Sorting In GridView ASP.Net

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

ViewState["SortExpression"] = sortExpression;

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

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