自定义sql server 聚合涵数

using System;
using System.Data;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;
using System.Text;
using System.Collections.Generic;

[Serializable]
[Microsoft.SqlServer.Server.SqlUserDefinedAggregate(Format.UserDefined,
    IsInvariantToDuplicates = false,
    IsInvariantToNulls = true,
    IsInvariantToOrder = false,
    IsNullIfEmpty = true,
    MaxByteSize = 8000
)]

public struct SumPart : IBinarySerialize 
{
    private StringBuilder _reuslt;
    //private List<string> _li ;

    public void Init()
    {
        // 在此处放置代码
        _reuslt = new StringBuilder();
        //_li = new List<string>(); 
    }

    public void Accumulate(SqlString Value)
    {
        // 在此处放置代码
        if (Value.IsNull)
        {
            return;
        }
        else
        {
            //_li.Add(Value.Value);
            if (_reuslt.Length > 0)
            {
                _reuslt.Append("$");
            }
            _reuslt.Append(Value.Value);

        }
    }

    public void Merge(SumPart Group)
    {
        // 在此处放置代码
        _reuslt.Append(Group._reuslt);
      
    }

    public SqlString Terminate()
    {
        // 在此处放置代码
        //if(li.Count>0)  为什么li老是==null???? 

        //if (_li == null)
        //{
        //    return new SqlString("");
        //}

        if (_reuslt.Length > 0)
        {
            string strReturn = "";
            string[] arrayStr = _reuslt.ToString().Split('$');
            List<string> li = new List<string>(); 
            for (int i = 0; i < arrayStr.Length; i++)
            {
                li.Add(arrayStr[i]);
            }
            li.Sort();
            foreach (string item in li)
            {
                strReturn += "$"+item ;
            }
            return new SqlString(strReturn);
            //return new SqlString(_reuslt.ToString());

        }
       
        return new SqlString("");
    }

    // 这是占位符成员字段
    private int var1;


    #region IBinarySerialize 成员

    public void Read(System.IO.BinaryReader r)
    {
        _reuslt = new StringBuilder(r.ReadString());
    }

    public void Write(System.IO.BinaryWriter w)
    {
        w.Write(_reuslt.ToString()); 
    }

    #endregion
}