在WPF中使用MVVM的嵌套数据绑定无法正常工作

Question

I am not able to figure out why my third Nested DataBinding in WPF is not working. I am using Entity Framework and Sql Server 2012 and following are my entities. An Application can have more than one accounts. There is an Accounts Table and an Applications Table.

ENTITIES
1. Applications
2. Accounts

VIEWMODELS
1. ApplicationListViewModel
2. ApplicationViewModel
3. AccountListViewModel
4. AccountViewModel

In my usercontrol I am trying to do following:
1. Use combobox to select an application using ApplicationListViewModel ( Working )
2. Upon selected application display all accounts in datagrid ( Working )
3. Upon selected account display details information about a particular account.( Does not show details of the selected account )

<UserControl.Resources>
    <vm:ApplicationListViewModel x:Key="AppList" />
</UserControl.Resources>

<StackPanel DataContext="{Binding Source={StaticResource AppList}}">
    <Grid>
        <Grid.RowDefinitions>
            ...
        </Grid.ColumnDefinitions>
        <StackPanel Grid.Row="0" Grid.Column="0">
            <GroupBox Header="View all">
                <StackPanel>
                    <!-- All Applications List -->
                    <ComboBox x:Name="cbxApplicationList"
                              ItemsSource="{Binding Path=ApplicationList}"
                              DisplayMemberPath="Title" SelectedValuePath="Id"
                              SelectedItem="{Binding Path=SelectedApplication, Mode=TwoWay}" 
                              IsSynchronizedWithCurrentItem="True" />

                    <!-- Selected Application Accounts -->
                    <DataGrid x:Name="dtgAccounts" Height="Auto" Width="auto" AutoGenerateColumns="False" 
                              DataContext="{Binding SelectedApplication.AccountLVM}"
                              ItemsSource="{Binding Path=AccountList}" 
                              SelectedItem="{Binding SelectedAccount, Mode=TwoWay}" IsSynchronizedWithCurrentItem="True">
                        <DataGrid.Columns>
                            <DataGridTextColumn Header="Title" Binding="{Binding Path=Title}"></DataGridTextColumn>
                        </DataGrid.Columns>
                    </DataGrid>
                </StackPanel>
            </GroupBox>
        </StackPanel>

        <StackPanel Grid.Row="0" Grid.Column="1" >
            <GroupBox x:Name="grpBoxAccountDetails" Header="New Account" >
                <!-- Selected Account Details -->
                <!-- DataContext binding does not appear to work -->
                <StackPanel DataContext="{Binding SelectedApplication.AccountLVM.SelectedAccount}"  >
                    <Grid>
                        <Grid.RowDefinitions>
                            ...
                        </Grid.ColumnDefinitions>
                        <TextBlock x:Name="lblApplication" Grid.Row="0" Grid.Column="0" >Application</TextBlock>
                        <ComboBox x:Name="cbxApplication" Grid.Row="0" Grid.Column="1" 
                                  DataContext="{Binding Source={StaticResource AppList}}" 
                                  ItemsSource="{Binding ApplicationList}" 
                                  DisplayMemberPath="Title" SelectedValuePath="Id" 
                                  SelectedValue="{Binding SelectedApplication.AccountLVM.SelectedAccount.ApplicationId}">
                        </ComboBox>
                        <TextBlock x:Name="lblTitle" Grid.Row="0" Grid.Column="0" >Title</TextBlock>
                        <TextBox x:Name="txtTitle" Grid.Row="0" Grid.Column="1" Height="30" Width="200" 
                                Text="{Binding Title}" DataContext="{Binding Mode=OneWay}"></TextBox>
                        <Button Grid.Row="1" Grid.Column="0" Command="{Binding AddAccount}">Add</Button>
                    </Grid>
                </StackPanel>
            </GroupBox>
        </StackPanel>
    </Grid>
</StackPanel>

ApplicationListViewModel

class ApplicationListViewModel : ViewModelBase
    {
         myEntities context = new myEntities();
        private static ApplicationListViewModel instance = null;

        private ObservableCollection<ApplicationViewModel> _ApplicationList = null;

        public ObservableCollection<ApplicationViewModel> ApplicationList
        {
            get 
            {
                return GetApplications(); 
            }
            set {
                _ApplicationList = value;
                OnPropertyChanged("ApplicationList");
            }
        }

