c# - DateTime and Offset Subtlety -


i'm researching "mechanics" of datetime , related classes in bcl. i'm interested in following odd behavior. consider following code snippet:

var dt1 = new datetime(2014, 10, 24, 15, 30, 00, datetimekind.unspecified); datetimeoffset dto2 = new datetimeoffset(dt1, timespan.fromhours(6)); string input = dto2.tostring("o"); datetime dt6 = datetime.parse(input, cultureinfo.invariantculture, datetimestyles.roundtripkind); console.writeline("dt6:" + dt6.tostring("o"));

i know should parse string datetimeoffset, i'm wondering why library allows me here , interesting thing why output be: "dt6:2014-10-24t13:30:00.00000000+04:00" did during parsing set offset actual offset @ given period of time in time zone of local machine , adjusted hours delta between initial offset , resulting one. no matter provide datetimestyles option. kind of resulting value set local, no matter either provide roundtrip value or not. question why sets kind local default , second question datetime stores offset , if stores it, why hell need datetimeoffset class?

a few things:

  • datetime , datetimeoffset both structs, not classes. value types.

  • your question boils down understanding behavior of datetime.parse method - not datetime struct. suggest reading the docs method. there lots of bits of information there describe subtlety of behaviors seeing.

  • the docs have return type in "return value" section (emphasis mine):

    generally, parse method returns datetime object kind property datetimekind.unspecified. however, parse method may perform time zone conversion , set value of kind property differently, depending on values of s , styles parameters:

    chart

    you're passing value of "2014-10-24t15:30:00.0000000+06:00" in s parameter, contains time zone offset, you're affected first row in chart, , result converted machine's local time zone, whatever may be.

  • the datetimestyles.roundtripkind flag not going have affect here. you'd see effect if input ended in z instead of offset, such third row in chart above applied. in case, roundtripkind flag ensures datetimekind.utc. without you'd still datetimekind.local.

  • the offset not stored. thing stored in datetime value combining ticks , kind properties 1 64-bit private field.

    as interesting side note, recognize since 2 bits of field reserved kind, there four possible states, though there 3 datetimekind values exposed. indeed, there hidden fourth kind, described under "datetime's deep dark secret" in this article, , in references sources. (basically, it's variation on datetimekind.local.)

  • if want date/time api makes better sense, consider using noda time.


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