myDateTime.ToString ("fffffff") should work. Is this a bit of a mistake?
Also, this is from the page you linked
Although you can display ten million parts of the second component of the time value, this value cannot be meaningful. The accuracy of the date and time value depends on the resolution of the system clock. In Windows NT 3.5 (and later) and Windows Vista operating systems, clock resolution is approximately 10-15 milliseconds.
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