sorting on searched records not working

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C#

ASP.NET

Gridview

I am having problem in sorting the searched records in a gridview to ascending and descending directions. I have applied the following code to normal gridview (all records) and it is working all fine but when I try to search any records from all records, and try to sort those searched records, following code neither performs any actions nor throws any errors: Following code is applied for sorting:

private const string ASCENDING = " ASC";
private const string DESCENDING = " DESC";

public SortDirection GridViewSortDirection
{
get
{
    if (ViewState["sortDirection"] == null)
        ViewState["sortDirection"] = SortDirection.Ascending;

    return (SortDirection)ViewState["sortDirection"];
}
set { ViewState["sortDirection"] = value; }
}

protected void GridView2_Sorting(object sender, GridViewSortEventArgs e)
{
string sortExpression = e.SortExpression;

if (GridViewSortDirection == SortDirection.Ascending)
{
    GridViewSortDirection = SortDirection.Descending;
    SortGridView1(sortExpression, DESCENDING);
}
else
{
    GridViewSortDirection = SortDirection.Ascending;
    SortGridView1(sortExpression, ASCENDING);
}
}

private void SortGridView1(string sortExpression, string direction)
{

DataTable dt = SearchTable().Tables[0];

DataView dv = new DataView(dt);
dv.Sort = sortExpression + direction;

GridView2.DataSource = dv;
GridView2.DataBind();

 }

Search function is :

public DataSet SearchTable()
{

    string sql1 = "SELECT * from dbo.Documents1";

    bool flag = false;

    if (!txtRef.Text.Equals(""))
    {
        if (flag == false)
        {
            sql1 = sql1 + " where Ref LIKE N'%" + txtRef.Text + "%'";
            flag = true;

        }
        else
        {
            sql1 = sql1 + "  and Ref LIKE N'%" + txtRef.Text + "%'";
        }
    }

    if (!txtSubject.Text.Equals(""))
    {
        if (flag == false)
        {
            sql1 = sql1 + " where Subject LIKE N'%" + txtSubject.Text + "%'";
            flag = true;

        }
        else
        {
            sql1 = sql1 + "  and Subject LIKE N'%" + txtSubject.Text + "%'";
        }
    }


    using (SqlConnection con = new SqlConnection("Data Source=MEHDI-PC\\SQLEXPRESS;Initial Catalog=PIMS;Integrated Security=True"))
    {
        using (SqlCommand cmd = new SqlCommand())
        {

            cmd.Connection = con;
            cmd.CommandText = sql1 + ";";
            //cmd.CommandType = CommandType.StoredProcedure;
            con.Open();
            //dataset object to get all select statement results
            DataSet ds = new DataSet();

            //sql dataadoptor to fill dataset
            using (SqlDataAdapter adp = new SqlDataAdapter(cmd))
            {
                adp.Fill(ds);
            }
            if (con.State == ConnectionState.Open)
            {
                con.Close();
            }

            return ds;

        }
    }
}

I cant understand where I am going wrong. when clicked on a column to sort, it took me out of searched records and sorted "All-Records" which are placed in GridView1 while searched-records are placed in GridView2 as shown in the code. what I am failed to understand is, why programs jumps to GridView1 and sort all-records there when I click to click to sort searched-records in GridView2. Any help will be much appreciated. Thanks in advance.