如何解析 DateTime 并将其转换为 RFC 822 日期时间格式？

Question

如何将 DateTime 结构转换为其等效的 RFC 822 日期时间 格式的字符串表示形式并将此字符串表示形式解析回 .NET 中的 DateTime 结构？ RFC-822 日期时间格式用于许多规范，例如 RSS 聚合格式 。

Answer 1

尝试一下：

  DateTime today = DateTime.Now;
  String rfc822 = today.ToString("r");
  Console.WriteLine("RFC-822 date: {0}", rfc822);

  DateTime parsedRFC822 = DateTime.Parse(rfc822);
  Console.WriteLine("Date: {0}", parsedRFC822);

传递到 DateTime 的 ToString() 方法的“r”格式说明符实际上会生成一个 RFC-1123 格式的日期时间字符串，但也会根据读取 http://www.w3.org/Protocols/rfc822/#z28。 我在创建 RSS 源时使用了这种方法，并且它们根据 http: 上提供的验证器通过验证。 //validator.w3.org/feed/check.cgi。

缺点是，在转换中，它将日期时间转换为 GMT。 要转换回当地时间，您需要应用当地时区偏移量。 为此，您可以使用 TimeZone 类来获取当前时区偏移量，并将“GMT”替换为时区偏移量字符串：

TimeZone tz = TimeZone.CurrentTimeZone;

String offset = tz.GetUtcOffset().ToString();
// My locale is Mountain time; offset is set to "-07:00:00"
// if local time is behind utc time, offset should start with "-".
// otherwise, add a plus sign to the beginning of the string.
if (!offset.StartsWith("-"))
  offset = "+" + offset; // Add a (+) if it's a UTC+ timezone
offset = offset.Substring(0,6); // only want the first 6 chars.
offset = offset.Replace(":", ""); // remove colons.
// offset now looks something like "-0700".
rfc822 = rfc822.Replace("GMT", offset);
// The rfc822 string can now be parsed back to a DateTime object,
// with the local time accounted for.
DateTime new = DateTime.Parse(rfc822);

Answer 2

这是 C# 中的实现，说明如何解析 DateTime 与其 RFC-822 表示形式之间的转换。 它的唯一限制是日期时间采用协调世界时 (UTC)。 我同意这不是非常优雅的代码，但它完成了工作。

/// <summary>
/// Provides methods for converting <see cref="DateTime"/> structures 
/// to and from the equivalent <a href="http://www.w3.org/Protocols/rfc822/#z28">RFC 822</a> 
/// string representation.
/// </summary>
public class Rfc822DateTime
{
    //============================================================
    //  Private members
    //============================================================
    #region Private Members
    /// <summary>
    /// Private member to hold array of formats that RFC 822 date-time representations conform to.
    /// </summary>
    private static string[] formats = new string[0];
    /// <summary>
    /// Private member to hold the DateTime format string for representing a DateTime in the RFC 822 format.
    /// </summary>
    private const string format     = "ddd, dd MMM yyyy HH:mm:ss K";
    #endregion

    //============================================================
    //  Public Properties
    //============================================================
    #region Rfc822DateTimeFormat
    /// <summary>
    /// Gets the custom format specifier that may be used to represent a <see cref="DateTime"/> in the RFC 822 format.
    /// </summary>
    /// <value>A <i>DateTime format string</i> that may be used to represent a <see cref="DateTime"/> in the RFC 822 format.</value>
    /// <remarks>
    /// <para>
    /// This method returns a string representation of a <see cref="DateTime"/> that utilizes the time zone 
    /// offset (local differential) to represent the offset from Greenwich mean time in hours and minutes. 
    /// The <see cref="Rfc822DateTimeFormat"/> is a valid date-time format string for use 
    /// in the <see cref="DateTime.ToString(String, IFormatProvider)"/> method.
    /// </para>
    /// <para>
    /// The <a href="http://www.w3.org/Protocols/rfc822/#z28">RFC 822</a> Date and Time specification 
    /// specifies that the year will be represented as a two-digit value, but the 
    /// <a href="http://www.rssboard.org/rss-profile#data-types-datetime">RSS Profile</a> recommends that 
    /// all date-time values should use a four-digit year. The <see cref="Rfc822DateTime"/> class 
    /// follows the RSS Profile recommendation when converting a <see cref="DateTime"/> to the equivalent 
    /// RFC 822 string representation.
    /// </para>
    /// </remarks>
    public static string Rfc822DateTimeFormat
    {
        get
        {
            return format;
        }
    }
    #endregion

