How do I get Pex to handle function using DateTime.Now or File.Exists

Question

I'm new to Pex and I can't see how to use it with machine specific routines like DateTime.Now and File.Exists().

I have a function for displaying a cut-off DateTime with the time zone offset.

public class CommonDateTime
{
    public static string ConvertToLongStringWithGmtOffset(DateTime cutoffData)
    {
        return String.Format(
            "{0} {1} GMT (local time is {2})", 
            cutoffData.ToLongDateString(), 
            cutoffData.ToShortTimeString(), 
            DateTime.Now.ToString("zzz"), // here is the problem...
            CultureInfo.InvariantCulture);
    }
}

I have a Pex parameterized test which gets generated by the Pex Explorer

[PexClass(typeof(CommonDateTime))]
[TestFixture]
public partial class CommonDateTime_Test
{
    /// <summary>Test stub for ConvertToLongStringWithGmtOffset(DateTime)</summary>
    [PexMethod]
    public string ConvertToLongStringWithGmtOffset(DateTime _cutOffData)
    {
        string result = CommonDateTime.ConvertToLongStringWithGmtOffset(_cutOffData);
        return result;
    }
}

However this generates a test which is machine-specific - it fails when the machine is in a non-GMT timezone.

public partial class CommonDateTime_Test
{
    [Test]
    [PexGeneratedBy(typeof(CommonDateTime_Test))]
    public void ConvertToLongStringWithGmtOffset156()
    {
        string s;
        s = this.ConvertToLongStringWithGmtOffset(default(DateTime));
        PexAssert.AreEqual<string>
            ("Monday, January 01, 0001 12:00 AM GMT (local time is +00:00)", s);
    }
}

What can I do in this situation? Can I tell it to skip exploring functions which refer to functions such as DateTime.Now or File.Exists(). Or can I tell it to always use a specific time zone somehow?

Answer 1

This is what the Moles project is for. It lets you mock basically anything, including built-in static functions like DateTime.Now.

The appropriate 'Moled' code would look something like this:

[PexMethod]
public string ConvertToLongStringWithGmtOffset(DateTime _cutOffData)
{
    MDateTime.NowGet = () => /* some value */;

    string result = CommonDateTime.ConvertToLongStringWithGmtOffset(_cutOffData);
    return result;
}

Here's a longer tutorial which actually uses DateTime.Now as its example.