视图中的MVVM WPF值为null

Question

我仍然是WPF的新手，并试图掌握MVVM模式。我有mainWindow.xaml文件，有2个datepickers

        <DatePicker Padding="5,5,5,5"
                    Text="{Binding Path=dateOptObj.openDate, Mode=TwoWay}"/>
        <TextBlock  Padding="5,5,5,5" Text=" to "/>
        <DatePicker Padding="5,5,5,5" 
                    Text="{Binding Path=dateOptObj.closeDate, Mode=TwoWay}"/>

当程序加载我的日期时，obj具有基于这些日期选择器的有效数据并加载匹配这两个日期的网格：

dateOptObj = new mdDateOptions();

现在这是加载该对象的实际类。

private ICollectionView _datOpts;
    public ObservableCollection<string> dates { get; private set; }
    public ICollectionView datesView
    {
        get
        {
            return _datOpts;
        }
    }
    public mdDateOptions()
    { 

        dates = new ObservableCollection<string>();
        dates.Add("Rolling Year");
        dates.Add("Year to Date");
        dates.Add("Last Year");

        _datOpts = CollectionViewSource.GetDefaultView(dates);
        _datOpts.CurrentChanged += new EventHandler(OnCurrentChanged);
    }

    public void OnCurrentChanged(object sender, EventArgs e)
    {
        string currSel = (string)_datOpts.CurrentItem;
        DateTime p_date = new DateTime();
        p_date = DateTime.Today;

        switch (currSel)
        {
            case "Rolling Year":
                openDate = DateTime.Today.AddYears(-1);
                closeDate = DateTime.Today;

                openDatePrevious = DateTime.Today.AddYears(-2);
                closeDatePrevious = DateTime.Today.AddYears(-1);

                break;
            case "Year to Date":
                openDate = new DateTime(DateTime.Today.Year, 1, 1);
                closeDate = DateTime.Today;

                openDatePrevious = new DateTime(DateTime.Today.AddYears(-1).Year, 1, 1);
                closeDatePrevious = DateTime.Today.AddYears(-1);
                break;
            case "Last Year":
                openDate = new DateTime(DateTime.Today.AddYears(-1).Year, 1, 1);
                closeDate = new DateTime(DateTime.Today.AddYears(-1).Year, 12, 31);

                openDatePrevious = new DateTime(DateTime.Today.AddYears(-2).Year, 1, 1);
                closeDatePrevious = new DateTime(DateTime.Today.AddYears(-2).Year, 12, 31);
                break;
            default:
                MessageBox.Show("Unknown Date Option");
                break;
        }
    }

我的问题是我在mainWindow.xaml上有一个userControl，它有一个按钮列表，用于在mainWindow.xaml上加载网格。当你点击userControl中的一个按钮时，我希望它根据日期选择器筛选出网格中的某些数据，但是当我的relay命令在viewmodel中触发时，上面的dateOptObj为null。谁能指出我正确的方向？以下是我加载UserControl的方法。

<vm:StatPanel DockPanel.Dock="Right" Loaded="StatPanel_Loaded" />

下面是我的UserControl面板，其中包含我想在MainWindow.xaml的主网格上过滤的按钮网格。

<DataGridTemplateColumn Header="Security">
                        <DataGridTemplateColumn.CellTemplate>
                            <DataTemplate>
                                <Button Name="Security" Content="{Binding Path=totalSecurities}" Command="{Binding Source={StaticResource viewModel}, Path=filterGridCommand}">
                                    <Button.CommandParameter>
                                        <MultiBinding Converter="{StaticResource PassThroughConverter}">
                                            <Binding Path="sector"/>
                                            <Binding ElementName="Security" Path="Name"/>
                                        </MultiBinding>
                                    </Button.CommandParameter>

                                </Button>
                            </DataTemplate>
                        </DataGridTemplateColumn.CellTemplate>
                    </DataGridTemplateColumn>

这是我的filterGridCommand

public RelayCommand filterGridCommand
    {
        get
        {
            if (_filterGridCommand == null)
            {
                _filterGridCommand = new RelayCommand(filterGrid);
            }

            return _filterGridCommand;
        }
    }
    private void filterGrid(object param)
    {
        var values = (object[])param;
        string sector = (string)values[0];
        string type = (string)values[1];


        EmployeeObj empObj = (EmployeeObj)_repsView.CurrentItem;
        dash.reFillLogNewAccounts(dateOptObj.openDate, dateOptObj.closeDate, empObj.RRNumber,sector,type);
    }