Why is my delimiter not appearing in the final output? It's initialized to be a comma, but I only get ~5 white spaces between each attribute using:

  SELECT [article_id]
         , dbo.GROUP_CONCAT(0, t.tag_name, ',') AS col
    FROM [AdventureWorks].[dbo].[ARTICLE_TAG_XREF] atx
    JOIN [AdventureWorks].[dbo].[TAGS] t ON t.tag_id = atx.tag_id
GROUP BY article_id

The bit for DISTINCT works fine, but it operates within the Accumulate scope...

Output:

article_id  |  col
-------------------------------------------------
1           |  a         a         b         c         

I only have rudimentary C# API knowledge...

C# Code:

using System;
using System.Data;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;
using System.Xml.Serialization;
using System.Xml;
using System.IO;
using System.Collections;
using System.Text;

[Serializable]
[SqlUserDefinedAggregate(Format.UserDefined, MaxByteSize = 8000)]
public struct GROUP_CONCAT : IBinarySerialize
{
    ArrayList list;
    string delimiter;

    public void Init()
    {
        list = new ArrayList();
        delimiter = ",";
    }

    public void Accumulate(SqlBoolean isDistinct, SqlString Value, SqlString separator)
    {
        delimiter = (separator.IsNull) ? "," : separator.Value ;

        if (!Value.IsNull)
        {
            if (isDistinct)
            {
                if (!list.Contains(Value.Value))
                {
                    list.Add(Value.Value);
                }
            }
            else
            {
                list.Add(Value.Value);
            }            
        }
    }

    public void Merge(GROUP_CONCAT Group)
    {
        list.AddRange(Group.list);
    }

    public SqlString Terminate()
    {
        string[] strings = new string[list.Count];

        for (int i = 0; i < list.Count; i++)
        {
            strings[i] = list[i].ToString();
        }

        return new SqlString(string.Join(delimiter, strings));
    }

    #region IBinarySerialize Members

    public void Read(BinaryReader r)
    {
        int itemCount = r.ReadInt32();
        list = new ArrayList(itemCount);

        for (int i = 0; i < itemCount; i++)
        {
            this.list.Add(r.ReadString());
        }
    }

    public void Write(BinaryWriter w)
    {
        w.Write(list.Count);
        foreach (string s in list)
        {
            w.Write(s);
        }
    }
    #endregion
}