MVVM Part 3: ViewModelBase

Note: This posting is part of a series. See MVVM-Library for other parts and download.

Class Diagram of ViewModelBase It’s time for our first real “ViewModel” class: ViewModelBase.

As a ViewModel is – I already said before – databound to the UI it inherits NotifyKnownPropertyChanged to provide base support for Data Binding.

Data Binding to a WPF UI brings up another problem: The PropertyChanged event may only be raised in the UI thread.

In order to handle forbidden accesses from other threads and to make “Invoke”-scenarios possible, ViewModelBase remembers the dispatcher of the thread which created it. The methods for this purpose should be self-explanatory.

Note: ViewModel hides NotifyKnownPropertyChanged.OnPropertyChanged in order to perform a (debug-only) check ensuring it was called from the right thread. I thought a while if it would be better to make NotifyKnownPropertyChanged.OnPropertyChanged virtual and implement an auto-invoke solution.  As ViewModel classes are not thread safe (in general), however, but may be able to deal with callbacks (etc.) executing in other threads, this approach is ok in my opinion. Derived classes must not call OnPropertyChanged from another thread – if they do though there will only be an error when using the debug build.

Today, I won’t post an sample project because ViewModelBase makes only sense if it is inherited. You can download the updated ViewModel project (without the sample for the last part) here: MVVM-Library.

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Change a Dependency Property in the setter of a bound property

ItemSelectionQuite a confusing title, but here is an example where this can be interesting (Note: I’m mot using my ViewModel library to keep this simpler).

Let’s say we have a list of values and the user can select one of them. Under special conditions (here in case that a checkbox is checked) we don’t want the user to change the selected item. It could look like in the screenshot on the right.

The following code would be logical to reach a behavior like that:


public sealed class Foo : INotifyPropertyChanged
    private string selectedItem;
    private bool denySelectionChange;

    public string SelectedItem
            return this.selectedItem;
            if (!String.Equals(this.selectedItem, value))
                if (!this.DenySelectionChange)
                    this.selectedItem = value;

    public bool DenySelectionChange
            return this.denySelectionChange;
            if (this.denySelectionChange != value)
                this.denySelectionChange = value;
    public ReadOnlyObservableCollection<string> Items
        private set;

    public event PropertyChangedEventHandler PropertyChanged;

    public Foo(ObservableCollection<string> items)
        this.Items = new ReadOnlyObservableCollection<string>(items);

    private void OnPropertyChanged(string p)
        var handler = this.PropertyChanged;
        if (handler != null)
            handler(this, new PropertyChangedEventArgs(p));


<Window x:Class="WpfApplication1.Window1"
        <CheckBox Content="Deny Selection change"
                  IsChecked="{Binding Path=DenySelectionChange}"
                  DockPanel.Dock="Bottom" />

        <ListBox ItemsSource="{Binding Path=Items}"
                 SelectedItem="{Binding Path=SelectedItem}" />

Window1.xaml.cs (only in order to fill the listbox):

public partial class Window1 : Window
    public Window1()

        Foo foo = new Foo(new ObservableCollection<string> { "Item 1", "Item 2", "Item 3", "Item 4" });
        this.DataContext = foo;

However, you will still be able to change the selected item when the checkbox is checked.

I didn’t find a reason for that. It seems the data binding engine of the WPF doesn’t like it when you change a property while its informing the bound properties about a previous change.

But I have a solution 😉

It’s quite easy: Just change the property back (= raise PropertyChanged) after we have left the Setter.

How to achieve that?

That’s also quite easy: Use Dispatcher.BeginInvoke:

public string SelectedItem
    get //...
        if (!String.Equals(this.selectedItem, value))
            if (!this.DenySelectionChange)
                this.selectedItem = value;

            Action action = () => this.OnPropertyChanged("SelectedItem");

It should work now.

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MVVM Part 2: Property Changing

Note: This posting is part of a series. See MVVM-Library for other parts and download.

Remember that ViewModels are databound to the View?

How are they bound?

You might know that the binding engine in WPF relies on INotifyPropertyChanged for that:

The INotifyPropertyChanged interface is used to notify clients, typically binding clients, that a property value has changed.

For example, consider a Person object with a property called FirstName. To provide generic property-change notification, the Person type implements the INotifyPropertyChanged interface and raises a PropertyChanged event when FirstName is changed.

Property Names as strings…

PropertyChanged expects an instance of PropertyChangedEventArgs which provides a property PropertyName containing the name of the property that has changed as a string.

Thinking about my recently published posting Ban string literals from your code! you might be able to comprehend why I not really like that way: You can easily misspell the property name.

My solution

Class Diagram

Class Diagram

The classes


The class Property represents a property (quite logical, isn’t it ;-)) with all its properties: The type that declares it, its name, its returning type and a callback that returns it value for an passed object.

It is the core of this project part.

Properties are immutable objects that are created via the static Factory-Method Create. You can either pass a PropertyInfo object or a Func<T, object> (speaking strictly a Expression<Func<T, object>>) which returns the value of the property. (Example later in this posting).


KnownPropertyChangedEventArgs is an EventArgs class that inherits PropertyChangedEventArgs and adds the property Property returning a Property 🙂 object.


INotifyKnowPropertyChanged implements INotifyPropertyChanged and hides its PropertyChanged event. Instead it declares an event with the same name but of type EventHandler<KnownPropertyChangedEventArgs>.

So INotifyKnowPropertyChanged also informs about property changes. However, it identifies the changed property not via a simple string but with an object of the described Property class.