    #region Rfc822DateTimePatterns
    /// <summary>
    /// Gets an array of the expected formats for RFC 822 date-time string representations.
    /// </summary>
    /// <value>
    /// An array of the expected formats for RFC 822 date-time string representations 
    /// that may used in the <see cref="DateTime.TryParseExact(String, string[], IFormatProvider, DateTimeStyles, out DateTime)"/> method.
    /// </value>
    /// <remarks>
    /// The array of the expected formats that is returned assumes that the RFC 822 time zone 
    /// is represented as or converted to a local differential representation.
    /// </remarks>
    /// <seealso cref="ConvertZoneToLocalDifferential(String)"/>
    public static string[] Rfc822DateTimePatterns
    {
        get
        {
            if (formats.Length > 0)
            {
                return formats;
            }
            else
            {
                formats = new string[35];

                // two-digit day, four-digit year patterns
                formats[0]  = "ddd',' dd MMM yyyy HH':'mm':'ss'.'fffffff zzzz";
                formats[1]  = "ddd',' dd MMM yyyy HH':'mm':'ss'.'ffffff zzzz";
                formats[2]  = "ddd',' dd MMM yyyy HH':'mm':'ss'.'fffff zzzz";
                formats[3]  = "ddd',' dd MMM yyyy HH':'mm':'ss'.'ffff zzzz";
                formats[4]  = "ddd',' dd MMM yyyy HH':'mm':'ss'.'fff zzzz";
                formats[5]  = "ddd',' dd MMM yyyy HH':'mm':'ss'.'ff zzzz";
                formats[6]  = "ddd',' dd MMM yyyy HH':'mm':'ss'.'f zzzz";
                formats[7]  = "ddd',' dd MMM yyyy HH':'mm':'ss zzzz";

                // two-digit day, two-digit year patterns
                formats[8]  = "ddd',' dd MMM yy HH':'mm':'ss'.'fffffff zzzz";
                formats[9]  = "ddd',' dd MMM yy HH':'mm':'ss'.'ffffff zzzz";
                formats[10] = "ddd',' dd MMM yy HH':'mm':'ss'.'fffff zzzz";
                formats[11] = "ddd',' dd MMM yy HH':'mm':'ss'.'ffff zzzz";
                formats[12] = "ddd',' dd MMM yy HH':'mm':'ss'.'fff zzzz";
                formats[13] = "ddd',' dd MMM yy HH':'mm':'ss'.'ff zzzz";
                formats[14] = "ddd',' dd MMM yy HH':'mm':'ss'.'f zzzz";
                formats[15] = "ddd',' dd MMM yy HH':'mm':'ss zzzz";

                // one-digit day, four-digit year patterns
                formats[16] = "ddd',' d MMM yyyy HH':'mm':'ss'.'fffffff zzzz";
                formats[17] = "ddd',' d MMM yyyy HH':'mm':'ss'.'ffffff zzzz";
                formats[18] = "ddd',' d MMM yyyy HH':'mm':'ss'.'fffff zzzz";
                formats[19] = "ddd',' d MMM yyyy HH':'mm':'ss'.'ffff zzzz";
                formats[20] = "ddd',' d MMM yyyy HH':'mm':'ss'.'fff zzzz";
                formats[21] = "ddd',' d MMM yyyy HH':'mm':'ss'.'ff zzzz";
                formats[22] = "ddd',' d MMM yyyy HH':'mm':'ss'.'f zzzz";
                formats[23] = "ddd',' d MMM yyyy HH':'mm':'ss zzzz";

                // two-digit day, two-digit year patterns
                formats[24] = "ddd',' d MMM yy HH':'mm':'ss'.'fffffff zzzz";
                formats[25] = "ddd',' d MMM yy HH':'mm':'ss'.'ffffff zzzz";
                formats[26] = "ddd',' d MMM yy HH':'mm':'ss'.'fffff zzzz";
                formats[27] = "ddd',' d MMM yy HH':'mm':'ss'.'ffff zzzz";
                formats[28] = "ddd',' d MMM yy HH':'mm':'ss'.'fff zzzz";
                formats[29] = "ddd',' d MMM yy HH':'mm':'ss'.'ff zzzz";
                formats[30] = "ddd',' d MMM yy HH':'mm':'ss'.'f zzzz";
                formats[31] = "ddd',' d MMM yy HH':'mm':'ss zzzz";

