I have the code:
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();
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?
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));
}
This would be equivalent to the more straightforward boolean logic:
var predicate = true;
foreach (var item in itemsToInclude) {
predicate = predicate && o.Items.Contains(item);
}
Which would be equivalent to
true && ((o.Items.Contains(itemsToInclude[0] && o.Items.Contains.itemsToInclude[1]) ...)
Or true && restOfPredicate
, which evaluates to true
if restOfPredicate
is true, and false
if restOfPredicate
is false
. Hence why it's considered neutral.
Starting out with PredicateBuilder.False
, however, would be equivalent false && restOfPredicate
, which would always evaluate to false
.
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
.
Bottom line: Use PredicateBuilder.True
as a neutral starting point with PredicateBuilder.And
, and PredicateBuilder.False
with PredicateBuilder.Or
.
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