指定DateTime.ToString（“o”）小数精度

Question

我有一个DateTime对象，我试图以ISO 8601兼容格式输出到xml文件中，以便在两个系统之间进行传输 - 我们无法控制收件人。 .net round trip format在很大程度上满足了这个要求，但强制精度为7dp。

有没有办法可以指明这个？例如像"o:0"这样的东西完全省略毫秒小数位，或者"o:3"将其设置为3dp。我知道我可以用yyyy'-'MM'-'dd'T'HH':'mm':'ssK（或yyyy'-'MM'-'dd'T'HH':'mm':'ss.fffK）自己格式化输出作为自定义格式说明符，以便在必要时控制小数位，但是想看看我是否错过了将小数精度简单地传递给"o"格式说明符的方法。

Answer 1

似乎答案是否定的，没有（至少在核心框架中）定制它。看看source code for DateTime roundtrip formatting这是硬编码到7dp：

// Omitted for brevity
...
AppendHHmmssTimeOfDay(result, dateTime);
result.Append('.');

long fraction = dateTime.Ticks % TimeSpan.TicksPerSecond;
AppendNumber(result, fraction, 7);

如果有人感兴趣，我的解决方案是实现一个自定义的ToFormattedString()扩展方法来处理这个问题，如果需要的话，它将用自定义格式字符串替换格式字符串，并用以下方法调用ToString：

using System.Globalization; // System.Globalization needed for IFormatProvider overload
using System.Text; // System.Text required for StringBuilder class

namespace Extensions
{
    public static class DateTimeFormatExtension
    {
        // Consts for building the custom format string
        private const string ROUNDTRIP_FORMAT_PREFIX = "yyyy'-'MM'-'dd'T'HH':'mm':'ss";
        private const char ROUNDTRIP_FORMAT_FRACTION = 'f';
        private const char ROUNDTRIP_FORMAT_SUFFIX = 'K';

        // Appending the 'f' custom format string maxes out at "fffffff"(7 dp) and will throw an exception if given more 
        private const int DATETIME_MAX_DP = 7;

        private static int GetRoundtripLength(int decimalPlaces) =>
            ROUNDTRIP_FORMAT_PREFIX.Length + decimalPlaces + 2; // +2 to account for the '.' and the 'K' suffix

        public static string ToFormattedString(this DateTime input) => input.ToString();

        public static string ToFormattedString(this DateTime input, string format)
        {
            var provider = DateTimeFormatInfo.CurrentInfo;
            return input.ToFormattedString(format, provider);
        }

                    public static string ToFormattedString(this DateTime input, string format, IFormatProvider provider)
        {
            string parsedFormat = format;
            if (!string.IsNullOrWhiteSpace(format))
            {
                switch (format[0])
                {
                    case 'o':
                    case 'O':
                        var precision = format.Substring(1);
                        // Only do this if we have a custom 'o' string, otherwise us the base functionality
                        if (!string.IsNullOrWhiteSpace(precision))
                        {
                            // If the custom addition to the format string is an integer, use that to determine dp
                            if (int.TryParse(precision, out int decimalCount))
                            {
                                // Build the format string
                                var formatBuilder = new StringBuilder(GetRoundtripLength(decimalCount));
                                formatBuilder.Append(ROUNDTRIP_FORMAT_PREFIX);

                                // Append '.' and 'f' chars to format string (Append nothing if 0 dp)
                                if (decimalCount > 0)
                                {
                                    formatBuilder
                                        .Append('.')
                                        .Append(ROUNDTRIP_FORMAT_FRACTION,
                                            // Cap max dp length to avoid exceptions
                                            Math.Min(decimalCount, DATETIME_MAX_DP));
                                }

                                // Append 'K' suffix
                                formatBuilder.Append(ROUNDTRIP_FORMAT_SUFFIX);
                                parsedFormat = formatBuilder.ToString();
                            }
                        }
                        break;
                    default:
                        break;
                }
            }
            return input.ToString(parsedFormat, provider);
        }
    }
}

然后可以像这样使用：

using Extensions;
...
DateTime currentDateTime = DateTime.UtcNow;
string result;
result = currentDateTime.ToFormattedString(); // empty just calls the default ToString()
// result: "2018-03-02 12:31:17 AM"
result = currentDateTime.ToFormattedString("dd/MM/yy ssmmhh"); // custom format strings still work
// result: "02-03-18 173112"
result = currentDateTime.ToFormattedString("d"); // standard format strings still work
// result: "2018-03-02"
result = currentDateTime.ToFormattedString("D");
// result: "Friday, March 2, 2018"
result = currentDateTime.ToFormattedString("F");
// result: "Friday, March 2, 2018 12:31:17 AM"
result = currentDateTime.ToFormattedString("o"); // standard format specifier uses default ToString("o") behaviour
// result: "2018-03-02T00:31:17.9818727Z"
result = currentDateTime.ToFormattedString("o0"); // no decimal places
// result: "2018-03-02T00:31:17Z"
result = currentDateTime.ToFormattedString("o3"); // 3 decimal places
// result: "2018-03-02T00:31:17.981Z"
result = currentDateTime.ToFormattedString("o100"); // too many decimals cap at 7
// result: "2018-03-02T00:31:17.9818727Z"

编辑：更新以从格式字符串中删除“：”以符合在numeric formatting上指定小数精度的方式：

标准数字格式字符串采用Axx形式，其中：

A是一个称为格式说明符的单个字母字符。

...

xx是一个可选的整数，称为精度说明符