsToInclude) {
  predicate = predicate.And(o => o.Items.Contains(item));
}

这等效于更简单的布尔逻辑：

This would be equivalent to the more straightforward boolean logic:

var predicate = true;

foreach (var item in itemsToInclude) {
  predicate = predicate && o.Items.Contains(item);
}

这将等同于

true && ((o.Items.Contains(itemsToInclude[0] && o.Items.Contains.itemsToInclude[1]) ...)

或 true& restOfPredicate ，如果 restOfPredicate 是true，如果 restOfPredicate 是<$ c，则 false

Or true && restOfPredicate, which evaluates to true if restOfPredicateis true, and false if restOfPredicate is false. Hence why it's considered neutral.

从 PredicateBuilder.False ，但是等同于 false& restOfPredicate ，该值始终会评估为 false 。

Starting out with PredicateBuilder.False, however, would be equivalent false && restOfPredicate, which would always evaluate to false.

类似地，对于或，以 false 开头到 false || restOfPredicate ，如果 restOfPredicate false 如果 restOfPredicate 是<$ c，则>为 false 和 true $ c> true 。而 true || restOfPredicate 始终将评估为 true 。

Similarly for or, starting out with false would be equivalent to false || restOfPredicate, which evaluate to false if restOfPredicate is false and true if restOfPredicate is true. And true || restOfPredicate would always evaluate to true.

底线：使用 PredicateBuilder.True 作为中性起点，并使用 PredicateBuilder.And 和 PredicateBuilder.False 与 PredicateBuilder.Or 。

Bottom line: Use PredicateBuilder.True as a neutral starting point with PredicateBuilder.And, and PredicateBuilder.False with PredicateBuilder.Or.

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