Question

I would like to display a grouped ListBox like this :

• Tag A
  • Post 1
  • Post 2
• Tag B
  • Post 2

Source collection is a list of Post, each post can have many Tags.
If user selects “Post 2”, it get highlighted twice.

I tried using an ICollectionView, but it need a Single value to filter, not multivalued one.

Answer 1

Assuming that this is your structure…

public class Post
{
    public string Name { get; set; }
    public string[] Tags { get; set; }
}

The only way you can achieve tags based grouping is by flattening it out

var PostTagList = myPosts.SelectMany(
                 p => p.Tags,
                      (owner, tag) => new {owner, tag}).Select(
                         ot => new Post() 
                         {
                             Name = ot.owner.Name,
                             Tags = new string[] { ot.tag }
                         }).ToList();

This way you will have a flatten hierarchy of repeating Post objects with each of their Tags (i.e. Post.Tags property will have only one individual Tag for that Post).

After this, the grouping will apply just fine.