更改页面大小或点击分页后，GridView丢失了搜索结果

我有一个gridview显示数据和文本框做搜索。当我搜索时，gridview可以正确显示数据，但当我点击更改页面大小或转到下一页时，gridview将重新加载到原始状态。有什么与ViewState有关的吗?感谢有人能帮上忙。下面是我的代码:

protected void Page_Load(object sender, EventArgs e)
        {

        }
    protected void ddPageSize_SelectedIndexChanged(object sender, EventArgs e)
    {
        // handle event                 
        DropDownList ddpagesize = sender as DropDownList;
        GridView1.PageSize = Convert.ToInt32(ddpagesize.SelectedItem.Text);
        ViewState["PageSize"] = ddpagesize.SelectedItem.Text;             
        GridView1.DataBind();
    }
    protected void btnSearch_Click(object sender, EventArgs e)
    {
        SqlDataSource1.SelectCommand = "" + txtSearchValue.Text + "'";                   
        GridView1.DataBind();            
    }
    protected void btnReload_Click(object sender, EventArgs e)
    {
        txtSearchValue.Text = "";
        lblSearchError.Text = "";
        GridView1.DataBind();
    }
    protected override void Render(System.Web.UI.HtmlTextWriter writer)
    {
        foreach (GridViewRow row in GridView1.Rows)
        {
            if (row.RowType == DataControlRowType.DataRow)
            {
                row.Attributes["onmouseover"] =
                   "this.style.backgroundColor;this.style.backgroundColor='#AED4EB';";
                row.Attributes["onmouseout"] =
                   "this.style.textDecoration='none';this.style.background='#ffffff';";
                // Set the last parameter to True 
                // to register for event validation. 
                row.Attributes["onclick"] =
                 Page.ClientScript.GetPostBackClientHyperlink(GridView1,
                    "Select$" + row.DataItemIndex, true);
            }
        }
        base.Render(writer);
    }
    protected void btnDate_Click(object sender, EventArgs e)
    {
        SqlDataSource1.SelectCommand = "";
        GridView1.DataBind();
    }
    protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
    {
        try
        {
            int rowindex = GridView1.SelectedIndex % GridView1.PageSize;
            GridViewRow row = GridView1.Rows[rowindex];
            Session["TSO"] = row.Cells[7].Text.Trim();
            string url = "http://localhost:60918/Requests.aspx";
            StringBuilder sb = new StringBuilder();
            sb.AppendLine("<script type='text/javascript'>");
            sb.AppendLine("window.open('" + url + "')");
            sb.AppendLine("<" + "/script>");
            ScriptManager.RegisterStartupScript(upGrdCustomers, upGrdCustomers.GetType(), "myjs", sb.ToString(), false);
        }
        catch (System.Threading.ThreadAbortException)
        {
            throw;
        }                  
    }

这是我的代码为GridView:

[ToolboxData("<{0}:GridView runat=server></{0}:GridView>")]
public class GridView : System.Web.UI.WebControls.GridView, IPageableItemContainer
{
    /// <summary>
    /// TotalRowCountAvailable event key
    /// </summary>
    private static readonly object EventTotalRowCountAvailable = new object();
    /// <summary>
    /// 
    /// </summary>
    /// <param name="dataSource"></param>
    /// <param name="dataBinding"></param>
    /// <returns></returns>
    protected override int CreateChildControls(IEnumerable dataSource, bool dataBinding)
    {
        int rows = base.CreateChildControls(dataSource, dataBinding);
        //  if the paging feature is enabled, determine
        //  the total number of rows in the datasource
        if (this.AllowPaging)
        {
            //  if we are databinding, use the number of rows that were created,
            //  otherwise cast the datasource to an Collection and use that as the count
            int totalRowCount = dataBinding ? rows : ((ICollection)dataSource).Count;
            //  raise the row count available event
            IPageableItemContainer pageableItemContainer = this as IPageableItemContainer;
            this.OnTotalRowCountAvailable(
                new PageEventArgs(
                    pageableItemContainer.StartRowIndex,
                    pageableItemContainer.MaximumRows,
                    totalRowCount
                )
            );
            //  make sure the top and bottom pager rows are not visible
            if (this.TopPagerRow != null)
            {
                this.TopPagerRow.Visible = false;
            }

        }
        return rows;
    }
    #region IPageableItemContainer Interface
    /// <summary>
    /// 
    /// </summary>
    /// <param name="startRowIndex"></param>
    /// <param name="maximumRows"></param>
    /// <param name="databind"></param>
    void IPageableItemContainer.SetPageProperties(
        int startRowIndex, int maximumRows, bool databind)
    {
        int newPageIndex = (startRowIndex / maximumRows);
        this.PageSize = maximumRows;
        if (this.PageIndex != newPageIndex)
        {
            bool isCanceled = false;
            if (databind)
            {
                //  create the event args and raise the event
                GridViewPageEventArgs args = new GridViewPageEventArgs(newPageIndex);
                this.OnPageIndexChanging(args);
                isCanceled = args.Cancel;
                newPageIndex = args.NewPageIndex;
            }
            //  if the event wasn't cancelled
            //  go ahead and change the paging values
            if (!isCanceled)
            {
                this.PageIndex = newPageIndex;
                if (databind)
                {
                    this.OnPageIndexChanged(EventArgs.Empty);
                }
            }
            if (databind)
            {
                this.RequiresDataBinding = true;
            }
        }
    }
    /// <summary>
    /// 
    /// </summary>
    int IPageableItemContainer.StartRowIndex
    {
        get { return this.PageSize * this.PageIndex; }
    }
    /// <summary>
    /// 
    /// </summary>
    int IPageableItemContainer.MaximumRows
    {
        get { return this.PageSize; }
    }
    /// <summary>
    /// 
    /// </summary>
    event EventHandler<PageEventArgs> IPageableItemContainer.TotalRowCountAvailable
    {
        add { base.Events.AddHandler(GridView.EventTotalRowCountAvailable, value); }
        remove { base.Events.RemoveHandler(GridView.EventTotalRowCountAvailable, value); }
    }
    /// <summary>
    /// 
    /// </summary>
    /// <param name="e"></param>
    protected virtual void OnTotalRowCountAvailable(PageEventArgs e)
    {
        EventHandler<PageEventArgs> handler = (EventHandler<PageEventArgs>)base.Events[GridView.EventTotalRowCountAvailable];
        if (handler != null)
        {
            handler(this, e);
        }
    }
    #endregion
}

}