無法訪問WPF中其他ViewModel類的成員

Question

我正在研究在我的應用程序中動態生成的單選按鈕集。 訪問班級成員時，我似乎面臨一個問題。 這是我到目前為止所做的：

XAML：

<GroupBox Header="Daughter Cards" Height="Auto" HorizontalAlignment="Stretch" Margin="20,5,20,20" Name="groupBox2" VerticalAlignment="Stretch" Width="Auto">
            <Grid>                   
                <Grid Grid.Column="0">
                    <ItemsControl ItemsSource="{Binding SlotChildren}">
                        <ItemsControl.ItemsPanel>
                            <ItemsPanelTemplate>
                                <UniformGrid Columns="3" Rows="8" />
                            </ItemsPanelTemplate>
                        </ItemsControl.ItemsPanel>

                        <ItemsControl.ItemTemplate>
                            <DataTemplate>
                                <RadioButton Content="{Binding SlotButtons}" Margin="0,10,0,0" IsChecked="{Binding IsChecked}" GroupName="SlotGroup" Height="15" Width="80" HorizontalAlignment="Center" VerticalAlignment="Center"/>
                            </DataTemplate>
                        </ItemsControl.ItemTemplate>
                    </ItemsControl>
                </Grid>                

                <Grid Grid.Column="1">                                                  
                        <ComboBox Grid.Row="0" ItemsSource="{Binding DaughterBoardBoxList}" SelectedItem="{Binding SelectedDaughterBoardBoxList, Mode=TwoWay}" SelectedIndex="0" Height="23" HorizontalAlignment="Center" Margin="0" Name="comboBox5" VerticalAlignment="Center" Width="158" />
                        <ComboBox Grid.Row="1" ItemsSource="{Binding DaughterVersionBoxList}" SelectedItem="{Binding SelectedDaughterVersionBoxList, Mode=TwoWay}" SelectedIndex="0" Height="23" HorizontalAlignment="Center" Margin="0" Name="comboBox6" VerticalAlignment="Center" Width="158" />
                        <ComboBox Grid.Row="2" ItemsSource="{Binding DaughterSerialBoxList}" SelectedItem="{Binding SelectedDaughterSerialBoxList, Mode=TwoWay}" SelectedIndex="0" Height="23" HorizontalAlignment="Center" Margin="0" Name="comboBox7" VerticalAlignment="Center" Width="158" />

                        <Button Grid.Row="1" Command="{Binding GetStringCommand}" Content="Get String" Height="23" HorizontalAlignment="Center" Margin="0" Name="RefreshDaughterCards" VerticalAlignment="Center" Width="90" />
                        <Button Grid.Row="2" Command="{Binding SetStringCommand}" Content="Set String" Height="23" HorizontalAlignment="Center" Margin="0" Name="WriteEEPROMDCBtn" VerticalAlignment="Center" Width="90" />
                        <Label Content="{Binding DaughterStatus}" Height="25" HorizontalAlignment="Center" Margin="0" Name="DaughterCardLabel" VerticalAlignment="Center" Width="170" />
                    </Grid>                                       
                </Grid> 
        </GroupBox>

EEPROMViewModel類：

public ObservableCollection<EEPROMSlotViewModel> SlotChildren { get; set; }

public EEPROMViewModel ()
{
        SlotChildren = new ObservableCollection<EEPROMSlotViewModel>();
        SlotChildren.Add(new EEPROMSlotViewModel() { ParentVM = this, SlotButtons = "0 : None", ID = 0 });
        SlotChildren.Add(new EEPROMSlotViewModel() { ParentVM = this, SlotButtons = "1 : None", ID = 1 });
        SlotChildren.Add(new EEPROMSlotViewModel() { ParentVM = this, SlotButtons = "2 : None", ID = 2 });
        SlotChildren.Add(new EEPROMSlotViewModel() { ParentVM = this, SlotButtons = "3 : None", ID = 3 });
        SlotChildren.Add(new EEPROMSlotViewModel() { ParentVM = this, SlotButtons = "4 : None", ID = 4 });
        SlotChildren.Add(new EEPROMSlotViewModel() { ParentVM = this, SlotButtons = "5 : None", ID = 5 });
        SlotChildren.Add(new EEPROMSlotViewModel() { ParentVM = this, SlotButtons = "6 : None", ID = 6 });
 }

