无法访问WPF中其他ViewModel类的成员

Question

I am working on set of radiobuttons which are dynamicaly generated in my app. I seem to be facing a issue when it comes to access the class members. Here is what I have done till now:

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 Class:

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 });
 }

generates 7 radiobuttons with ID related to each.

EEPROMSlotViewModel Class:

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;
    }

thus whenever I select a radiobutton and click SETSTRING button, the following code gets executed:

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 
        }

Lets say I clicked 1st radio button, ID is supposed to be 0. It calls getShortName() method which does the following:

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

I am facing few issues here:

• Is mSlotVM right way to access other class members/function??
• Once control enters getShortname() , it throws the excetion as follows: Object reference not set to an instance of an object. at ParentVM.DaughterBoardBoxList[0]; .
• Even if I comment the first 3 statements in getShortName(), When getShortName gets called and once the control comes back, value of label is "", i should be "hello".

I feel mSlotVm this is the reason behind the exception. Please help :)

Answer 1

1. No, you just create a new instance of the EEPROMSlotViewModel -Class, youre not accessing any of the RadioButtons ViewModels.
2. It would be helpful if you could show us your EEPROMViewModel -Class. I think the problem is, that your ParentVM. -Lists are null.

3. To achieve what you want, your getShortname() -method has to look like:

    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"; } 

EDIT: Through EEPROMSlotViewModel mSlotVM = new EEPROMSlotViewModel(); you create a new instance of EEPROMSlotViewModel , but youre not getting the checked RadioButtons ViewModel. So at the point were you call mSlotVM.getShortName(0, label); mSlotVM has no ParentVM, thats what raises the exception. What you could do is go through your SlotChildren-List and take the EEPROMSlotViewModel whose IsChecked-property is true.

Example:

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