WPF MVVM passing data between viewmodel but property not updating - wpf

Question

I'm making a Window to manage the users who using laptop. I have the window named "LaptopWindow" which contain a TextBox to display the user id of the one using it. I have made a button to open new UserControl named "FindEmployeeUC" to find the "EmpID" by select the row in DataGrid of UserControl and pass it back to the TextBox in "LaptopWindow".
I got the selected row of the DataGrid and use the property name "SelectedUA" to hold it inside the view model "UserAccountViewModel".
When OnPropertyChanged event fire I call the instance of "LaptopManagementViewModel" (this view model is bound with "LaptopWindow") and set the EmpID to the TextBox in "LaptopWindow" by the property named "ReceiverID"
The property "ReceiverID" got value but the UI of "LaptopWindow" didn't get update.
I tried to use Delegate, Singleton pattern, It had the same result.
Here is some code to explain more what I'm facing
The "LaptopWindow" xaml:
<StackPanel Grid.Row="2" Grid.Column="1" Style="{StaticResource inputControl}">
<TextBlock Text="Người nhận"/>
<TextBox Name="txtReceiver" Text="{Binding ReceiverID,Source={StaticResource vmLaptopManagement}}" Margin="0,0,30,0"/>
</StackPanel>
<!--Button open FindEmpUC -->
<Button Grid.Row="2" Grid.Column="1" Width="30" Height="29" HorizontalAlignment="Right" VerticalAlignment="Bottom" Background="Transparent" Margin="0,4,4,4" Command="{Binding CmdFindEmp}">
<Image Source="/imgs/find-48.png" Stretch="Uniform" />
</Button>
The "LaptopManagementViewModel":
//the userAccountVM
UserAccountViewModel userAccountVM;
//the constructor
public LaptopManagementViewModel(UserAccountViewModel userAccountVM)
{
LstDVUS = LaptopManagementBLL.Instance.GetDVUsageStatuses();
LstLaptop = LaptopManagementBLL.Instance.GetLaptopsInfo();
this.userAccountVM = userAccountVM;
ReceiverID = this.userAccountVM.SelectedUA.EmpID;
}
//the ReceiverID property
string receiverID;
public string ReceiverID
{
get { return receiverID; }
set
{
receiverID = value;
OnPropertyChanged("ReceiverID");
}
}
//function open FindEmployeeUC
private void FindEmployee(object obj)
{
//show findEmployee UC
Window wd = new Window()
{
Content = new FindEmployeeUC(),
};
wd.ShowDialog();
}
The "FindEmployeeUC" xaml:
<DataGrid Grid.Row="1" ItemsSource="{Binding LstUA}" CanUserAddRows="False" SelectedItem="{Binding SelectedUA,Mode=TwoWay}" AutoGenerateColumns="False" ColumnWidth="*" IsReadOnly="True">
<DataGrid.Columns>
<DataGridTextColumn Header="ID" Binding="{Binding ID}"></DataGridTextColumn>
<DataGridTextColumn Header="EmpID" Binding="{Binding EmpID}"></DataGridTextColumn>
<DataGridTextColumn Header="EmpName" Binding="{Binding EmpName}"></DataGridTextColumn>
<DataGridTextColumn Header="Position" Binding="{Binding Position}"></DataGridTextColumn>
<DataGridTextColumn Header="LineGroup" Binding="{Binding LineGroup}"></DataGridTextColumn>
</DataGrid.Columns>
</DataGrid>
The "UserAccountViewModel":
//The property "SelectedUA"
UserAccountModel selectedUA;
public UserAccountModel SelectedUA
{
get { return selectedUA; }
set
{
if(selectedUA!=value)
{
selectedUA = value;
LaptopManagementViewModel laptopVM = new LaptopManagementViewModel(this);
OnPropertyChanged("SelectedUA");
}
}
}
I expect to get the EmpID for the TextBox in "LaptopWindow". I attach a picture for more detail:
Thanks in advance!

