C# Casting vs. Parse

If g[0]["MyUntypedDateField"] is really a DateTime object, then the cast is the better choice. If it's not really a DateTime, then you have no choice but to use the Parse (you would get an InvalidCastException if you tried to use the cast)

Casting is the only good answer.

You have to remember, that ToString and Parse results are not always exact - there are cases, when you cannot safely roundtrip between those two functions.

The documentation of ToString says, it uses current thread culture settings. The documentation of Parse says, it also uses current thread culture settings (so far so good - they are using the same culture), but there is an explicit remark, that:

Formatting is influenced by properties of the current DateTimeFormatInfo object, which by default are derived from the Regional and Language Options item in Control Panel. One reason the Parse method can unexpectedly throw FormatException is if the current DateTimeFormatInfo.DateSeparator and DateTimeFormatInfo.TimeSeparator properties are set to the same value.

So depending on the users settings, the ToString/Parse code can and will unexpectedly fail...