The Series

Level: Moderate

This series assumes the developer is familiar with .NET MAUI, the Model-View-ViewModel pattern, and has worked through one or more tutorials such as the excellent James Montemagno's Workshop.

Source: https://github.com/bladewolf55/net-maui-progression

.NET MAUI's testing problem

At the time of this writing, .NET MAUI apps don't support unit testing. At least, not in any way I could find. And I tried, I really tried, to fool .NET by playing with target frameworks and conditionally including a Program Main method, and other goofy stuff, but to no avail.

So, that's the bad news, a real bummer, and in my opinion a terrible oversight on the .NET MAUI team's part. There's no talk of unit testing, no guidance, no explanation.

Here's the good news: you can unit test .NET MAUI class libraries. Examples can be found at the Community Toolkit repository (thank you, open source!).

This means we need to separate anything we want to unit test into a library. That's not ideal, especially for smaller projects, but we'll make do. I have confidence Microsoft is working on a better way.

Add the ViewModels library

To keep things organized, let's add a ViewModels library. Later, we'll add a Models library.

You could also have a single library for testable code, broken into namespaces. I would do that here, but, frankly, I wasn't sure what to name it! Plus, separate libraries reenforces our separation of concerns.

  1. Solution > Add > New Project > .NET MAUI Class Library named "Maui.Progression.ViewModels".
  2. Install the Microsoft.Toolkit.Mvvm package, and uninstall that package from the app project.
  3. Delete the Class1.cs file.
  4. Copy or move the Counter.cs file to the root of the ViewModels library.
  5. In the app project, delete the ViewModels folder.
  6. In the app project, add the ViewModels project as a project reference.
  7. Update the MainPage.xaml viewmodels namespace. 
    xmlns:viewmodels="clr-namespace:Maui.Progression.ViewModels;assembly=Maui.Progression.ViewModels"

At this point you may need to do a Solution Clean and Rebuild, or even close Visual Studio, delete the bin/obj folders, and restart then build.

For example, I got this warning:

Warning	WMC1006	Cannot resolve Assembly or Windows Metadata file 'C:\Users\charl\source\repos\dotnet-maui-progression\src\Maui.Progression.02\Maui.Progression.ViewModels\bin\Debug\net6.0-windows10.0.19041.0\Maui.Progression.ViewModels.dll'	Maui.Progression	C:\Users\charl\source\repos\dotnet-maui-progression\src\Maui.Progression.02\Maui.Progression\Maui.Progression.csproj

Running the app should behave as before.

Add the testing project

My preferred unit testing stack is xUnit, FluentAssertions, and NSubstitute. I'm keeping this tutorial simple by using vanilla xUnit.

  1. Solution > Add > New Project > xUnit Test Project named "Maui.Progression.UnitTests"
  2. Add Maui.Progression.ViewModels as a project reference.
  3. Rename UnitTest1.cs to Counter_Should.cs

    This is just my preferred naming convention.

  4. Update Counter_Should.cs with this code. 
    using Maui.Progression.ViewModels;

namespace Maui.Progression.UnitTests;

public class Counter_Should
{
    [Fact]
    public void Increment_counter_by_one_using_command()
    {
        // arrange
        int expected = 1;
        var vm = new Counter();

        // verify the starting count
        Assert.True(vm.Count == 0, "Starting count should have been zero.");

        // act
        vm.IncreaseCounterCommand.Execute(null);

        // assert
        Assert.Equal(expected, vm.Count);
    }
}

Run the test, which should pass.

As a final check, rerun the app.

Wrap Up

We've separated out our View Model into a testable library, and written a simple passing test. We haven't written any kind of UI test, but that's OK for now. In a future part, I hope to try out the Visual Studio App Center, which appears purpose-built for multi-platform UI testing.

Next up: adding a Model!

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