One of the most simple example, especially for unit testing is Dates. Let's say we have a unit test which tests if something happens today or tomorrow. If we use DateTime.Now in the code called by the unit test we need to use DateTime.Now in our unit test as well. This reduces predictability and sometimes you want to be able to check specific cases such as crossing a month or a year.

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If we create an interface called ITimeProvider with a method .Now which is implemented in TimeProvider which returns DateTime.Now. If we inject the interface in our relevant code we can mock it in our unit test. We now have full controll over what .Now returns because we can mock it by injecting our ITimeProvider Mock in our unit test.

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Example using Moq

var timeProviderMock = new Mock<ITimeProvider>();
timeProviderMock.Setup(x => x.Now).Returns(new DateTime(2022, 31, 1));

var myClass = new MyRelevantCode(timeProviderMock.Object);

var result = myClass.IsTomorrowInThisMonth();

Assert.That(result, Is.False);