Question

My Linq2Sql statements gaves me what I what I required. Below is my method returning anonymous list of category and all its related subcategories:

public IQueryable GetAllCategoriesAndSubcategories()
{
    return from p in _context.Categories
    let relatedchilds = from c in _context.SubCategories
                        where c.CategoryId == p.Id
                        select c
                        select new
                               {
                                  p,
                                  relatedchilds
                               };
}

In my code behind i am using this method to fetch category and all its subcategory:

 private void WriteCategories()
        {
            var repository = new CategoryRepository();
            var dt = repository.GetAllCategoriesAndSubcategories();
            var sb = new StringBuilder();
            sb.AppendLine(" <div class='widget_box' id='category'>");
            sb.AppendLine("     <div class='wintitle'>");
            sb.AppendLine("         <div class='inner_wintitle'>Categories</div>");
            sb.AppendLine("     </div>");
            sb.AppendLine("     <div class='winbody'>");
            sb.AppendLine("         <ul class='categories'>");
            int i = 1;
            foreach (Category category in dt.OfType<Category>())
            {
                sb.AppendLine(
                 string.Format("<li class='catetitle' id='catetitle{0}'><a href='/category/{1}/{2}'>{3}</a></li>", i,
                                 category.Id, Common.ReplaceUnwantedChars(category.Name), category.Name));
                sb.AppendLine("<li style='display:none;' class='category_sub' ><div><ul>");
                foreach (var subCategory in dt.OfType<SubCategory>())
                {
                    sb.AppendLine(string.Format("<li><a href='category/{0}/{1}/{2}'>{3}</a></li>", category.Id,
                                                subCategory.Id, Common.ReplaceUnwantedChars(subCategory.Name),
                                                subCategory.Name));

                }
                i++;
            }
            sb.AppendLine("</div></ul></div>");
            ltlCategories.Text = sb.ToString();
        }

Adding watch I got the stuff give below:

[0] { p = {InnovativeTechnosoft.BusinessBazaar.Web.UI.Core.Category}, relatedchilds = {System.Collections.Generic.List`1[InnovativeTechnosoft.BusinessBazaar.Web.UI.Core.SubCategory]} }    <Anonymous Type>

Requirement : From code itself its clear that I need to iterate through the category and its subcategory. I am trouble and condition check where I am using dt.OfType<Category>(). If I use simply foreach(Category c in dt), it gives me casting exception.

Please help. Where and what I am doing wrong.

Answer 1

What your method GetAllCategoriesAndSubcategories() returns is an IQueryable of an anonymous type – instances of this type are not Category objects so the cast will always fail, and OfType<Category>() will simply return an empty collection.

Instead projecting to an anonymous type use a helper class that will allow you to use it later on, i.e.:

public class CategoryWithSubcategories
{
   public Category SomeCategory {get;set;}
   public IEnumerable<SubCategory> RelatedSubCategories {get;set;}
}

Then change the method signature of GetAllCategoriesAndSubcategories to:

public IQueryable<CategoryWithSubcategories> GetAllCategoriesAndSubcategories()
{
    return from p in _context.Categories
    let relatedchilds = from c in _context.SubCategories
                        where c.CategoryId == p.Id
                        select c
                        select new CategoryWithSubcategories
                               {
                                  SomeCategory = p,
                                  RelatedSubCategories = relatedchilds
                               };
}

Now you can query the returned enumeration like:

foreach (CategoryWithSubcategories category in dt)
{
   //your code here
   foreach (var subCategory in category.RelatedSubCategories)
   {
     //more code here
   }
}