LINQ - Dynamic Where Clause on "Query Syntax"

Question

I have been searching about how to make dynamic queries and everything that I found was using "Method Syntax".

Is it possible create dynamic predicates for "Query syntax" ?

I tried to use something like

Expression<Func<TEntity, bool>>

inside the predicate but the compiler return the following message

"Cannot convert Expression<Func<TEntity, bool>> to bool"

it works on 'Method Syntax' but not on 'Query Syntax'

It works:

Expression<Func<Localization, bool>> locClause = (l => l.id == locId);

var results = UnitOfWork.Localization.AsQueryable().Where(locClause).ToList();

It doesn't work:

Expression<Func<Localization, bool>> locClause = (l => l.id == locId);

var result = from l in UnitOfWork.Localization.AsQueryable()
                          where locClause 
                          select l;

Is there a way to do this ?

The Expression can't get passed into the LINQ subsystem in query syntax and get processed at run time, you have to deal with it at compile time (i.e. compile it). But, it would be easier to just use the `Func` (or `Predicate`) and do `where locClause(l)` — Peter Ritchie, Sep 24 '14 at 14:12

score 1 · Accepted Answer · answered Sep 24 '14 at 19:12

Not knowing what you need Expression for, I can't be sure this will do everything you need it to. The use of AsQueryable has me suspect you don't (if you're directly querying a provider, you should already be IQueryable<Localization>); but, you'll have to confirm. But, if you don't need to use Expression, you could do something like this:

Func<Localization, bool> locClause = (l => l.id == locId);

var result = from l in UnitOfWork.Localization.AsQueryable()
                          where locClause(l)
                          select l;

Or with a Predicate<T>:

Predicate<Localization> locClause = l => l.id == locId;

var result = from l in UnitOfWork.Localization.AsQueryable()
                          where locClause(l)
                          select l;

This, of course, means the delegate is executed on the client and not translated into something run on the server (i.e. part of the T-SQL generated by the provider, if that is indeed happening now). If you need that, you'd need to continue using the Expression and continue to use the method chain syntax:

var result = UnitOfWork.Localization.AsQueryable().Where(locClause);

I don't believe there's any reason to choose Predicate<T> over Func<T,bool>, other than Predicate<T> is more explicit w.r.t. to intention.

There's also no functional benefit to using query syntax over method chain syntax--just readability/maintainability. I frequently find that to do much in the way of anything complex with providers usually results in dropping down to method chain syntax. I usually just use query syntax with LINQ To Objects.

score 0 · Answer 2 · answered Sep 24 '14 at 14:19

0

No, you're going to need to use method syntax. There is no way to provide an expression that you have in a variable as the parameter to a method using query syntax.

answered Sep 24 '14 at 14:19

Servy

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Really ? So there is no way to make dynamic queries with query syntax ? – vdefeo Sep 24 '14 at 18:51
@ViniciusDefeo Not dynamic in the way you're trying to use them, no. – Servy Sep 24 '14 at 18:53

LINQ - Dynamic Where Clause on "Query Syntax"

2 Answers2