Converting Lambda Functions into Lambda Expression

Question

I am trying to combine a multiple selection with a lambda function into an lambda expression. How do I do that? I know the last line is wrong, but giving you an idea of what I mean.

Func<Event, bool> where = null;

if (!string.IsNullOrWhiteSpace(searchToken))
    where = q => q.Name.ToUpper().Contains(searchToken.ToUpper());

where += q => q.Hidden = false;

Expression<Func<Event, bool>> where1 = q => where; <-- Erroring

It's not clear what you're really aiming for. Are you trying to perform multiple filters? What's the bigger picture? — Jon Skeet
– Jon Skeet, Commented Oct 15, 2011 at 22:58
The bigger picture is I have a generic function that takes Expression<Func<Event, bool>>. I have conditional statements that I need to add or not. I am using the Func<Event,boo> to build my criteria but want to send that to the generic function as Expression<Func<Event, bool>>. — Mike Flynn
– Mike Flynn, Commented Oct 16, 2011 at 5:26
I noticed something called Expression Builder, but is there a default way to do it? — Mike Flynn
– Mike Flynn, Commented Oct 16, 2011 at 5:38
You can build an Expression<Func<Event, bool>> from a delegate, but it almost certainly won't do what you need it to. — Jon Skeet
– Jon Skeet, Commented Oct 16, 2011 at 7:27
Note that building a multicast delegate like that almost certainly wouldn't do what you wanted even in LINQ to Objects - only the return value from the last condition would be used. — Jon Skeet
– Jon Skeet, Commented Oct 16, 2011 at 7:28

Jon Skeet · Accepted Answer · 2011-10-16 07:30:54Z

4

I suspect you want PredicateBuilder. (The source is available on that page.) You'd use it like this:

var predicate = q => !q.Hidden;
if (!string.IsNullOrWhiteSpace(searchToken))
{
    predicate = predicate.And(q => q.Name.ToUpper()
                                         .Contains(searchToken.ToUpper());
}
return predicate;

That's assuming you want to "and" the conditions - you never made that clear...

Note that that is not a good way to compare in a case-insensitive way, either. If you could tell us what's going to consume the query (e.g. LINQ to SQL, LINQ to EF) we could suggest a provider-compatible way of performing a case-insensitive query.

answered Oct 16, 2011 at 7:30

Jon Skeet

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2 Comments

Mike Flynn Over a year ago

What is var predicate = q => !q.Hidden;? Is that an Expression of Func?

Jon Skeet Over a year ago

@MikeFlynn: That wouldn't compile - you can't use assign a lambda expression to an implicitly typed variable. You could either have Expression<Func<Foo, bool>> predicate = q => !q.Hidden; or Func<Foo, bool> predicate = q => !q.Hidden;. The first will give you an expression tree, the latter a delegate.

pero · Accepted Answer · 2011-10-16 07:17:52Z

0

Look at http://msdn.microsoft.com/en-us/library/bb882637.aspx. How to use expression trees to build dynamic queries.

AFAIK when using Expression <> like that the expression must be known in compile time, because the compiler then build AST abstract syntax three and stores it as data in your Expression <> instance.

answered Oct 16, 2011 at 7:17

pero

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