ASP.NET

protected void ListBox1_SelectedIndexChanged(object sender, EventArgs e)<br />
{<br />
   count = 0;<br />
}

int presentCount = 0;
protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
   YourType  obj =  e.Row.DataItem as YourType;
  if ( obj.Property = "P")
	presentCount++;
}
else if (e.Row.RowType == DataControlRowType.Footer)
{
e.Row.Cells[3].Text = presentCount.ToString();
}
}