Lucene facet computed field

IndexType

namespace MyProject
{
    public class IndexType : IComputedIndexField
    {
        /// <inheritdoc />
        public string FieldName { get; set; }
        /// <inheritdoc />
        public string ReturnType { get; set; }

        /// <inheritdoc />
        public object ComputeFieldValue(IIndexable indexable)
        {
            Item item = indexable as SitecoreIndexableItem;

            if (item != null) 
            {
                try
                {
                    if (item.Paths.IsMediaItem)
                    {
                        return "Doc";
                    }
                    else
                    {
                        return "Page";
                    }
                }
                catch (Exception)
                {
                    return null;
                }
            }
           
            return null; // Return null if nothing to index
        }
    }
}

XML

<!-- Type for faceting-->
  <field fieldName="typefacet" storageType="yes" indexType="untokenized"
  patch:after="field[last()]">MyProject.IndexType, MyProject</field>