I have a singleton model class (Project) shared application-wide through dependency injection. Works great in the application. I have another model class (Analysis) that accesses this singleton that I am trying to unit test. But because of the DI, I am struggling to unit test this.

My current code looks like:

public partial class App : Application
{
    public IHost AppHost;

    /// <summary>
    /// Default constructor
    /// </summary>
    public App()
    {
        AppHost = Host.CreateDefaultBuilder()
            .ConfigureServices(services =>
            {
                services.AddSingleton<IProject, Project>();
            })
            .Build();
...
    }
}

public partial class Analysis
{
    public void AnalysisMethod()
    {
        IProject project = ((App)Application.Current).AppHost.Services
            .GetRequiredService<IProject>();
    }
}

I'm trying to unit test AnalysisMethod but of course it fails because there is no Application.Current. How should I be coding/testing this, keeping in mind that I'd like to keep a singleton IProject created by DI?

I have a singleton model class (Project) shared application-wide through dependency injection. Works great in the application. I have another model class (Analysis) that accesses this singleton that I am trying to unit test. But because of the DI, I am struggling to unit test this.

My current code looks like:

public partial class App : Application
{
    public IHost AppHost;

    /// <summary>
    /// Default constructor
    /// </summary>
    public App()
    {
        AppHost = Host.CreateDefaultBuilder()
            .ConfigureServices(services =>
            {
                services.AddSingleton<IProject, Project>();
            })
            .Build();
...
    }
}

public partial class Analysis
{
    public void AnalysisMethod()
    {
        IProject project = ((App)Application.Current).AppHost.Services
            .GetRequiredService<IProject>();
    }
}

I'm trying to unit test AnalysisMethod but of course it fails because there is no Application.Current. How should I be coding/testing this, keeping in mind that I'd like to keep a singleton IProject created by DI?

• c#
• wpf
• unit-testing
• dependency-injection

1 Answer 1

The usual approach to DI is constructor injection, i.e. all classes should take all their dependencies as constructor parameters. All the classes are registered in the DI container, not just singletons, and as you ask the container for an object it will recursively resolve all the dependencies. So your Analysis class should look something like:

public partial class Analysis
{
    private IProject _project;
    public void AnalysisMethod(IProject project)
    {
        _project = project;
    }
}

This gives you several options when testing:

1. Inject a Mock of the IProject
2. Replace selected classes in the DI container with mocks. Typically classes that deal with database or file access or other things that are external to the application. This lets you test larger parts of the application at once.

Both approaches can be useful, depending on the circumstances, and how tightly the classes are coupled.