Using TimeProvider and FakeTimeProvider in WinForms

Each new version of .NET brings great new tools. We got generics and LINQ in .NET 2 and 3, the async/await model in .NET 4.5, and string interpolation in .NET 4.6. Okay, that last one's not in the same league as the other ones, but I use string interpolation all the time.

Unfortunately for those of us working on legacy WinForms apps, we don't often get to use the latest and greatest, like generic math support or list patterns from .NET 7. One new feature from .NET 8 though – the TimeProvider class – is available to .NET Framework users. Let's see how.

Backporting TimeProvider

Without getting into all the basics here (I've already written about TimeProvider, testing TimeProvider, and testing TimeProvider timers before), one of the nice things the .NET team did for us was to backport TimeProvider.

It's available for use in earlier .NET versions, including .NET Framework 4.62 and above, thanks to the Microsoft.Bcl.TimeProvider NuGet package:

Microsoft.Bcl.TimeProvider provides time abstraction support for apps targeting .NET 7 and earlier, as well as those intended for the .NET Framework. For apps targeting .NET 8 and newer versions, referencing this package is unnecessary, as the types it contains are already included in the .NET 8 and higher platform versions.

Using TimeProvider in WinForms

I'll try to keep this fairly simple, but we should setup a few things first before we get to the good stuff.

Reference the TimeProvider Package

The first thing we need to add is a reference to Microsoft.Bcl.TimeProvider – won't get very far without that. 😄

Configure Dependency Injection

And while we could just reference TimeProvider.System directly to access the new class, that makes it tough to do any meaningful testing later on. So let's configure the app for Dependency Injection, by referencing the Microsoft.Extensions.DependencyInjection package and then creating a separate class to register any dependencies.

Right now, all we need is TimeProvider.System, so that whenever a request is made for the abstract TimeProvider class, the singleton instance is returned instead:

public class Services
{
    public static IServiceProvider ServiceProvider { get; set; }

    public static void RegisterServices()
    {
        var services = new ServiceCollection();

        services.AddSingleton(TimeProvider.System);

        ServiceProvider = services.BuildServiceProvider();
    }

    public static T Get<T>() where T : class
    {
        return (T)ServiceProvider.GetService(typeof(T));
    }
}

We can call the above code from the Program.cs file, right before showing the main form:

static void Main()
{
    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);
    Services.RegisterServices();  // register the dependencies
    Application.Run(new Form1());
}

Using TimeProvider (directly from the Form)

After requesting an instance of TimeProvider once in the constructor, getting the local time from anywhere is a simple one-liner. While we're at it, let's create a timer too, to update the displayed time every second.

readonly TimeProvider tp;

public Form1()
{
    InitializeComponent();

    tp = Services.Get<TimeProvider>();

    var timer = tp.CreateTimer(
        callback: (state) => Invoke(new Action(() =>
            tslCurrentTimeUpdate.Text = tp.GetLocalNow().ToString("T"))),
        state: null,
        dueTime: TimeSpan.Zero,
        period: TimeSpan.FromSeconds(1));
}

private void Form1_Shown(object sender, EventArgs e)
{
    tslCurrentTimeOnce.Text = tp.GetLocalNow().ToString("T");
}

Here's the StatusStrip area of the app while running the app. The time on the left never changes, while the time on the right is refreshed every second.

Using TimeProvider (from a separate class)

Ideally, we should try to keep business logic out of code-behind files as much as possible (using patterns like MVP), so let's create a DiscountLogic class to play around with TimeProvider some more.

It has one property (for returning a discount percentage based on the current date) and one method (for returning a discounted price, based on the discount percentage). By accepting a TimeProvider in the constructor, and implementing an interface, we set things up nicely later on for testing.

public class DiscountLogic : IDiscountLogic
{
    readonly TimeProvider _timeProvider;

    public DiscountLogic(TimeProvider timeProvider)
    {
        _timeProvider = timeProvider;
    }

    public decimal DailyDiscount
    {
        get
        {
            var now = _timeProvider.GetLocalNow();
            var discountPercent = 0;

            if (now.DayOfWeek != DayOfWeek.Saturday && now.DayOfWeek != DayOfWeek.S
                discountPercent += 10;

            discountPercent +=
                now.Month >= 11 || now.Month <= 2 ? 30 :      // Nov, Dec, Jan, Feb
                now.Month >= 3 && now.Month <= 5 ? 20 :       // Mar, Apr, May
                now.Month >= 9 && now.Month <= 10 ? 10 : 0;   // Sep, Oct

            return discountPercent;
        }
    }

    public decimal GetDiscountPrice(decimal originalPrice)
    {
        return originalPrice * ((100 - DailyDiscount) / 100);
    }
}

public interface IDiscountLogic
{
    decimal DailyDiscount { get; }
    decimal GetDiscountPrice(decimal originalPrice);
}

