Ruthlessly Helpful

Stephen Ritchie's offerings of ruthlessly helpful software engineering practices.

Four Ways to Fake Time, Part 2

In Part 1 of this four part series you learned how a code’s implicit dependency on the system clock can make the software difficult to test. The first post presented a very simple solution, pass in the clock as a method parameter. It is effective, however, adding a new parameter to every method of a class isn’t always the best solution.

Fake Time 2: Brute Force Property Injection

Here is a second way to fake time. It is brute force in the sense that it is rudimentary. Using full-blown dependency injection with an IoC container is left as an exercise for the reader. The goal of this post is to illustrate the principle and provide you with a technique you can use today.

Perhaps an example would be helpful …

using System;
using Lender.Slos.Utilities.Configuration;

namespace Lender.Slos.Financial
    public class ModificationWindow
        private readonly IModificationWindowSettings _settings;

        public ModificationWindow(
            IModificationWindowSettings settings)
            _settings = settings;

        // This property is for testing use only
        private DateTime? _now;
        public DateTime Now
            get { return _now ?? DateTime.Now; }
            internal set { _now = value; }

        public bool Allowed()
            var now = this.Now;

            // Start date's month & day come from settings
            var startDate = new DateTime(

            // End date is 1 month after the start date
            var endDate = startDate.AddMonths(1);

            if (now >= startDate &&
                now < endDate)
                return true;

            return false;

In this example code, the Allowed method changed very little from how it was written at the end of the first post. The primary difference is that there isn’t any clock optional argument. The value of the now variable comes from the new class property named Now.

Let’s take a closer look at the Now property. First, it has a backing variable named _now, which is declared as a nullable DateTime. Second, since _now defaults to null, this means that the Now property getter will return System.DateTime.Now if the property is never set. In other words, if the Now property is never set then that property behaves like a call to System.DateTime.Now.

Note that the null coalescing operator (??) expression in the getter can be rewritten as follows:

    return _now == null ? DateTime.Now : _now.Value;

And so, if our test code sets the Now property to a specific DateTime value then that property returns that DateTime value, instead of System.DateTime.Now. This allows the test code to “freeze the clock” before calling the method-under-test.

The following is the revised test method. It sets the Now property to the currentTime value at the end of the arrangement section. This, in effect, fakes the Allowed method, and establishes a known value for the clock.

public void Allowed_WhenCurrentDateIsInsideModificationWindow_ExpectTrue(
    int startMonth)
    // Arrange
    var settings = new Mock<IModificationWindowSettings>();
        .SetupGet(e => e.StartMonth)
        .SetupGet(e => e.StartDay)

    var currentTime = new DateTime(

    var classUnderTest = new ModificationWindow(settings.Object);

    classUnderTest.Now = currentTime; // Set the value of Now; freeze the clock

    // Act
    var result = classUnderTest.Allowed();

    // Assert
    Assert.AreEqual(true, result);

There is one more subtlety to mention. The test method cannot set the class-under-test’s Now property without being allowed access. This is accomplished by adding the following line to the end of the AssemblyInfo.cs file in the Lender.Slos.Financial project, which declares the class-under-test.

[assembly: InternalsVisibleTo("Tests.Unit.Lender.Slos.Financial")]

The use of InternalsVisibleTo establishes a friend assembly relationship.


  1. A straightforward, KISS approach
  2. Can work with .NET Framework 2.0
  3. No impact on class-users and method-callers
  4. Isolated change, minimal risk
  5. Testability is greatly improved


  1. Improves testability only one class at a time
  2. Adds a testing-use-only property to the class

I use this approach when working with legacy or Brownfield code. It is a minimally invasive technique.

In the next part of this Fake Time series we’ll look at the IClock interface and a constructor injection approach.


2 responses to “Four Ways to Fake Time, Part 2

  1. Pingback: Four Ways to Fake Time « Ruthlessly Helpful

  2. Pingback: Four Ways to Fake Time, Part 3 « Ruthlessly Helpful

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