Sorting and Paging a GridView

Sorting can be tricky. But maintaining the sort order on paging is trickier
Here is a sample code for that

DataBase used is Northwind.
Table used is Products.

The ASPX

<asp:GridView ID="GridView1" runat="server"
                AllowPaging="true"
                AllowSorting="true"
                AutoGenerateColumns="false"
                DataKeyNames="ProductID"
                PageSize="10"
                OnPageIndexChanging="GridView1_PageIndexChanging"
                OnSorting="GridView1_Sorting">
    <Columns>
        <asp:TemplateField HeaderText="ID" SortExpression="ProductID">
            <ItemTemplate>
                <asp:Label ID="lblProductID" runat="server"
                            Text='<%#Eval("ProductID") %>'>
                </asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Name" SortExpression="ProductName">
            <ItemTemplate>
                <asp:Label ID="lblProductName" runat="server"
                            Text='<%#Eval("ProductName") %>'>
                </asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Stock" SortExpression="UnitsInStock">
            <ItemTemplate>
                <asp:Label ID="lblUnitsInStock" runat="server"
                            Text='<%#Eval("UnitsInStock") %>'>
                </asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView>

The ASPX.CS

private const string ASCENDING = " ASC";
private const string DESCENDING = " DESC";
static private DataView dvProducts;

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        dvProducts = new DataView(GetProductsFromDataTable());
        BindGridView();
    }
}

protected void GridView1_Sorting(object sender, GridViewSortEventArgs e)
{
    string sortExpression = e.SortExpression;

    if (GridViewSortDirection == SortDirection.Ascending)
    {
        GridViewSortDirection = SortDirection.Descending;
        dvProducts.Sort = sortExpression + DESCENDING;
        BindGridView();
    }
    else
    {
        GridViewSortDirection = SortDirection.Ascending;
        dvProducts.Sort = sortExpression + ASCENDING;
        BindGridView();
    }   
}

protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
{
    GridView1.PageIndex = e.NewPageIndex;
    BindGridView();
}

private void BindGridView()
{
    GridView1.DataSource = dvProducts;
    GridView1.DataBind();
}

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

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

private DataTable GetProductsFromDataTable()
{
    DataTable dtProductsTemp = new DataTable();
    using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["NorthwindConnectionString"].ConnectionString))
    {
        using (SqlDataAdapter adapProducts = new SqlDataAdapter("Select * from Products", conn))
        {
            adapProducts.Fill(dtProductsTemp);
        }
    }
    return dtProductsTemp;
}

Extra namespace used

using System.Data.SqlClient;

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