DataTable 排序问题

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;

namespace MdyTest
{
public partial class WebForm1 : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
DataTable dt = GetDataTable();
ViewState["DataTable"] = dt;
GridView1.DataSource = dt;
GridView1.DataBind();
}
}

private DataTable GetDataTable()
{
DataTable dt = new DataTable();
dt.Columns.Add("A",typeof(int));
for (int i = 1; i <= 50; i++)
{
DataRow dr = dt.NewRow();
dr["A"] = i;
dt.Rows.Add(dr);
}
return dt;
}

protected void btnRandom_Click(object sender, EventArgs e)
{
DataTable dt = ViewState["DataTable"] as DataTable;
ViewState["DataTable"] = GetRandomDataTable(dt, "A");
GridView1.DataSource = ViewState["DataTable"] as DataTable;
GridView1.DataBind();
}

#region 生成随机的DataTable
/// <summary>
/// 生成随机的DataTable
/// </summary>
/// <param name="dt"></param>
/// <param name="columnName"></param>
/// <returns></returns>
private DataTable GetRandomDataTable(DataTable dt,string columnName)
{
List<int> list = GetRandomList(dt.Rows.Count);
if (columnName == "A")
{
for (int i = 0; i < list.Count; i++)
{
dt.Rows[i]["A"] = list[i];
}
}
else if (columnName == "B")
{
for (int i = 0; i < list.Count; i++)
{
dt.Rows[i]["B"] = list[i];
}
}
return dt;
}
#endregion

protected void btnAddColumn_Click(object sender, EventArgs e)
{
DataTable dt = ViewState["DataTable"] as DataTable;
dt.Columns.Add("B",typeof(int));
ViewState["DataTable"] = GetRandomDataTable(dt, "B");
GridView1.DataSource = ViewState["DataTable"] as DataTable;
GridView1.DataBind();
}

protected void btnDesc_Click(object sender, EventArgs e)
{
DataTable dt = ViewState["DataTable"] as DataTable;
DataTable newDt = new DataTable();
newDt = dt.Clone();//拷贝表dt结构到新表newDt
for (int i = 0; i < dt.Rows.Count; i++)
{
newDt.ImportRow(dt.Rows[i]);
}
newDt.DefaultView.Sort = "A desc,B desc";
GridView1.DataSource = newDt;
GridView1.DataBind();
}

#region 生成不重复的随机List集合
/// <summary>
/// 生成不重复的随机List集合
/// </summary>
/// <param name="codeLength"></param>
/// <returns></returns>
private List<int> GetRandomList(int codeLength)
{
List<int> randList = new List<int>();
int i = 0;
Random random = new Random();
while (i < codeLength)
{
int randomNumber = random.Next(1, codeLength+1);
if (!randList.Contains(randomNumber))
{
i++;
randList.Add(randomNumber);
}
}
return randList;
}
#endregion

}
}


newDt.DefaultView.Sort = "A desc,B desc";
最后B列数据并没有降序排列,这是为什么