CODEWITHSUNDEEP
c#linqlinq-to-entities
Rodney S. Foley
Rodney S. Foley

Reputation: 10330

Need help with adding Where clauses in loop with Linq

Below is a rough and simplified code example of what I am trying to do and it is not working as expected. I am using Linq to Entity in case that matters.

Basically in the first loop with the DateClause's ends up returning nothing if I have two date clauses, works great with one date clause. In the TextClause's loop it seems to ignore every text clause after the first one.

My expectation is that the clauses would be ANDed together, so that if I do a text clause and then do another they should be added and I would get a more focused results, especially with using the Contains method.

Expectations for the dates are that I would expect to be able to do a date range with this and but if there are two dates it always returns nothing, even though I know there are records with the right dates in the range.

I am sure I am doing something wrong or plain stupid, but I can't see it. 

enum Operators
{
    Contains,
    DoesNotContain,
    GreaterThan,
    LessThan
}

public void DoSomething(List<DateClauses> dateClauses, List<TextClauses> textClauses)
{
    var query = from t in context.Table
                where t.Enabled = true
                select new
                {
                    title = t.title
                    date = t.date
                }


    foreach (DateClause clause in dateClauses)
    {
        switch (clause.Operator)
        {
            case Operator.GreaterThan:
                query = query.Where(l => l.date > clause.Date);
                break;
            case Operator.LessThan
                query = query.Where(l => l.date < clause.Date);
                break;
        }       
    }

    foreach (TextClause clause in textClauses)
    {
        switch (clause.Operator)
        {
            case Operator.Contains:
                query = query.Where(l => l.title.Contains(clause.Text));
                break;
            case Operator.DoesNotContain
                query = query.Where(l => !l.title.Contains(clause.Text));
                break;
        }
    }
}

EDIT: Updated example code to show use of enum in the process. When I extrapolated the use of the enum in to some of the very nice solutions (that otherwise would work with a bool) cause an exception shown in a comment from me in Joel's response.

I want to say I like what I have seen in responses so far, and I have learned a few new Linq tricks, I apologize for the example as I didn't think bool vs enum would matter much with Linq. I hope the changes will help find a solution that can work for me. Thanks again for the great responses to date.

Upvotes: 2

Views: 4027

Answers (3)

Ruben
Ruben

Reputation: 15515

Just a wild guess, but could it be that changing your loops to the following has different results?

foreach (DateClause clause in dateClauses)
{
    var capturedClause = clause;
    switch (clause.Operator)
    {
            case Operator.GreaterThan:
                    query = query.Where(l => l.date > capturedClause.Date);
                    break;
            case Operator.LessThan
                    query = query.Where(l => l.date < capturedClause.Date);
                    break;
    }               
}

You're creating Where criteria capturing the loop variable, not the value of the loop variable for each iteration. So in the end, each Where will be the same, using the value for the last iteration of the loop, as the query is executed after the last iteration. By introducing a temp variable you'll capture the variable for each iteration.

See also Eric Lippert's blog about this subject.

Upvotes: 4

Joel Coehoorn
Joel Coehoorn

Reputation: 415735

 var query = context.Table
         .Where(t => t.Enabled 
             && textClauses.Where(c => c.Contains).All(c => t.title.Contains(c.Text) )
             && textClauses.Where(c => c.DoesNotContain).All(c => !t.title.Contains(c.Text) )
             && dateClauses.Where(c => c.GreaterThan).All(c => t.date > c.Date) )
             && dateClauses.Where(c => c.LesserThan).All(c => t.date < c.Date) )
          ).Select(t =>  new {
               title = t.title
               date = t.date
      });

The main thing here is that each of your current foreach loops can be refactored to use .All() instead. Or, rather, each if condition inside the loop can be. If you really want to you can still break out each of the where's:

var query = context.Table.Where(t => t.Enabled).Select(t =>  new {
               title = t.title
               date = t.date
      });

query = query.Where(t => textClauses.Where(c => c.Contains).All(c => t.title.Contains(c.Text) );
query = query.Where(t => textClauses.Where(c => c.DoesNotContain).All(c => !t.title.Contains(c.Text) );
query = query.Where(t => dateClauses.Where(c => c.GreaterThan).All(c => t.date > c.Date) );
query = query.Where(t => dateClauses.Where(c => c.LesserThan).All(c => t.date < c.Date) );

Upvotes: 2

nitzmahone
nitzmahone

Reputation: 13940

I've done this before with no issues- perhaps you should try moving the .Select to the end (after the Wheres are done). Also, I use PredicateBuilder heavily for added flexibility with dynamic query stuff- right now you're getting "and" semantics for your appended wheres, where I'd guess you want "or" semantics for at least some of them. PredicateBuilder lets you do that easily (and it's free!).

Upvotes: 0

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