生成7個與每個ID相關的單選按鈕。

EEPROMSlotViewModel類：

private string _SlotButtons;
    public string SlotButtons
    {
        get; set;
    }

    private EEPROMViewModel _parentVm;
    public EEPROMViewModel ParentVM
    {
        get; set;
    }

    private int _ID;
    public int ID
    {
        get; set;
    }

    private bool _isChecked;
    public bool IsChecked
    {
        get; set;
    }

因此，每當我選擇一個SETSTRING按鈕並單擊SETSTRING按鈕時，就會執行以下代碼：

EEPROMSlotViewModel mSlotVM = new EEPROMSlotViewModel();

        string label;
        if (mSlotVM.ID == 0) //Accessing the 1st radiobutton clicked
        {
            label = string.Empty;
            mSlotVM.getShortName(0, label);
            if (label == string.Empty)
            {
                label = "None";
            }

            mSlotVM.SlotButtons = Convert.ToString(0 + ":" + label); // Setting CONTENT of radiobutton selected 
        }

可以說我單擊了第一個單選按鈕，ID應該為0。它調用getShortName()方法，該方法執行以下操作：

ParentVM.SelectedDaughterBoardBoxList = ParentVM.DaughterBoardBoxList[0];
ParentVM.SelectedDaughterVersionBoxList = ParentVM.DaughterVersionBoxList[0];
ParentVM.SelectedDaughterSerialBoxList = ParentVM.DaughterSerialBoxList[0];
shortlabel = "Hello";

我在這里面臨幾個問題：

• mSlotVM是訪問其他類成員/函數的正確方法嗎？
• 一旦控件進入getShortname() ，它將引發如下異常： Object reference not set to an instance of an object. 在ParentVM.DaughterBoardBoxList[0]; 。
• 即使我注釋了getShortName（）中的前3條語句，當getShortName被調用並且控件返回時， label值為“”，我也應該為“ hello”。

我覺得這是mSlotVm異常的原因。 請幫忙 ：）

Answer 1

1. 不，您只需創建EEPROMSlotViewModel的新實例，就不會訪問任何RadioButtons ViewModels。
2. 如果您可以向我們展示您的EEPROMViewModel ，將很有幫助。 我認為問題是您的ParentVM. -列表為空。

3. 要實現所需的功能，您的getShortname() -方法必須類似於：

    public void getShortname(int i, ref string shortlabel) { ParentVM.SelectedDaughterBoardBoxList = ParentVM.DaughterBoardBoxList[0]; ParentVM.SelectedDaughterVersionBoxList = ParentVM.DaughterVersionBoxList[0]; ParentVM.SelectedDaughterSerialBoxList = ParentVM.DaughterSerialBoxList[0]; shortlabel = "Hello"; } 

編輯：通過EEPROMSlotViewModel mSlotVM = new EEPROMSlotViewModel(); 您創建了一個EEPROMSlotViewModel的新實例，但是您沒有得到選中的RadioButtons ViewModel。 因此，此時您調用了mSlotVM.getShortName(0, label); mSlotVM沒有ParentVM，這就是引發異常的原因。 您可以做的是瀏覽SlotChildren-List並獲取IsChecked屬性為true的EEPROMSlotViewModel。

例：

EEPROMSlotViewModel checkedVM;    
string label = string.Empty;
foreach (EEPROMSlotViewModel vm in SlotChildren)
{
    if (vm.IsChecked)
    {
        checkedVM = vm;
    }
    else
    {
        vm.SlotButtons = vm.ID + " : NONE"
    }

} 
checkedVM.getShortName(0, ref label);
if (label == string.Empty)
{
    label = "None";
}
checkedVM.SlotButtons = Convert.ToString(0 + ":" + label); // Setting CONTENT of radiobutton selected