Question

[Test]
public void Sadness()
{
   var dateTime = DateTime.UtcNow;
   Assert.That(dateTime, Is.EqualTo(DateTime.Parse(dateTime.ToString())));
}

Failed :

 Expected: 2011-10-31 06:12:44.000
 But was:  2011-10-31 06:12:44.350

I wish to know what is happening behind the scenes in ToString() etc to cause this behavior.

EDIT After seeing Jon’s Answer :

[Test]
public void NewSadness()
{
    var dateTime = DateTime.UtcNow;
    Assert.That(dateTime, Is.EqualTo(DateTime.Parse(dateTime.ToString("o"))));
}

Result :

Expected: 2011-10-31 12:03:04.161
But was:  2011-10-31 06:33:04.161

Same result with capital and small ‘o’ . I’m reading up the docs, but still unclear.

Answer 1

The default format specifier is “G” – the general-purpose format – which has limited fidelity. If you want to reproduce exactly the same thing, use the roundtrip specifier, “O”.

string s = dateTime.ToString("O", CultureInfo.InvariantCulture);
Assert.That(dateTime, Is.EqualTo(DateTime.ParseExact(
       s, "O", CultureInfo.InvariantCulture, DateTimeStyles.RoundtripKind)));