LINQ - 动态“查询语法”的子句

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 ?

Answer 1

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 (ie 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 wrt 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.

Answer 2

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.