var wardList = (from ward in alarmInfoList
                            select new
                            {
                                ward.AlarmJQ,
                                ward.AlarmJQName
                            }).Distinct();


            List<WardInfo> wiList = new List<WardInfo>();
            foreach (var ward in wardList)
            {
                WardInfo wi = new WardInfo();
                wi.AlarmJQ = ward.AlarmJQ;
                wi.AlarmJQName = ward.AlarmJQName;
                wiList.Add(wi);
                
            }
             cboWard.DataSource = wiList;
             cboWard.ValueMember = "AlarmJQ";
             cboWard.DisplayMember = "AlarmJQName";
 

 private void cboWard_SelectedIndexChanged(object sender, EventArgs e)
        {            
            var departmentList = (from department in alarmInfoList
                                  where department.AlarmJQ == cboWard.SelectedValue.ToString()
                            select new
                            {
                                department.AlarmArea,
                                department.AlarmAreaName
                            }).Distinct();

            List<DepartmentInfo> diList = new List<DepartmentInfo>();
            DepartmentInfo diAll = new DepartmentInfo();
            diAll.AlarmArea = -1;
            diAll.AlarmAreaName = "全部";
            diList.Add(diAll);

            foreach (var department in departmentList)
            {
                DepartmentInfo di = new DepartmentInfo();
                di.AlarmArea = department.AlarmArea;
                di.AlarmAreaName = department.AlarmAreaName;
                diList.Add(di);
            }
            cboDepartment.DataSource = diList;
            cboDepartment.DisplayMember = "AlarmAreaName";
            cboDepartment.ValueMember = "AlarmArea";
        }
 

List<WardInfo> wiList = new List<WardInfo>();

裡面的WardInfo物件，而不是想要的

cboWard.ValueMember = "AlarmJQ";

解決方式：

            List<WardInfo> wiList = new List<WardInfo>();
            foreach (var ward in wardList)
            {
                WardInfo wi = new WardInfo();
                wi.AlarmJQ = ward.AlarmJQ;
                wi.AlarmJQName = ward.AlarmJQName;
                wiList.Add(wi);
                
            }
            //去除委託
            cboWard.SelectedIndexChanged -= new EventHandler(cboWard_SelectedIndexChanged);
            cboWard.DataSource = wiList;
            cboWard.ValueMember = "AlarmJQ";
            cboWard.SelectedIndexChanged += new EventHandler(cboWard_SelectedIndexChanged);
            cboWard.DisplayMember = "AlarmJQName";
 

            cboWard.SelectedIndexChanged += new EventHandler(cboWard_SelectedIndexChanged);
            cboWard.DisplayMember = "AlarmJQName";

如有什麼問題，或者更好的方式，希望大家指正！