sorting and paging with gridview asp.net

I'm trying to get a gridview to sort and page manually with no success.

The problem is that when a user clicks the column they want to sort, it sorts that page, but doesn't sort the datasource (dataview) behind the gridview. So when they progress to a different page, their sort is lost. Pretty much I'm looking for a sort that will actually sort the datasource behind the gridview. Here is what I have so far:

protected void GridView_OnSort(object sender, GridViewSortEventArgs e)
    {
        String sortExpression = e.SortExpression;

        if (GridViewSortDirection == SortDirection.Ascending)
        {
            DataView myDataView = new DataView(mybll.GetItemsOrdered());
            myDataView.Sort = sortExpression + " DESC";
            GridView.DataSource = myDataView;
            GridView.DataBind();
        }
        else
        {
            DataView myDataView = new DataView(mybll.GetItemsOrdered());
            myDataView.Sort = sortExpression + " ASC";
            GridView.DataSource = myDataView;
            GridView.DataBind();
        }
    }

Any help would be appreciated. Thanks.

Save your sorting order in a ViewState.

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 GridView_Sorting(object sender, GridViewSortEventArgs e)
{
    string sortExpression = e.SortExpression;

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

}

private void SortGridView(string sortExpression,string direction)
{
    //  You can cache the DataTable for improving performance
    DataTable dt = GetData().Tables[0]; 

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

    GridView1.DataSource = dv;
    GridView1.DataBind();         
}

Why you don't want to use existing sorting functionality? You can always customize it.

Sorting Data in a GridView Web Server Control at MSDN

Here is an example with customization:

http://www.netomatix.com/development/GridViewSorting.aspx

<asp:GridView 
    ID="GridView1" runat="server" AutoGenerateColumns="false" AllowSorting="True" onsorting="GridView1_Sorting" EnableViewState="true"> 
    <Columns>
        <asp:BoundField DataField="bookid" HeaderText="BOOK ID"SortExpression="bookid"  />
        <asp:BoundField DataField="bookname" HeaderText="BOOK NAME" />
        <asp:BoundField DataField="writer" HeaderText="WRITER" />
        <asp:BoundField DataField="totalbook" HeaderText="TOTALBOOK" SortExpression="totalbook"  />
        <asp:BoundField DataField="availablebook" HeaderText="AVAILABLE BOOK" />
    </Columns>
</asp:GridView>

Code behind:

protected void Page_Load(object sender, EventArgs e) {
        if (!IsPostBack) {
            string query = "SELECT * FROM book";
            DataTable DT = new DataTable();
            SqlDataAdapter DA = new SqlDataAdapter(query, sqlCon);
            DA.Fill(DT);

            GridView1.DataSource = DT;
            GridView1.DataBind();
        }
    }

    protected void GridView1_Sorting(object sender, GridViewSortEventArgs e) {

        string query = "SELECT * FROM book";
        DataTable DT = new DataTable();
        SqlDataAdapter DA = new SqlDataAdapter(query, sqlCon);
        DA.Fill(DT);

        GridView1.DataSource = DT;
        GridView1.DataBind();

        if (DT != null) {
            DataView dataView = new DataView(DT);
            dataView.Sort = e.SortExpression + " " + ConvertSortDirectionToSql(e.SortDirection);

            GridView1.DataSource = dataView;
            GridView1.DataBind();
        }
    }

    private string GridViewSortDirection {
        get { return ViewState["SortDirection"] as string ?? "DESC"; }
        set { ViewState["SortDirection"] = value; }
    }

    private string ConvertSortDirectionToSql(SortDirection sortDirection) {
        switch (GridViewSortDirection) {
            case "ASC":
                GridViewSortDirection = "DESC";
                break;

            case "DESC":
                GridViewSortDirection = "ASC";
                break;
        }

        return GridViewSortDirection;
    }
}

Tarkus's answer works well. However, I would suggest replacing VIEWSTATE with SESSION.

The current page's VIEWSTATE only works while the current page posts back to itself and is gone once the user is redirected away to another page. SESSION persists the sort order on more than just the current page's post-back. It persists it across the entire duration of the session. This means that the user can surf around to other pages, and when he comes back to the given page, the sort order he last used still remains. This is usually more convenient.

There are other methods, too, such as persisting user profiles.

I recommend this article for a very good explanation of ViewState and how it works with a web page's life cycle: https://msdn.microsoft.com/en-us/library/ms972976.aspx

To understand the difference between VIEWSTATE, SESSION and other ways of persisting variables, I recommend this article: https://msdn.microsoft.com/en-us/library/75x4ha6s.aspx