下拉组合框不处理选择

我有一个Silverlight WPF应用程序，可以显示数据的选项卡在这种情况下，我有一个新的状态下拉菜单，下面是下拉菜单的样子:

屏幕截图http://www.mediafire.com/download.php?vkwvj6qq6q6zjoz

处理这个的代码在这里:它没有这样做。我们希望用户能够根据组合框中的状态过滤详细视图。知道为什么它不显示任何东西吗?我尝试唯一的状态，我们正在显示"未开始-迟到"，它显示一个空白的屏幕。

private String _Type;
private String AssessmentType
{
     get { return _Type; }
     set { if (this.SetReferenceProperty("AssessmentType", ref _Type, value)) 
                 { RefreshData(); } 
         }
 }

评估类型定义:

private readonly String[] _assessmentType = new String[]   
       { "All", "UnCompleted", "Completed", "Incomplete Late", 
         "NOT STARTED - LATE", "Submitted", "Submitted Late" };
RibbonControlHelper.CreateComboBox("Assessment", "Assessment", 
       "Select Assessment to show.", _assessmentType, (type) => 
            { AssessmentType = type; }))
protected override void RefreshData()
{
        if (FacilitiesAreChanging) { return; }
        Loading = true;
        SchedulesRepository.Details(FacilitySelectionService.SelectedFacilities, 
                                 UnitCode, AssessmentType, StartDate, EndDate,
        (schedules) =>
        {
            var data = new ListCollectionView(schedules);
            data.GroupDescriptions.Add(new PropertyGroupDescription
                                                             ("FACILITY_KEY"));
            data.GroupDescriptions.Add(new PropertyGroupDescription
                                                             ("UNIT"));
            Data = data;
            Loading = false;
        });
 }