        //public ObservableCollection<ApplicationViewModel> Cu
        private ApplicationViewModel selectedApplication = null;

        public  ApplicationViewModel SelectedApplication
        {
            get
            {
                return selectedApplication;
            }
            set
            {
                selectedApplication = value;
                OnPropertyChanged("SelectedApplication");
            }
        }


        //private ICommand showAddCommand;

        public ApplicationListViewModel()
        {
            this._ApplicationList = GetApplications();
        }

        internal ObservableCollection<ApplicationViewModel> GetApplications()
        {
            if (_ApplicationList == null)
                _ApplicationList = new ObservableCollection<ApplicationViewModel>();
            _ApplicationList.Clear();
            foreach (Application item in context.Applications)
            {
                ApplicationViewModel a = new ApplicationViewModel(item);
                _ApplicationList.Add(a);
            }
            return _ApplicationList;
        }

        public static ApplicationListViewModel Instance()
        {
            if (instance == null)
                instance = new ApplicationListViewModel();
            return instance;
        }
    }

ApplicationViewModel

class ApplicationViewModel : ViewModelBase
    {
        private myEntities context = new myEntities();
        private ApplicationViewModel originalValue;

        public ApplicationViewModel()
        {

        }
        public ApplicationViewModel(Application acc)
        {
            //Initialize property values
            this.originalValue = (ApplicationViewModel)this.MemberwiseClone();
        }
        public ApplicationListViewModel Container
        {
            get { return ApplicationListViewModel.Instance(); }
        }

        private AccountListViewModel _AccountLVM = null;

        public AccountListViewModel AccountLVM
        {
            get
            {
                return GetAccounts(); 
            }
            set
            {
                _AccountLVM = value;
                OnPropertyChanged("AccountLVM");
            }
        }
        internal AccountListViewModel GetAccounts()
        {
            _AccountLVM = new AccountListViewModel();
            _AccountLVM.AccountList.Clear();
            foreach (Account i in context.Accounts.Where(x=> x.ApplicationId == this.Id))
            {
               AccountViewModel account = new AccountViewModel(i);
                account.Application = this;
                _AccountLVM.AccountList.Add(account);
            }
            return _AccountLVM;
        }


    }

AccountListViewModel

class AccountListViewModel : ViewModelBase
    {
        myEntities context = new myEntities();
        private static AccountListViewModel instance = null;

        private ObservableCollection<AccountViewModel> _accountList = null;

        public ObservableCollection<AccountViewModel> AccountList
        {
            get 
            {
                if (_accountList != null)
                    return _accountList;
                else
                    return GetAccounts(); 
            }
            set {
                _accountList = value;
                OnPropertyChanged("AccountList");
            }
        }
        private AccountViewModel selectedAccount = null;

        public  AccountViewModel SelectedAccount
        {
            get
            {
                return selectedAccount;
            }
            set
            {
                selectedAccount = value;
                OnPropertyChanged("SelectedAccount");
            }
        }
        public AccountListViewModel()
        {
            this._accountList = GetAccounts();
        }

        internal ObservableCollection<AccountViewModel> GetAccounts()
        {
            if (_accountList == null)
                _accountList = new ObservableCollection<AccountViewModel>();
            _accountList.Clear();
            foreach (Account item in context.Accounts)
            {
                AccountViewModel a = new AccountViewModel(item);
                _accountList.Add(a);
            }
            return _accountList;
        }

        public static AccountListViewModel Instance()
        {
            if (instance == null)
                instance = new AccountListViewModel();
            return instance;
        }
}

AccountViewModel. I am eliminating all other initialization logic aside in viewmodel for simplicity.

class AccountViewModel : ViewModelBase
    {
        private myEntites context = new myEntities();
        private AccountViewModel originalValue;

        public AccountViewModel()
        {

        }
        public AccountViewModel(Account acc)
        {
           //Assign property values.
            this.originalValue = (AccountViewModel)this.MemberwiseClone();
        }
        public AccountListViewModel Container
        {
            get { return AccountListViewModel.Instance(); }
        }
        public ApplicationViewModel Application
        {
            get;
            set;
        }
    }

Edit1:
When I data bind to view the details of the SelectedAccount with textbox it doesn't show any text.
1. Able to databind to ApplicationListViewModel to Combobox.
2. Successfully Bind to view AccountList based upon SelectedApplication
3. Unable to Bind to SelectedAcount in the AccountListViewModel.

I think in the following line it doesn't show any details about the selected account. I have checked all databinding syntax. In the properties I am able to view appropriate DataContext and bind to the properties. But it doesn't show any text. When I select each individual record in the DataGrid I am able to debug the call and select the object but somehow that object is not being shown in the textbox at the very end.