                // Fall back patterns
                formats[32] = "yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffffffK"; // RoundtripDateTimePattern
                formats[33] = DateTimeFormatInfo.InvariantInfo.UniversalSortableDateTimePattern;
                formats[34] = DateTimeFormatInfo.InvariantInfo.SortableDateTimePattern;

                return formats;
            }
        }
    }
    #endregion

    //============================================================
    //  Public Methods
    //============================================================
    #region Parse(string s)
    /// <summary>
    /// Converts the specified string representation of a date and time to its <see cref="DateTime"/> equivalent.
    /// </summary>
    /// <param name="s">A string containing a date and time to convert.</param>
    /// <returns>
    /// A <see cref="DateTime"/> equivalent to the date and time contained in <paramref name="s"/>, 
    /// expressed as <i>Coordinated Universal Time (UTC)</i>.
    /// </returns>
    /// <remarks>
    /// The string <paramref name="s"/> is parsed using formatting information in the <see cref="DateTimeFormatInfo.InvariantInfo"/> object.
    /// </remarks>
    /// <exception cref="ArgumentNullException"><paramref name="s"/> is a <b>null</b> reference (Nothing in Visual Basic).</exception>
    /// <exception cref="ArgumentNullException"><paramref name="s"/> is an empty string.</exception>
    /// <exception cref="FormatException"><paramref name="s"/> does not contain a valid RFC 822 string representation of a date and time.</exception>
    public static DateTime Parse(string s)
    {
        //------------------------------------------------------------
        //  Validate parameter
        //------------------------------------------------------------
        if (String.IsNullOrEmpty(s))
        {
          throw new ArgumentNullException("s");
        }

        DateTime result;
        if (Rfc822DateTime.TryParse(s, out result))
        {
            return result;
        }
        else
        {
            throw new FormatException(String.Format(null, "{0} is not a valid RFC 822 string representation of a date and time.", s));
        }
    }
    #endregion

    #region ConvertZoneToLocalDifferential(string s)
    /// <summary>
    /// Converts the time zone component of an RFC 822 date and time string representation to its local differential (time zone offset).
    /// </summary>
    /// <param name="s">A string containing an RFC 822 date and time to convert.</param>
    /// <returns>A date and time string that uses local differential to describe the time zone equivalent to the date and time contained in <paramref name="s"/>.</returns>
    /// <exception cref="ArgumentNullException"><paramref name="s"/> is a <b>null</b> reference (Nothing in Visual Basic).</exception>
    /// <exception cref="ArgumentNullException"><paramref name="s"/> is an empty string.</exception>
    public static string ConvertZoneToLocalDifferential(string s)
    {
        string zoneRepresentedAsLocalDifferential   = String.Empty;

        //------------------------------------------------------------
        //  Validate parameter
        //------------------------------------------------------------
        if (String.IsNullOrEmpty(s))
        {
          throw new ArgumentNullException("s");
        }

        if(s.EndsWith(" UT", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" UT") + 1) ), "+00:00");
        }
        else if (s.EndsWith(" GMT", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" GMT") + 1 ) ), "+00:00");
        }
        else if (s.EndsWith(" EST", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" EST") + 1)), "-05:00");
        }
        else if (s.EndsWith(" EDT", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" EDT") + 1)), "-04:00");
        }
        else if (s.EndsWith(" CST", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" CST") + 1)), "-06:00");
        }
        else if (s.EndsWith(" CDT", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" CDT") + 1)), "-05:00");
        }
        else if (s.EndsWith(" MST", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" MST") + 1)), "-07:00");
        }
        else if (s.EndsWith(" MDT", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" MDT") + 1)), "-06:00");
        }
        else if (s.EndsWith(" PST", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" PST") + 1)), "-08:00");
        }
        else if (s.EndsWith(" PDT", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" PDT") + 1)), "-07:00");
        }
        else if (s.EndsWith(" Z", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" Z") + 1)), "+00:00");
        }
        else if (s.EndsWith(" A", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" A") + 1)), "-01:00");
        }
        else if (s.EndsWith(" M", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" M") + 1)), "-12:00");
        }
        else if (s.EndsWith(" N", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" N") + 1)), "+01:00");
        }
        else if (s.EndsWith(" Y", StringComparison.OrdinalIgnoreCase))
        {
            zoneRepresentedAsLocalDifferential  = String.Concat(s.Substring(0, (s.LastIndexOf(" Y") + 1)), "+12:00");
        }
        else
        {
            zoneRepresentedAsLocalDifferential  = s;
        }

