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.

1. Setup the base query in the 'SetupArticleQuery()' method which saves the query to 'this.articles'.
2. 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;

            }
        }
}

Answer 1

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

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