Domain.cs

public class Domain
{
    public event EventHandler<MyArgs> MyEvent;

    public void Invoke()
    {
        var sw = new Stopwatch();
        sw.Start();
        Console.WriteLine($"{sw.ElapsedMilliseconds}: Entering delay");
        while (sw.ElapsedMilliseconds < 5000)
        {
            
        }
        Console.WriteLine($"{sw.ElapsedMilliseconds}: Invoking event");
        MyEvent?.Invoke(this, new MyArgs { TheAnswer = 42 });
        Console.WriteLine($"{sw.ElapsedMilliseconds}: Event invoked");
        sw.Stop();
    }
}

public class MyArgs : EventArgs
{
    public int TheAnswer = 69;
}

Test.cs

[TestClass]
public class UnitTest1
{
    [TestMethod]
    public void TestMethod1()
    {
        var d = new Domain();
        d.Invoke();
        d.MyEvent += (o, e) =>
        {
            Assert.AreEqual(e.TheAnswer, "42");
        };
    }
}