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c#entity-frameworklinq
Anshul Nigam
Anshul Nigam

Reputation: 1628

Compilation error when using expression in linq

I have some linq query like

  Expression<Func<sometable, bool>> whereClauseDynamic= t => true;

 if (#somecondition)
        whereClauseDynamic= t => t.ID == #somevalue;

 var temp= (from tax in db.sometable
              join x in db.y on #someid
              where trans.Finished >= start
              where trans.Finished <= end
              where whereClauseDynamic
               .................

This works fine if i use Entity Framework type syntax

(from t in db.sometable
           .Where(t => t.Finished >= start)
           .Where(whereClauseLocation)
            .................

But throws a compilation error in 1st case

Error CS0029 Cannot implicitly convert type 'System.Linq.Expressions.Expression>' to 'bool'

Error CS1662 Cannot convert query expression to intended delegate type because some of the return types in the block are not implicitly convertible to the delegate return type

What am I missing?

Upvotes: 1

Views: 1147

Answers (1)

Zein Makki
Zein Makki

Reputation: 30022

One dirty trick is to use a default (always true) Expression for every table in the query. And based on the condition you have, you change one (or more) of them to a real where clause. In your query, you apply them all by default.

Expression<Func<X, bool>> whereClauseDynamicX = t => true;
Expression<Func<Y, bool>> whereClauseDynamicY = t => true;
Expression<Func<Z, bool>> whereClauseDynamicZ = t => true;
Expression<Func<someTable, bool>> whereClauseDynamicSomeTable = t => true;


from tax in db.sometable.Where(whereClauseDynamicSomeTable )
join x in db.x.Where(whereClauseDynamicX) on #someid
/* Continue the query */

Another way:

Build your query on the way with the conditions, meaning:

IQueryable<someTable> someTableQuery = db.sometable;
IQueryable<X> xQuery = db.x;
IQueryable<Y> yQuery = db.y;
IQueryable<Z> zQuery = db.z;

if(condition1)
    someTableQuery = someTableQuery.Where(x=> /* some condition */)
//.
//.
//.


var query = from tax in someTableQuery
            join x in xQuery
            /* REST */

The above approaches have not negative effect of performance whatsoever, because the query is only executed when Enumerated (using a loop or other methods like ToList() - ToArray())

Upvotes: 2

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