i have a comboBox with drop list,i want to display the selected item information to the Datagridview

ok man i want to try to help you, as i understand you have combobox and datagridview and you need to bing the selected items from combo to grid....it is easy man:
first you must ckeck the selected or checked items of the combo

List<string> lstString = new List<string>();
        foreach (ListItem lItem in combo.Items)
        {
            if (lItem.Selected)
            {
                lstString.Add(lItem.Value);
            }
        }

in this way you are adding the IDs of the selected items in combo into a list of strings(list of IDs), then you have to get the objects item of these IDs:

List<items> lst = new List<items>();//insted of Items put your class
        Items item = new Items();
        foreach(string s in lstString)
        {
            item = lstItem.FirstOrDefault(p => p.ID == s);
            lst.Add(item);
        }</items></items>

now you must bind the grid , to give it datasource which is the list of items
grid.DataSource = lst;

Note: when you bind you combobox you have to put items in
session[""] in order to keep data alive becuese we need to search via it

best regards