NotifyKnownPropertyChanged provides a default implementation of the INotifyKnowPropertyChanged interface.

It maximizes comfort by providing two different methods:

  • OnPropertyChanged: Derived classes will call this method in order to notify about property changes.
  • NotifyPropertyChanged: This method will be called whenever a property changes (respectively when OnPropertyChanged is called). You can override it in order to be notified about property changes.

Sample project

The sample project today is quite small: It’s a simple console application to show how to use those classes. You can find it at MVVM-Library.

Code Snippet

I created a small snippet that helps declaring properties using this way of property change-tracking.

Your opinion

What do you think?

Any bugs / issues / suggestions?

Looking forward to your comments!

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MVVM Part 1: Overview

There have already been quite a few articles, HOW-TOs and tutorials to the Model-View-ViewModel (MVVM) architecture.

One of the best is WPF Apps With The Model-View-ViewModel in my opinion.

However, I have different or advanced solutions for some problems. So I’ll start a series with an unknown number of parts about this topic.

Still, I explicitly recommend reading the others, too. Only this will help you to find your interpretation of MMVM.

What is MVVM?

MVVM is a pattern that helps to separate the behavior of an User Interface from its design.

Why doing so?

There are several reasons for that:

  • Separation: UI and logic are separated for changing them without affecting the other one
  • Testing: Plain UIs can not be tested easily. ViewModels can.
  • Support of different views: You want to create multiple UIs for an application? That’s much easier
  • Ease of the use of WPF Controls: Ever tried to fill a Treeview in WPF without MVVM? It’s quite uncomfortable.
  • Taking advantage of WPF-Features like Data Templates, Data Binding and dynamic Control creation

The three players

The Model


In short, the Model is the “real application”. It represents all the business logic.

Do not see the Model as something passive or as something that contains no logic!

Where are its limits?

The Model shouldn’t contain any logic to directly interact with the user. This shouldn’t be any new information.

This includes displaying of Windows / Forms / MessageBoxes but also formatting data. That’s none of their business!


A little “checklist” for a good Model (of course, there may be exceptions here); arbitrary order:

  • Did I use no class contained in System.Windows (and inferior namespaces)? Thus, I need no reference to PresentationFramework.dll, System.Windows.Forms, PresentationCore, …?
  • Is it neither laid out for Windows Forms, nor WPF? Could I use everything in a Console Application theoretically?
  • Am I using content that needs to be translated? It’s only OK for messages that won’t displayed to the user. (Exceptions – if you prefer using localized ones – and Logfiles can be exceptions)
  • Is there no need of additional logic concerning working with my Model-classes? That means: Every kind of manipulation the application itself does to its data (e.g. functions like “Load the latest stocks from our homepage” or “Do an automatic cleanup and delete all Customers that made no order during the last year”) should be implemented here.
  • Does my Model perform validation?
  • Did I implement code to fetch objects from my data store and save it there? Is this procedure abstracted by my Model? Did I also abstract the specific queries so the user of my classes doesn’t need to know about the structure of my database?
  • Did I use events when requiring feedback from the user?

The ViewModel

Well, what is the ViewModel then?

According to my definition it is a layer which is databound to the view and prepares the model in order to be displayed by it. It does so by providing a kind of proxy classes. Thereby it simplifies complicated situations by changing the structure of the model.

What does it do?

As already said in the definition: Its main task is simplifying and restructuring data in a form that can’t be visualized directly by the view. Everything is supplied in a bindable way by implementing INotifyPropertyChanged.

Every action you can perform (this also includes object creation) is exposed as a command.

There are three “base” types of patterns: Selecting one or more items, Changing a property, Invoking a method.

Where are its limits?

The ViewModel should not know about the concrete view. Only the structure of a view is considered when creating it. By saying “structure” I neither mean the layout nor the real “looking”. “Structure” describes what a user can actually do – something like “There is a possibility to change the name of a person.” or “Items can be deleted, added, and reordered here”.

In my opinion, formatting (of primitive values – e.g. setting the number of decimal points or combining two or more properties to a new one) is also not a part of the ViewModel.

One important thing, a ViewModel should never do: Do not use classes from “System.Windows”. That means: No MessageBox.Show, no creation of Windows, …

  • Is the ViewModel “easy” to use?
  • Does it track property changes and report them via INotifyPropertyChanged?
  • Do all properties either return another ViewModel, a primitive data type or a collection of one of these?
  • Does it not contain any String.Concats, String.Formats, … in order to combine two or more properties to a new one?
  • Are there commands for every action you can do with the ViewModel?
  • Is it independent of the current UI?
  • Did I use no classes declared in “PresentationFramework.dll” (which includes all controls)?
  • Is every action exposed as a command?

The View


The view is the “real UI” or simply “that thing you can really see”. For each ViewModel there is exactly one UserControl representing it.

This UserControl is assembled out of existing controls (of course there can be user generated controls – however, these controls should be independent). It has no code-behind and only consist of XAML-Code. Data Templates, Data Binding, Styles and Resources are used to a great extent.

All formatting, arrangement and visualizing of data is done here.

Where are its limits?

Although the View displays everything, it’s designed more passive than active. It only invokes commands, and changes databound properties. It will never change something by code.

Therefore, it contains no program logic.

  • Does the View only use the ViewModel (and not the Model) classes?
  • Is the View free of logic that affects more than the UI itself?
  • Is there exactly one UserControl per each ViewModel class?
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