How to write a ViewModelBase in MVVM

It's worth nothing to use MVVM frameworks if you don't know what's going on inside.

So let's go step by step and build your own ViewModelBase class.

1. ViewModelBase is class common for all your viewmodels. Let's move all common logic to this class.

2. Your ViewModels should implement INotifyPropertyChanged (do you understand why?)

    public abstract class ViewModelBase : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
   
        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }
   

   the [CallerMemberName] attribute is not required, but it will allow you to write: OnPropertyChanged(); instead of OnPropertyChanged("SomeProperty");, so you will avoid string constant in your code. Example:

    public string FirstName
    {
        set
        {
            _firstName = value;
            OnPropertyChanged(); //instead of OnPropertyChanged("FirstName") or OnPropertyChanged(nameof(FirstName))
        }
        get{ return _firstName;}
    }
   

   Please note, that OnPropertyChanged(() => SomeProperty) is no more recommended, since we have nameof operator in C# 6.

3. It's common practice to implement properties that calls PropertyChanged like this:

    public string FirstName
    {
        get { return _firstName; }
        set { SetProperty(ref _firstName, value); }
    }
   

   Let's define SetProperty in your viewmodelbase:

    protected virtual bool SetProperty<T>(ref T storage, T value, [CallerMemberName] string propertyName = "")
    {
        if (EqualityComparer<T>.Default.Equals(storage, value))
            return false;
        storage = value;
        this.OnPropertyChanged(propertyName);
        return true;
    }
   

   It simply fires PropertyChanged event when value of the property changes and returns true. It does not fire the event when the value has not changed and returns false. The basic idea is, that SetProperty method is virtual and you can extend it in more concrete class, e.g to trigger validation, or by calling PropertyChanging event.

This is pretty it. This is all your ViewModelBase should contain at this stage. The rest depends on your project. For example your app uses page base navigation and you have written your own NavigationService for handling navigation from ViewModel. So you can add NavigationService property to your ViewModelBase class, so you will have access to it from all your viewmodels, if you want.

In order to gain more reusability and keep SRP, I have class called BindableBase which is pretty much the implementation of INotifyPropertyChanged as we have done here. I reuse this class in every WPF/UWP/Silverligt/WindowsPhone solution because it's universal.

Then in each project I create custom ViewModelBase class derived from BindableBase:

public abstract ViewModelBase : BindableBase
{
    //project specific logic for all viewmodels. 
    //E.g in this project I want to use EventAggregator heavily:
    public virtual IEventAggregator () => ServiceLocator.GetInstance<IEventAggregator>()   
}

if I have app, that uses page based navigation I also specify base class for page viewmodels.

public abstract PageViewModelBase : ViewModelBase
{
    //for example all my pages has title:
    public string Title {get; private set;}
}

I could have another class for dialogs:

public abstract DialogViewModelBase : ViewModelBase
{
    private bool? _dialogResult;

    public event EventHandler Closing;

    public string Title {get; private set;}
    public ObservableCollection<DialogButton> DialogButtons { get; }

    public bool? DialogResult
    {
        get { return _dialogResult; }
        set { SetProperty(ref _dialogResult, value); }
    }

    public void Close()
    {
        Closing?.Invoke(this, EventArgs.Empty);
    }
}

The below class can be used as a ViewModelBase in WPF projects:

public abstract class ViewModelBase : INotifyPropertyChanged
{
    /// <summary>
    /// Multicast event for property change notifications.
    /// </summary>
    public event PropertyChangedEventHandler PropertyChanged;

    /// <summary>
    /// Checks if a property already matches the desired value.  Sets the property and
    /// notifies listeners only when necessary.
    /// </summary>
    /// <typeparam name="T">Type of the property.</typeparam>
    /// <param name="storage">Reference to a property with both getter and setter.</param>
    /// <param name="value">Desired value for the property.</param>
    /// <param name="propertyName">Name of the property used to notify listeners.This
    /// value is optional and can be provided automatically when invoked from compilers that
    /// support CallerMemberName.</param>
    /// <returns>True if the value was changed, false if the existing value matched the
    /// desired value.</returns>
    protected virtual bool SetProperty<T>(ref T storage, T value, [CallerMemberName] string propertyName = null)
    {
        if (object.Equals(storage, value)) return false;
        storage = value;
        // Log.DebugFormat("{0}.{1} = {2}", this.GetType().Name, propertyName, storage);
        this.OnPropertyChanged(propertyName);
        return true;
    }

    /// <summary>
    /// Notifies listeners that a property value has changed.
    /// </summary>
    /// <param name="propertyName">Name of the property used to notify listeners.  This
    /// value is optional and can be provided automatically when invoked from compilers
    /// that support <see cref="CallerMemberNameAttribute"/>.</param>
    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        var eventHandler = this.PropertyChanged;
        if (eventHandler != null)
            eventHandler(this, new PropertyChangedEventArgs(propertyName));
    }
}

And an example of ViewModel class is:

public class MyViewModel : ViewModelBase
{
    private int myProperty;
    public int MyProperty
    {
        get { return myProperty; }
        set { SetProperty(ref myProperty, value); }
    }
}

For ease of writing, below snippet can be used:

<?xml version="1.0" encoding="utf-8"?>  
<CodeSnippets  
    xmlns="http://schemas.microsoft.com/VisualStudio/2005/CodeSnippet">  
    <CodeSnippet Format="1.0.0">  
        <Header>  
            <Title>OnPropertyChanged</Title>  
        </Header>      
      <Snippet>
          <SnippetTypes>
            <SnippetType>SurroundsWith</SnippetType>
            <SnippetType>Expansion</SnippetType>
          </SnippetTypes>
          <Declarations>
            <Literal>
              <ID>TYPE</ID>
              <ToolTip>Property type</ToolTip>
              <Default>int</Default>
            </Literal>
            <Literal>
              <ID>NAME1</ID>
              <ToolTip>Property name</ToolTip>
              <Default>MyProperty</Default>
            </Literal>
          </Declarations>
            <Code Language="CSharp">  
                <![CDATA[private $TYPE$ _$NAME1$;
public $TYPE$ $NAME1$
{
    get => _$NAME1$; 
    set => SetProperty(ref _$NAME1$, value);    
}]]>  
            </Code>  
        </Snippet>  
    </CodeSnippet>  
</CodeSnippets>  

The full code could be downloaded from here.