Unit testing a time - based component?

Question

So I'm writing a cache system (not too complicated, but it has very nice semantics) with a ttl (time to live) for items.

Of course the ttl must be unit tested, but since one can't inject a new implementaton of "IDateTime" or "ITimeSpan" in C# (because there is no such thing), how would you go about that?

Write a new component "IDateTimeProvider" and "DateTimeProvider" first, which one can mockup then?

Isn't reimplementing parts of the .NET runtime library... overkill?

Edit: Thank you all for your amazing answers! I know now exactly what I'm going to do!

Answer 1

I usually use a variation of Ayende's solution.

public static class SystemTime
{
    public static Func<DateTime> DateProvider = () => DateTime.Now;

    public static DateTime Now { get { return DateProvider(); } }
}

In the code under test you can now use SystemTime.Now to get the current time:

var currentTime = SystemTime.Now;

And in the test you can set the current time to a known value:

SystemTime.DateProvider = () => new DateTime(2010,6,25);

Answer 2

You might want to see this question:

How do I MOQ the System.IO.FileInfo class... or any other class without an interface?

In a nutshell you'll either need to make a wrapper class yourself, or use a library that already provides wrapper classes for DateTime, like SystemWrapper.

Answer 3

how would writing an IDateTimeProvider be reimplementing parts of the .Net Framework?

You'd just have two implementations, one that returns DateTime.Now and another that returns a value you specified before.

Answer 4

An IDateTime provider might prove useful in other ways -- if your cache is to be used between client and server apps running on separate machines, you will one one single time-source for both machines (usually the server) otherwise the times may be out of step. I you use the IDateTimeProvider interface everywhere, then the client can use a version that gets the time from the server.

You would get that plus the Unit-testing de-coupling.