CODEWITHSUNDEEP
c#wpfmvvmpropertiesconstructor
Andy
Andy

Reputation: 3682

Property that changes in constructor or as property

I am working with C# using the MVVM pattern. I have two WPF windows, each with a view model. Basically I need to pass a property of the main view model to the 'child' view model. At the minute, I do this by setting a private variable equal to the new view model in the main view model's constructor, and in doing so passing the property in the constructor of the child view model.

However, there is a dependency property linked to the property as it used as a binding for the selected item in a combobox. Therefore, it is likely to change after the child view model is initialized, but by passing the property in the constructor, the change is not made in my child view model.

So, is there anyway for me to pass the property into the constructor and have it change in the child view model when it does in the main view model? Or would I have to create a property in the child view model which is updated everytime the property in the main view model is set?

Hope that makes sense.

Edit Inside my main view model, I declare the following:

public readonly DependencyProperty CurrentDatabaseManagedProperty = DependencyProperty.Register("CurrentDatabaseManaged", typeof(DatabaseInfo), typeof(MainViewModel));
public DatabaseInfo CurrentDatabaseManaged {
    get { return (DatabaseInfo)GetValue(CurrentDatabaseManagedProperty); }
    set { SetValue(CurrentDatabaseManagedProperty, value); }
}

public DatabaseInfo CurrentDatabaseManagedSelection {
    get { return CurrentDatabaseManaged; }
    set {
        if (CurrentDatabaseManaged != null &&
            (String.Equals(value.Name, CurrentDatabaseManaged.Name, StringComparison.OrdinalIgnoreCase))) return;

            CurrentDatabaseManaged = (value.IsUsable ? value : dbm.ReadDatabase(value.FileName));
        }
}

Where CurrentDatabaseManagedSelection is the SelectedItem of the combobox. In the constructor of the main view model, I have the following:

_DatabaseVM = new ChildViewModel(CurrentDatabaseManaged);

And the constructor of ChildViewModel looks like this:

public ChildViewModel( DatabaseInfo SelectedDatabase)
  {
    if (SelectedDatabase != null)
         _SelectedDatabase = SelectedDatabase;
    }
  }

Basically I would like _SelectedDatabase to be updated whenever CurrentDatabaseManagedSelection is.

Upvotes: 0

Views: 704

Answers (2)

Tim S.
Tim S.

Reputation: 56536

It looks like you want to bind to the CurrentDatabaseManagedSelection property. The simplest way to simulate this is to add this to the setter of that property:

_DatabaseVM._SelectedDatabase = value;

To do it with actual bindings, you'd need to

  1. make ChildViewModel._SelectedDatabase a dependency property,
  2. make MainViewModel implement INotifyPropertyChanged, and
  3. call the PropertyChanged event in the setter of CurrentDatabaseManagedSelection.
  4. make ChildViewModel extend DependencyObject

  5. instead of just setting the property, set a binding, e.g.

    BindingOperations.SetBinding(this, _SelectedDatabaseProperty,
  new Binding("CurrentDatabaseManagedSelection") { Source = mainViewModel });

Upvotes: 0

user1228
user1228

Reputation:

You have to change the value later, after bindings are set up in the UI.

Use the Dispatcher.BeginInvoke method to put off updating the property until later on.

public MyClass(object someValue)
{
    Dispatcher.BeginInvoke(
        (Action)(() => Property = someValue),  // the cast may not be required
        DispatcherPriority.ApplicationIdle);  // runs after everything is loaded
}

Upvotes: 1

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