Better way to group elements?

Question

this.Items is a list of products. Each product has a category (product.Attributes["category"]).

I want to render a drop down list, where all the products are grouped by category. This works well, but how could I improve it? Could there be a way to not use a dictionary?

protected override void RenderContents(System.Web.UI.HtmlTextWriter writer)
{
 var groups = new Dictionary<string, List<ListItem>>();
 // sort by category
 foreach (ListItem product in this.Items)
 {
 if (!groups.ContainsKey(category))
 {
 groups.Add(category, new List<ListItem>());
 }
 groups[category].Add(product);
 }
 // render each category
 foreach (var group in groups)
 {
 // <optgroup>
 RenderOptionGroupBeginTag(group.Key, writer);
 // each option 
 group.Value.ForEach(p => this.RenderListItem(p, writer));
 // </optgroup>
 RenderOptionGroupEndTag(writer);
 }
}

Answer 1

Since ListItemCollection implements IEnumerable, but not IEnumerable<ListItem>, you have to use Cast before you can use other LINQ methods.

After you do that, you can use GroupBy, as Steve Michael suggested and then work with the result almost exactly like with your groups:

var categoryGroups = this.Items.Cast<ListItem>()
 .GroupBy(i => i.Attributes["category"]);
foreach (var categoryGroup in categoryGroups)
{
 RenderOptionGroupBeginTag(group.Key, writer);
 foreach (var item in group)
 {
 this.RenderListItem(item, writer);
 }
 RenderOptionGroupEndTag(writer);
}

Some other changes I made to the code:

• I used foreach instead of ForEach(). ForEach() looks like a LINQ method, but it behaves very differently, since it doesn't do any querying. I think that in cases like this, foreach in the most suitable option.
• Changing groups (and group) to categoryGroups means that the code is clearer and so you can get rid of the loop comment.
• I also removed the other comments, because I think the code is clear about what it does. If you want to emphasize that RenderOptionGroupBeginTag creates <optgroup>, maybe you should rename it? Probably to something like RenderOptgroupBeginTag.

Answer 2

You could do something like this:

var groupedList = this.Items.GroupBy(i => i.Attributes["category"])
 .Select(g =>new { Name = g.Key, Items = g.ToList()})
 .ToList();

Is that along the lines of what you're looking for?

• user3744187: It could be but this won't compile, System.Web.UI.WebControls.ListItemCollection does not contain a definition for GroupBy...
• Jedidja: You probably need to add 'using System.Linq;' at the top of your class. ListItemCollection implements IList and ICollection so it should work. But I'm not near Visual Studio right now.
• Steve Michael: Yeah it's probably just because of the using statement. Also if you use ReSharper it makes it very clear what is missing and allows you to add the reference easily. Check it out! Makes life easier :)
• user3744187: I added using System.Linq; and I already use Resharper (which is indeed awesome :) ) but compilation still fails...
• svick: @Jedidja ListItemCollection implements only the non-generic versions of those interfaces, so it won't work with LINQ like this.