提示字符串格式不正确
 private void cmb2_SelectedIndexChanged(object sender, EventArgs e)
        {
            string id = cmb2.SelectedValue.ToString();
            foreach(Friend f in friends)
            {
                if (f.iID ==Convert.ToInt32( id))//这里有一个问题 说字符串格式不正确!!!
                {
                    txtid.Text = f.iID.ToString();
                    txtname.Text = f.sName;
                    txttel.Text = f.sTel;
                    txtmob.Text = f.sMob;
                    txtdanwei.Text = f.sDanwei;
                    txtadress.Text = f.sAdress;
                    txtqq.Text = f.sQQ;
                    txtemail.Text = f.sEmail;

                     
                }
            }
        }
------解决方案--------------------
如果你单单要cmb中的序号的话改成:
string id = cmb2.SelectedIndex.ToString();