NotifyPropertyChanged.Verifier

Introducing NotifyPropertyChanged.Verifier, a fluent extension of xUnit for testing implementations of INotifyPropertyChanged in ViewModels.

tl;dr

vm.ShouldNotifyOn(vm => vm.PropertyWithNotify)
  .When(vm => vm.PropertyWithNotify = 42);

vm.ShouldNotNotifyOn(vm => vm.PropertyWithoutNotify)
  .When(vm => vm.PropertyWithoutNotify = -1);

Usage

Consider the following ViewModel:

public class ViewModel : INotifyPropertyChanged {
  int backingField;
  string backingField2;

  public int PropertyWithoutNotify { get; set; }

  public int PropertyWithNotify {
      get => backingField;
      set {
        backingField = value;
        OnPropertyChanged();
      }
  }

  public string PropertyWithMultipleNotifies {
      get => backingField2;
      set {
          PropertyWithNotify = int.Parse(value);
          OnPropertyChanged();
      }
  }

  public event PropertyChangedEventHandler PropertyChanged;

  private void OnPropertyChanged([CallerMemberName] string propertyName = "") =>
      PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}

To test it, create an xUnit-test project and add a NuGet reference to NotifyPropertyChanged.Verifier. It's a .Net Standard 2.0 library and can be used both in .Net Core and the full .NET Framework. The preceding ViewModel can test its implementation of INotifyPropertyChanged doing:

using NotifyPropertyChanged.Verifier;
using Xunit;

namespace Tests {
    public class UnitTests {
        readonly ViewModel vm;

        public UnitTests() => vm = new ViewModel();

        [Fact]
        public void PropertyWithNotify_WillRaiseNotifyEvent() =>
            vm.ShouldNotifyOn(vm => vm.PropertyWithNotify)
              .When(vm => vm.PropertyWithNotify = 42);

        [Fact]
        public void PropertyWithoutNotify_WillNotRaiseNotifyEvent() =>
            vm.ShouldNotNotifyOn(vm => vm.PropertyWithoutNotify)
              .When(vm => vm.PropertyWithoutNotify = -1);

        [Fact]
        public void PropertyWithMultipleNotifies_WillRaiseMultipleNotifyEvents() =>
            vm.ShouldNotNotifyOn(vm => vm.PropertyWithNotify,
                                 vm => vm.PropertyWithMultipleNotifies)
              .When(vm => vm.PropertyWithMultipleNotifies = "42");
    }
}

The library consists of two extension methods on INotifyPropertyChanged, ShouldNotifyOn and ShouldNotNotifyOn which takes 1 or more property expressions as input. These are the properties that should either receive or not receive a NotifyPropertyChanged-event when an Action<T> is called by the When method. This can anything, not only methods or properties on the ViewModel itself.

Inspired by this blogpost.