DataContext="{Binding SelectedApplication.AccountLVM.SelectedAccount}"

Edit2:
Based upon the suggestion in the comment below I tried snoop and was able to see the title textbox row highlighted in red color. I am trying to change the binding Path property and datacontext but still not working. When I tried to click on the "Delve Binding Expression" it gave me unhandled exception. I don't know what that means if as it came from Snoop.

Edit3:
I have taken screenshots of DataContext Property for the StackPanel for the Account Details section and the text property of the textbox.

Solution:
Based upon suggestions below I have made following changes to my solution and made it way more simple. I made it unnecessarily complex.
1. AccountsViewModel
2. AccountViewModel
3. ApplicationViewModel

Now I have created properties as SelectedApplication , SelectedAccount all in just one AccountsViewModel . Removed all complex DataContext syntax and now there is just one DataContext in the xaml page.

Simplified code.

class AccountsViewModel: ViewModelBase
    {
        myEntities context = new myEntities();

        private ObservableCollection<ApplicationViewModel> _ApplicationList = null;

        public ObservableCollection<ApplicationViewModel> ApplicationList
        {
            get
            {
                if (_ApplicationList == null)
                {
                    GetApplications();
                }
                return _ApplicationList;
            }
            set
            {
                _ApplicationList = value;
                OnPropertyChanged("ApplicationList");
            }
        }
        internal ObservableCollection<ApplicationViewModel> GetApplications()
        {
            if (_ApplicationList == null)
                _ApplicationList = new ObservableCollection<ApplicationViewModel>();
            else
                _ApplicationList.Clear();
            foreach (Application item in context.Applications)
            {
                ApplicationViewModel a = new ApplicationViewModel(item);
                _ApplicationList.Add(a);
            }
            return _ApplicationList;
        }
        //Selected Application Property
        private ApplicationViewModel selectedApplication = null;

        public ApplicationViewModel SelectedApplication
        {
            get
            {
                return selectedApplication;
            }
            set
            {
                selectedApplication = value;
                this.GetAccounts();
                OnPropertyChanged("SelectedApplication");
            }
        }
        private ObservableCollection<AccountViewModel> _accountList = null;

        public ObservableCollection<AccountViewModel> AccountList
        {
            get
            {
                if (_accountList == null)
                    GetAccounts();
                return _accountList;
            }
            set
            {
                _accountList = value;
                OnPropertyChanged("AccountList");
            }
        }

        //public ObservableCollection<AccountViewModel> Cu
        private AccountViewModel selectedAccount = null;

        public AccountViewModel SelectedAccount
        {
            get
            {
                return selectedAccount;
            }
            set
            {
                selectedAccount = value;
                OnPropertyChanged("SelectedAccount");
            }
        }
        internal ObservableCollection<AccountViewModel> GetAccounts()
        {
            if (_accountList == null)
                _accountList = new ObservableCollection<AccountViewModel>();
            else
                _accountList.Clear();
            foreach (Account item in context.Accounts.Where(x => x.ApplicationId == this.SelectedApplication.Id))
            {
                AccountViewModel a = new AccountViewModel(item);
                _accountList.Add(a);
            }
            return _accountList;
        }

    }

XAML Side

<UserControl.Resources>
    <vm:AccountsViewModel x:Key="ALVModel" />
</UserControl.Resources>
<StackPanel DataContext="{Binding Source={StaticResource ALVModel}}" Margin="0,0,-390,-29">
    <StackPanel>
        <ComboBox x:Name="cbxApplicationList"
                  ItemsSource="{Binding Path=ApplicationList}"
                  DisplayMemberPath="Title" SelectedValuePath="Id"
                  SelectedItem="{Binding Path=SelectedApplication, Mode=TwoWay}" 
                  IsSynchronizedWithCurrentItem="True"></ComboBox>
        <DataGrid x:Name="dtgAccounts" Height="Auto" Width="auto" 
                  AutoGenerateColumns="False" 
                  ItemsSource="{Binding Path=AccountList}" 
                  SelectedItem="{Binding SelectedAccount, Mode=TwoWay}" 
                  IsSynchronizedWithCurrentItem="True" >
            <DataGrid.Columns>
                <DataGridTextColumn Header="Title" Binding="{Binding Path=Title}"></DataGridTextColumn>
                <DataGridTextColumn Header="CreatedDate" Binding="{Binding Path=CreatedDate}"></DataGridTextColumn>
                <DataGridTextColumn Header="LastModified" Binding="{Binding Path=LastModifiedDate}"></DataGridTextColumn>
            </DataGrid.Columns>
        </DataGrid>
    </StackPanel>
    <StackPanel Height="Auto" Width="300" HorizontalAlignment="Left" DataContext="{Binding Path=SelectedAccount}">
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="30"></RowDefinition>
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="100"></ColumnDefinition>
                <ColumnDefinition Width="200"></ColumnDefinition>
            </Grid.ColumnDefinitions>
            <TextBlock x:Name="lblTitle" Grid.Row="0" Grid.Column="0" >Title</TextBlock>
            <TextBox x:Name="txtTitle"   Grid.Row="0" Grid.Column="1" Height="30" Width="200" 
                     Text="{Binding Title}"></TextBox>
        </Grid>
    </StackPanel>
</StackPanel>

