How can I use this extension method in a linq to entities query using query syntax?

Question

I am creating a query to return some data from a database. I am passing in a list of ids that I want to filter on. If the filter list is null or empty I want to return all the things.

I have an extension method that lets me do this query is an IQueryable, Ids is a list of nullable ints (don't ask!) ListHasElements is a method that returns true if the list is non-null and has something in it.

 var filteredList = query.WhereIf(ListHasElements(Ids), s => Ids.Contains(s.Id.Value));

However when I build query I use my prefered query syntax

var query = from a in dbContext.as
            join b in dbContext.bs on a.Id.ToString() equals b.Id
            join cin dbContext.cs on b.Id equals c.Id into bcJoin
            from items in bcJoin.DefaultIfEmpty()
            where b.Sent >= fromDate
            where b.Sent <= toDate
            select new{a=a.thing, b=b.thingy, q=items.q,Id=a.Id}

Then I have to insert the initial line to do my magic WhereIf filter. and finally a further select to group by and create my output object (code not shown!)

The extension method looks like this.

public static IQueryable<TSource> WhereIf<TSource>(this IQueryable<TSource> source, bool condition,
    Expression<Func<TSource, bool>> predicate)
{
    return condition ? source.Where(predicate) : source;
}

Can I use this method directly in my query syntax query?

The method `WhereIf` has a parameter `condition` of type `bool`. How are you passing `Ids` which is a list? — Yacoub Massad, Nov 03 '15 at 17:11
Youve hit upon one reason I much prefer the extension method syntax to the query syntax. — Jamiec, Nov 03 '15 at 17:15
What exactly do you want to filter? `dbContext.as`? `dbContext.bs`? — Yacoub Massad, Nov 03 '15 at 17:16
Why do people like so much the query syntax? I find it a lot more difficult to understand. — Matias Cicero, Nov 03 '15 at 17:17
@MatiasCicero Joins are _much_ cleaner in query syntax. Adding intermediate results with `let` is handy in some cases as well. That's about it, though. — D Stanley, Nov 03 '15 at 17:18

score 1 · Answer 1 · answered Nov 03 '15 at 17:16

1

Quote from MSDN:

Some queries must be expressed as method calls

This is one of those cases. Even some of their own extension methods must be called via method syntax. For example, Count.

answered Nov 03 '15 at 17:16

test

2,589
2
24
52

D Stanley · Accepted Answer · 2015-11-03T17:19:46.717

0

You can't. The compiler takes well-known query syntax keywords and converts them to method syntax behind-the-scenes. It knows nothing of your custom extension and thus has no equivalent in query syntax. Note that other "built-in" linq functions have no query syntax equivalent:

Skip
Count
Take
Single
FirstOrDefault
Max
etc.

Although I would note that your extension is basically equivalent to

where !condition || predicate(b)

edited Nov 03 '15 at 17:19

answered Nov 03 '15 at 17:17

D Stanley

149,601
11
178
240

My answer works just fine, and uses his .WhereIf in his query. – Robert McKee Nov 03 '15 at 18:30
1

@RobertMcKee but not as the question asks. – Jon Hanna Nov 03 '15 at 18:40
@RobertMcKee What do you do if `predicate` depends on the results of the join instead of just the original set? – D Stanley Nov 03 '15 at 19:04
It answers the question he asked `Can I use this method directly in my query syntax query?`, which is yes. I used the method in his query syntax query, and it gives the results he wants. Seems pretty straight forward to me. – Robert McKee Nov 03 '15 at 19:33
@DStanley That wasn't the question. – Robert McKee Nov 03 '15 at 19:41

Robert McKee · Answer 3 · 2015-11-03T18:31:27.103

YES!

var query = from a in dbContext.as.WhereIf(ListHasElements(Ids), s => Ids.Contains(s.Id.Value))
        join b in dbContext.bs on a.Id.ToString() equals b.Id
        join cin dbContext.cs on b.Id equals c.Id into bcJoin
        from items in bcJoin.DefaultIfEmpty()
        where b.Sent >= fromDate
        where b.Sent <= toDate
        select new{a=a.thing, b=b.thingy, q=items.q,Id=a.Id}

My test code (Listings is a table with an Id field):

void Main()
{
   var list=new int []{1,2,3,4};
   var query=from l in Listings.WhereIf(ListHasElements(list),s=>list.Contains(s.Id))
   select l;
   query.Dump();
}
public bool ListHasElements<T>(IEnumerable<T> it)
{
    return it.Any();
}

public static class Ext {
    public static IQueryable<TSource> WhereIf<TSource>(this IQueryable<TSource> source, bool condition,
        Expression<Func<TSource, bool>> predicate)
    {
        return condition ? source.Where(predicate) : source;
    }
}

The filtering happens at the end, not the beginning. You're also not doing that in query syntax, you're using method syntax. (You're just avoiding creation of a second variable.) — Servy, Nov 03 '15 at 18:53
I used the method in his query syntax. Apparently he has no issues doing so since is using `.ToString` and `.DefaultIfEmtpy` which are both methods in his otherwise query syntax. As to doing the filtering at the end or beginning, it makes no difference in the results of his query. I used it in his query, and it gives the results you would expect. — Robert McKee, Nov 03 '15 at 19:31

How can I use this extension method in a linq to entities query using query syntax?

3 Answers3