        return zoneRepresentedAsLocalDifferential;
    }
    #endregion

    #region ToString(DateTime utcDateTime)
    /// <summary>
    /// Converts the value of the specified <see cref="DateTime"/> object to its equivalent string representation.
    /// </summary>
    /// <param name="utcDateTime">The Coordinated Universal Time (UTC) <see cref="DateTime"/> to convert.</param>
    /// <returns>A RFC 822 string representation of the value of the <paramref name="utcDateTime"/>.</returns>
    /// <exception cref="ArgumentException">The specified <paramref name="utcDateTime"/> object does not represent a <see cref="DateTimeKind.Utc">Coordinated Universal Time (UTC)</see> value.</exception>
    public static string ToString(DateTime utcDateTime)
    {
        if (utcDateTime.Kind != DateTimeKind.Utc)
        {
            throw new ArgumentException("utcDateTime");
        }

        return utcDateTime.ToString(Rfc822DateTime.Rfc822DateTimeFormat, DateTimeFormatInfo.InvariantInfo);
    }
    #endregion

    #region TryParse(string s, out DateTime result)
    /// <summary>
    /// Converts the specified string representation of a date and time to its <see cref="DateTime"/> equivalent.
    /// </summary>
    /// <param name="s">A string containing a date and time to convert.</param>
    /// <param name="result">
    /// When this method returns, contains the <see cref="DateTime"/> value equivalent to the date and time 
    /// contained in <paramref name="s"/>, expressed as <i>Coordinated Universal Time (UTC)</i>, 
    /// if the conversion succeeded, or <see cref="DateTime.MinValue">MinValue</see> if the conversion failed. 
    /// The conversion fails if the s parameter is a <b>null</b> reference (Nothing in Visual Basic), 
    /// or does not contain a valid string representation of a date and time. 
    /// This parameter is passed uninitialized.
    /// </param>
    /// <returns><b>true</b> if the <paramref name="s"/> parameter was converted successfully; otherwise, <b>false</b>.</returns>
    /// <remarks>
    /// The string <paramref name="s"/> is parsed using formatting information in the <see cref="DateTimeFormatInfo.InvariantInfo"/> object. 
    /// </remarks>
    public static bool TryParse(string s, out DateTime result)
    {
        //------------------------------------------------------------
        //  Attempt to convert string representation
        //------------------------------------------------------------
        bool wasConverted   = false;
        result              = DateTime.MinValue;

        if (!String.IsNullOrEmpty(s))
        {
            DateTime parseResult;
            if (DateTime.TryParseExact(Rfc822DateTime.ConvertZoneToLocalDifferential(s), Rfc822DateTime.Rfc822DateTimePatterns, DateTimeFormatInfo.InvariantInfo, DateTimeStyles.AdjustToUniversal, out parseResult))
            {
                result          = DateTime.SpecifyKind(parseResult, DateTimeKind.Utc);
                wasConverted    = true;
            }
        }

        return wasConverted;
    }
    #endregion
}

Answer 3

按照 Kirk 的想法，我反编译了 System.ServiceModel.Synmination.Rss20FeedFormatter 类 (System.ServiceModel.dll) 的源代码，这里是 RFC 822 日期的 Microsoft 内部解析器格式（我稍微简化了他们的异常处理逻辑以减少依赖性）：