Answer 1

In your OnPropertyChanged event invocator you are always creating a new instance of UserAccountViewModel. This instance is never referenced in your XAML code, therefore your view can't see this new instances.
Since view model have a state you typically use a single instance for a binding target.
I removed the parameterized constructor to enable the instantiation in XAML (the instance is assigned to the UserAccountVM property from XAML) and also removed the reference to LaptopManagementViewModel from UserAccountViewModel. I created the view model instances and added them to the ResourceDictionary of App.xaml.
I also added a PropertyChanged event handler to the LaptopManagementViewModel to listen for changes of UserAccountViewModel.SelectedUA.
It is also highly recommended to avoid string literals. Instead of calling OnPropertyChanged("MyProperty") you should use the free compiler support by applying nameof(): OnPropertyChanged(nameof(MyClass.MyProperty)). I replaced the corresponding code. You now get rid of typos and get full support of compiler checks and refactoring tools (e.g. renaming).
Also stay away from Singletons. They smell strong.
Last complaint: make fields always private (or protected), especially when they are property backing fields. If you don't use any access modifier then internal will apply implicitly. Which is equivalent to public inside a shared assembly and fields should never be exposed.
Microsoft Docs recommends :
Generally, you should use fields only for variables that have private or protected accessibility. Data that your class exposes to client code should be provided through methods, properties and indexers. By using these constructs for indirect access to internal fields, you can guard against invalid input values. A private field that stores the data exposed by a public property is called a backing store or backing field.
App.xaml
<Application x:class="App">
<Application.Resources>
<ResourceDictionary>
<UserAccountViewModel x:Key="UserAccountViewModel" />
<LaptopManagementViewModel x:Key="LaptopManagementViewModel">
<LaptopManagementViewModel.UserAccountVM>
<StaticResource ResourceKey="UserAccountViewModel" />
</LaptopManagementViewModel.UserAccountVM>
</LaptopManagementViewModel>
</Application.Resources>
</Application>
LaptopWindow.xaml
<Window x:class="LaptopWindow">
<Window.DataContext>
<StaticResource ResourceKey="LaptopManagementViewModel" />
</Window.DataContext>
...
</Window>
FindEmployeeUC.xaml
<Window x:class="FindEmployeeUC">
<Window.DataContext>
<StaticResource ResourceKey="UserAccountViewModel" />
</Window.DataContext>
<DataGrid
...
</DataGrid>
</Window>
LaptopManagementViewModel.cs
public class LaptopManagementViewModel
{
private UserAccountViewModel userAccountVM;
public UserAccountViewModel UserAccountVM
{
get => userAccountVM;
set
{
userAccountVM = value;
OnPropertyChanged(nameof(UserAccountVM));
if (userAccountVM != null)
{
// Always clean up event handlers to avoid memory leaks
userAccountVM.PropertyChanged -= UpdateReceiverIdOnPropertyChanged;
}
userAccountVM.PropertyChanged += UpdateReceiverIdOnPropertyChanged;
}
}
// The constructor is now parameterless for the use in XAML
public LaptopManagementViewModel()
{
LstDVUS = LaptopManagementBLL.Instance.GetDVUsageStatuses();
LstLaptop = LaptopManagementBLL.Instance.GetLaptopsInfo();
}
// UserAccountVM.PropertyChanged event handler
private void UpdateReceiverIdOnPropertyChanged(object sender, PropertyChangedEventArgs e)
{
if (e.PropertyName.Equals(nameof(UserAccountViewModel.SelectedUA), StringComparison.OrdinalIgnoreCase))
{
ReceiverID = UserAccountVM.SelectedUA.EmpID;
}
}
private string receiverID;
public string ReceiverID
{
get { return receiverID; }
set
{
receiverID = value;
OnPropertyChanged(nameof(ReceiverID));
}
}
}
UserAccountViewModel.cs
public class UserAccountViewModel
{
private UserAccountModel selectedUA;
public UserAccountModel SelectedUA
{
get => selectedUA;
set
{
if(selectedUA!=value)
{
// Removed wrong creation of LaptopManagementViewModel instances
selectedUA = value;
OnPropertyChanged(nameof(SelectedUA));
}
}
}
}