sql异步查询问题

那么,为什么这不会成为回调函数呢？

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;

namespace sqlAsyncTesting {
    public partial class Form1 : Form {

    public Form1() {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e) {
        using (SqlConnection conn = new SqlConnection(@"Data Source = bigapple; Initial Catalog = master; Integrated Security = SSPI; Asynchronous Processing = true;")) {
            conn.Open();
            SqlCommand cmd = new SqlCommand(@"WAITFOR DELAY '00:03'; Select top 3 * from sysobjects;", conn);
            IAsyncResult result = cmd.BeginExecuteReader(new AsyncCallback(HandleCallback), cmd, CommandBehavior.CloseConnection);
        }
    }

    private void HandleCallback(IAsyncResult result) {
        SqlDataReader dr;
        SqlCommand _this = (SqlCommand)result.AsyncState;

        if (result.IsCompleted) {
            dr = _this.EndExecuteReader(result);
        } else dr = null;

        DataTable dt = new DataTable();
        DataSet ds = new DataSet();

        dt.Load(dr);
        ds.Tables.Add(dt);
        dr.Close();
        Complete(ds);
    }

    private void Complete(DataSet ds) {
        string output = string.Empty;
        foreach (DataColumn c in ds.Tables[0].Columns) {
            output += c.ColumnName + "\t";
        }
        output += "\r\n";
        foreach (DataRow dr in ds.Tables[0].Rows) {
            foreach (object i in dr.ItemArray) {
                output += i.ToString() + "\t";
            }
            output += "\r\n";
        }
    }
}

}