public static class DateTimeParser
{
    public static DateTimeOffset ParseDateTimeRFC822(string dateTimeString)
    {
        StringBuilder dateTimeStringBuilder = new StringBuilder(dateTimeString.Trim());
        if (dateTimeStringBuilder.Length < 18)
        {
            throw new FormatException("Invalid date format. Expected date in RFC 822 format");
        }
        if (dateTimeStringBuilder[3] == ',')
        {
            // There is a leading (e.g.) "Tue, ", strip it off
            dateTimeStringBuilder.Remove(0, 4);
            // There's supposed to be a space here but some implementations dont have one
            RemoveExtraWhiteSpaceAtStart(dateTimeStringBuilder);
        }
        ReplaceMultipleWhiteSpaceWithSingleWhiteSpace(dateTimeStringBuilder);
        if (char.IsDigit(dateTimeStringBuilder[1]))
        {
            // two-digit day, we are good
        }
        else
        {
            dateTimeStringBuilder.Insert(0, '0');
        }
        if (dateTimeStringBuilder.Length < 19)
        {
            throw new FormatException("Invalid date format. Expected date in RFC 822 format");
        }
        bool thereAreSeconds = (dateTimeStringBuilder[17] == ':');
        int timeZoneStartIndex;
        if (thereAreSeconds)
        {
            timeZoneStartIndex = 21;
        }
        else
        {
            timeZoneStartIndex = 18;
        }
        string timeZoneSuffix = dateTimeStringBuilder.ToString().Substring(timeZoneStartIndex);
        dateTimeStringBuilder.Remove(timeZoneStartIndex, dateTimeStringBuilder.Length - timeZoneStartIndex);
        bool isUtc;
        dateTimeStringBuilder.Append(NormalizeTimeZone(timeZoneSuffix, out isUtc));
        string wellFormattedString = dateTimeStringBuilder.ToString();

        DateTimeOffset theTime;
        string parseFormat;
        if (thereAreSeconds)
        {
            parseFormat = "dd MMM yyyy HH:mm:ss zzz";
        }
        else
        {
            parseFormat = "dd MMM yyyy HH:mm zzz";
        }
        if (DateTimeOffset.TryParseExact(wellFormattedString, parseFormat,
            CultureInfo.InvariantCulture.DateTimeFormat,
            (isUtc ? DateTimeStyles.AdjustToUniversal : DateTimeStyles.None), out theTime))
        {
            return theTime;
        }
        throw new FormatException("Invalid date format. Expected date in RFC 822 format");
    }

    static string NormalizeTimeZone(string rfc822TimeZone, out bool isUtc)
    {
        isUtc = false;
        // return a string in "-08:00" format
        if (rfc822TimeZone[0] == '+' || rfc822TimeZone[0] == '-')
        {
            // the time zone is supposed to be 4 digits but some feeds omit the initial 0
            StringBuilder result = new StringBuilder(rfc822TimeZone);
            if (result.Length == 4)
            {
                // the timezone is +/-HMM. Convert to +/-HHMM
                result.Insert(1, '0');
            }
            result.Insert(3, ':');
            return result.ToString();
        }
        switch (rfc822TimeZone)
        {
            case "UT":
            case "Z":
                isUtc = true;
                return "-00:00";
            case "GMT":
                return "-00:00";
            case "A":
                return "-01:00";
            case "B":
                return "-02:00";
            case "C":
                return "-03:00";
            case "D":
            case "EDT":
                return "-04:00";
            case "E":
            case "EST":
            case "CDT":
                return "-05:00";
            case "F":
            case "CST":
            case "MDT":
                return "-06:00";
            case "G":
            case "MST":
            case "PDT":
                return "-07:00";
            case "H":
            case "PST":
                return "-08:00";
            case "I":
                return "-09:00";
            case "K":
                return "-10:00";
            case "L":
                return "-11:00";
            case "M":
                return "-12:00";
            case "N":
                return "+01:00";
            case "O":
                return "+02:00";
            case "P":
                return "+03:00";
            case "Q":
                return "+04:00";
            case "R":
                return "+05:00";
            case "S":
                return "+06:00";
            case "T":
                return "+07:00";
            case "U":
                return "+08:00";
            case "V":
                return "+09:00";
            case "W":
                return "+10:00";
            case "X":
                return "+11:00";
            case "Y":
                return "+12:00";
            default:
                return "";
        }
    }

    static void RemoveExtraWhiteSpaceAtStart(StringBuilder stringBuilder)
    {
        int i = 0;
        while (i < stringBuilder.Length)
        {
            if (!char.IsWhiteSpace(stringBuilder[i]))
            {
                break;
            }
            ++i;
        }
        if (i > 0)
        {
            stringBuilder.Remove(0, i);
        }
    }