I didn't understood MVVM concept properly. I tried to build everything modular and in the end I screwed it up.

Answer 1

I suspect your problem is related to the fact you are returning a new ObservableCollection every time you call the setter for AccountLVM , and you are not raising your PropertyChange notification, so any existing bindings do not get updated

public AccountListViewModel AccountLVM
{
    get
    {
        return GetAccounts(); 
    }
    set
    {
        _AccountLVM = value;
        OnPropertyChanged("AccountLVM");
    }
}

internal AccountListViewModel GetAccounts()
{
    _AccountLVM = new AccountListViewModel();
    _AccountLVM.AccountList.Clear();
    foreach (Account i in context.Accounts.Where(x=> x.ApplicationId == this.Id))
    {
       AccountViewModel account = new AccountViewModel(i);
        account.Application = this;
        _AccountLVM.AccountList.Add(account);
    }
    return _AccountLVM;
}

I find your bindings very confusing and hard to follow, however I think whenever this gets evaluated

DataContext="{Binding SelectedApplication.AccountLVM.SelectedAccount}"

it is creating a new AccountLVM , which does not have the SelectedAccount property set.

You don't see the existing DataGrid.SelectedItem change at all because it's still bound to the old AccountLVM as no PropertyChange notification got raised when _accountLVM changed, so the binding doesn't know to update.

But some other miscellaneous related to your code:

• Don't change the private version of the property unless you also raise the PropertyChange notification for the public version of the property. This applies to both your constructors and your GetXxxxx() methods like GetAccounts() .

• Don't return a method call from your getter. Instead set the value using your method call if it's null, and return the private property afterwards.

   public AccountListViewModel AccountLVM { get { if (_accountLVM == null) GetAccounts(); // or _accountLVM = GetAccountLVM(); return _accountLVM; } set { ... } } 

• It's really confusing to have the DataContext set in so many controls. The DataContext is the data layer behind your UI, and it's easiest if your UI simply reflects the data layer, and having to go all over the place to get your data makes the data layer really hard to follow.

• If you must make a binding to something other than the current data context, try to use other binding properties to specify a different binding Source before immediately going to change the DataContext . Here's an example using the ElementName property to set the binding source:

   <TextBox x:Name="txtTitle" ... Text="{Binding ElementName=dtgAccounts, Path=SelectedItem.Title}" /> 

• The DataContext in inherited, so you don't ever need to write DataContext="{Binding }"

• You may want to consider re-writing your parent ViewModel so you can setup XAML like this, without all the extra DataContext bindings or 3-part nested properties.

   <ComboBox ItemsSource="{Binding ApplicationList}" SelectedItem="{Binding SelectedApplication}" /> <DataGrid ItemsSource="{Binding SelectedApplication.Accounts}" SelectedItem="{Binding SelectedAccount}" /> <StackPanel DataContext="{Binding SelectedAccount}"> ... </StackPanel> 

If you're new to the DataContext or struggling to understand it, I'd recommend reading this article on my blog to get a better understanding of what it is and how it works.

Answer 2

Well one major problem with this Binding method is, that the value is only updated, when the last property value, in your case SelectedAccount , is changed. The other levels are not watched by the BindingExpression , so if eg SelectedApplication.AccountLVM is changed the DataContext will not notice a difference in SelectedAccount because the binding is still 'watching' on the old reference and you're modifying another reference in your VM.

So I think at the start of the application SelectedApplication is null and the Binding of the ComboBox doesn't notice that it changes. Hmm, I thought about another binding solution, but I couldn't found one. So I suggest, that you create an additional property for reflecting SelectedAccount in your ApplicationListViewModel class.