Cannot convert lambda expression to type 'string' because it is not a delegate type

Question

I'm building a page that shows a LINQ query result as a table.

Setup the base query in the 'SetupArticleQuery()' method which saves the query to 'this.articles'.
Run another method, 'UpdateFilter()' to do some filtering on the results that are saved in 'this.articles'.

I get the error

Cannot convert lambda expression to type 'string' because it is not a delegate type

at the line with the code

this.articles = from art in this.articles
                where art.category_id == this.categoryId
                select art;

Any ideas how to fix the code below?

namespace WebApplication3 {
    public partial class _Default : System.Web.UI.Page {
        private IQueryable articles;

        protected void Page_Load(object sender, EventArgs e) {
            this.SetupArticleQuery();
            this.UpdateFilter();
        }

        private void SetupArticleQuery() {
            this.articles = from a in KB.Articles
                            join t in KB.Teams on a.primary_team_id equals t.id
                            join cat in KB.Categories on a.category_id equals cat.id
                            join scat in KB.SubCategories on a.subcategory_id equals scat.id
                            join top in KB.Topics on a.topic_id equals top.id
                            select new {
                                a.id,
                                a.title,
                                a.view_count,
                                a.created_at,
                                a.created_by,
                                a.primary_team_id,
                                primary_team_name = t.name,
                                category_id = cat.id,
                                category_name = cat.name,
                                subcategory_id = scat.id,
                                subcategory_name = scat.name,
                                topic_id = top.id,
                                topic_name = top.name
                            };
        }

        private void UpdateFilter() {
            if (this.categoryId > 0) {
                this.articles = from art in this.articles
                                where art.category_id == this.categoryId
                                select art;

            }
        }
}

I had the same problem becuase I was simply missing my using System.Linq; — markmnl
– markmnl, Commented Jun 25, 2012 at 4:00

Asa Julian · Accepted Answer · 2012-01-27 21:40:30Z

28

I had to add the following to get this error to go away.

using System.Data;
using System.Data.Entity;

answered Jan 27, 2012 at 21:40

Asa Julian

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Comments

Community · Accepted Answer · 2017-05-23 11:53:56Z

13

Actually I haven't found any problem in your code.

From this answer, I would suggest to confirm that you have added:

Using System.Linq;

Good luck!

edited May 23, 2017 at 11:53

CommunityBot

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answered Mar 31, 2011 at 14:54

Homam

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Comments

abatishchev · Accepted Answer · 2011-04-28 05:44:07Z

This query:

this.articles = from a in KB.Articles
                join t in KB.Teams on a.primary_team_id equals t.id
                join cat in KB.Categories on a.category_id equals cat.id
                join scat in KB.SubCategories on a.subcategory_id equals scat.id
                join top in KB.Topics on a.topic_id equals top.id
                select new {
                    a.id,
                    a.title,
                    a.view_count,
                    a.created_at,
                    a.created_by,
                    a.primary_team_id,
                    primary_team_name = t.name,
                    category_id = cat.id,
                    category_name = cat.name,
                    subcategory_id = scat.id,
                    subcategory_name = scat.name,
                    topic_id = top.id,
                    topic_name = top.name
                };

Won't work like this since it returns an anonymous type. As a result, you'd have to type it as var, which is invalid for class-level members; you can only use var for local variables.

What you need to do is create an actual class to hold your projection, and have something like:

...
join top in KB.Topics on a.topic_id equals top.id
select new LocalDTO()
{
    id = a.id,
    ...
};

From there you could just have:

private void UpdateFilter()
{
    if (this.categoryId > 0)
        this.articles = this.articles.Where(a => art.category_id);
}

And of course this.articles would be declares as IQueryable<LocalDTO>.

Josh Anderson · Accepted Answer · 2011-03-31 14:49:53Z

1

If it is a problem with anonymous types, can you do something like this?

private void UpdateFilter() {
    if (this.categoryId > 0) {
        this.articles = this.articles.Where(a => a.category_id == this.categoryId).AsQueryable();
    }
}

This may be off base, since the underlying lambda is the same.

answered Mar 31, 2011 at 14:49

Josh Anderson

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1 Comment

Damiro Over a year ago

I tried that one, too, but I received the same error message.

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