    static void ReplaceMultipleWhiteSpaceWithSingleWhiteSpace(StringBuilder builder)
    {
        int index = 0;
        int whiteSpaceStart = -1;
        while (index < builder.Length)
        {
            if (char.IsWhiteSpace(builder[index]))
            {
                if (whiteSpaceStart < 0)
                {
                    whiteSpaceStart = index;
                    // normalize all white spaces to be ' ' so that the date time parsing works
                    builder[index] = ' ';
                }
            }
            else if (whiteSpaceStart >= 0)
            {
                if (index > whiteSpaceStart + 1)
                {
                    // there are at least 2 spaces... replace by 1
                    builder.Remove(whiteSpaceStart, index - whiteSpaceStart - 1);
                    index = whiteSpaceStart + 1;
                }
                whiteSpaceStart = -1;
            }
            ++index;
        }
        // we have already trimmed the start and end so there cannot be a trail of white spaces in the end
        Debug.Assert(builder.Length == 0 || builder[builder.Length - 1] != ' ', "The string builder doesnt end in a white space");
    }
}

第一个看起来不寻常的事情是它们返回 [DateTimeOffset][1] 类而不是 DateTime< /代码>。 但当我们阅读更多相关内容时，它似乎完全合乎逻辑 - DateTimeOffset 存储日期、时间和时区信息（与 RFC 822 格式的字符串完全相同）。 如果您仅返回 DateTime 对象，则该对象位于哪个时区：UTC、本地或解析字符串中指定的时区 - 在某些情况下任何答案都将是错误的。 因此 DateTimeOffset 解决了一个重要的不确定性问题。 您可以使用 DateTimeOffset.ToUniversalTime()、DateTimeOffset.ToLocalTime() 方法将其转换为稍后需要的时区。

我在几个案例中测试了它，看起来它完美地完成了工作。

但我不确定为什么微软决定将这个实现私有化——它似乎不需要很多支持。

Answer 4

以下是 Microsoft 在 Rss20FeedFormatter 中的做法。 Oppositional 的代码并没有去掉 GMT 偏移部分中的“:”。 杰夫·伍德曼 (Jeff Woodman) 似乎就是这么做的。 下面的代码也执行此操作（如果不使用 Atom10FeedFormatter.zeroOffset）。

private string AsString(DateTimeOffset dateTime)
{
    if (dateTime.Offset == Atom10FeedFormatter.zeroOffset)
    {
        return dateTime.ToUniversalTime().ToString("ddd, dd MMM yyyy HH:mm:ss Z", CultureInfo.InvariantCulture);
    }
    StringBuilder builder = new StringBuilder(dateTime.T)oString("ddd, dd MMM yyyy HH:mm:ss zzz", CultureInfo.InvariantCulture));
    builder.Remove(builder.Length - 3, 1);
    return builder.ToString();
}

Answer 5

这是我使用扩展方法的实现：

namespace MyNamespace
{
    public static partial class ExtensionMethods
    {
        public static string ToRFC822String(this DateTime timestamp)
        {
            return timestamp.ToString("ddd',' d MMM yyyy HH':'mm':'ss")
                + " "
                + timestamp.ToString("zzzz").Replace(":", "");
        }
    }
 }

使用：

 using MyNamespace;

 ....

 string MyRFC822String = DateTime.Now.ToRFC822String();

Answer 6

根据 Kirk Liemohn 的回答，我成功地使用了这种方法：

private DateTimeOffset? ParseDate(string date)
    {
        const string FORMAT = "ddd, d MMM yyyy HH:mm:ss zzz";
        const string FORMAT2 = "ddd, dd MMM yyyy HH:mm:ss zzz";
        const string FORMAT3 = "dd MMM yyyy HH:mm:ss zzz";
        const string FORMAT4 = "d MMM yyyy HH:mm:ss zzz";
        DateTimeOffset d;
        if (DateTimeOffset.TryParseExact(date, new string[] { FORMAT, FORMAT2, FORMAT3, FORMAT4 }, CultureInfo.InvariantCulture, DateTimeStyles.AllowLeadingWhite | DateTimeStyles.AllowTrailingWhite, out d))
            return d;
        return null;
    }

示例：

DateTimeOffset? date = ParseDate("Thu, 5 Apr 2012 23:47:37 +0200");
Console.WriteLine(date.ToString());
// => 05/04/2012 23:47:37 +02:00

它不尊重 RFC 的完整规范，但它适用于我的用例。

具体来说，它不适用于时区表达，如：“GMT”、“CST”等（请参阅 RFC822 第 5.1 节中的“区域”）。 请参阅 Oleksandr Pshenychnyy 的更好答案。