One more line in the Services.cs class registers the new class:

services.AddTransient<IDiscountLogic, DiscountLogic>();

And then back in the Form, we can get an instance of DiscountLogic and display a message about today's discount:

private void Form1_Shown(object sender, EventArgs e)
{
    var dl = Services.Get<IDiscountLogic>();
    lblDiscount.Text =
        $"The discount for {tp.GetLocalNow().ToString("dddd, MMM d")} is {dl.DailyDiscount}%. " +
        $"A $5.00 icecream costs ${dl.GetDiscountPrice(5):N2} today.";
}

Testing TimeProvider Using FakeTimeProvider

There's a FakeTimeProvider class that helps us test TimeProvider, provided by the Microsoft.Extensions.TimeProvider.Testing package. We'll add that to an NUnit test project, which happens to target .NET 6. I'd prefer to be able to target the .NET Framework but I'm not sure it's possible – more on that later.

Fast-forwarding through time

Here's where the power of all this really shows. Testing code that uses time has always been difficult.. and limited.

We might write a test that simply calls the DailyDiscount property to see what the percentage is for today. Of course, based on the logic, if it's not a summer weekend when the following test runs, it'll fail. We need a way to fake out what time it is.

[Test]
public void GivenSummer_WhenWeekend_ThenNoDiscount()
{
    Assert.That(discountLogic.DailyDiscount, Is.EqualTo(0));
}

The FakeTimeProvider class lets us jump to a future date, like the tests below that jump forward (in the US) to a summer weekend and then a winter weekday, to make sure the appropriate discount (if any) is applied for different times of the year.

[TestFixture]
public class DiscountTests
{
    private FakeTimeProvider fakeTimeProvider;
    private DiscountLogic discountLogic;

    [SetUp]
    public void Setup()
    {
        fakeTimeProvider = new FakeTimeProvider();
        fakeTimeProvider.SetLocalTimeZone(TimeZoneInfo.Utc);

        discountLogic = new DiscountLogic(fakeTimeProvider);
    }

    [Test]
    public void GivenSummer_WhenWeekend_ThenNoDiscount()
    {
        fakeTimeProvider.SetUtcNow(DateTimeOffset.Parse("7/27/2024"));

        Assert.That(discountLogic.DailyDiscount, Is.EqualTo(0));
        Assert.That(discountLogic.GetDiscountPrice(5), Is.EqualTo(5m));
    }

    [Test]
    public void GivenWinter_WhenWeekday_ThenLargeDiscount()
    {
        fakeTimeProvider.SetUtcNow(DateTimeOffset.Parse("2/2/2024"));

        Assert.That(discountLogic.DailyDiscount, Is.EqualTo(40));
        Assert.That(discountLogic.GetDiscountPrice(8), Is.EqualTo(4.8m));
    }

It's worth noting in the above code that I set the local time zone to UTC (something else FakeTimeProvider lets us do), to avoid any weirdness with the test suite running in different timezones on different systems and potentially failing.

It doesn't work with .NET Framework... maybe

The TimeProvider works with .NET Framework 4.6.2, so it makes sense that FakeTimeProvider would too – and it claims it does on the NuGet package page. But when I created a test project using that version, it wouldn't run my tests.

There was a warning in the Error List panel that suggested setting a flag, which seems like overkill (assuming it works at all).

Microsoft.Extensions.TimeProvider.Testing 8.1.0 doesn't support net462 and has not been tested with it. Consider upgrading your TargetFramework to net6.0 or later. You may also set <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings> in the project file to ignore this warning and attempt to run in this unsupported configuration at your own risk.

So as far as I can tell, we can't use FakeTimeProvider in a .NET Framework test suite, although a test suite running a newer version of .NET shouldn't have a problem testing a legacy WinForms app. Targeting .NET 6 in the test suite works fine.

If someone finds a workaround, or that I missed something, I'd love to know what it is!