blazor-calendar/BlazorApp/Data/Event.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using NodaTime;

namespace BlazorApp.Data
{
    public class Event
    {
        public string Uid { get; set; }
        public string Summary { get; set; }
        public DateTime? DtStart { get; set; }
        public DateTime? DtEnd { get; set; }
        public string Description { get; set; }
        public TimeSpan? Duration { get; set; }

        public List<(string, string, string)> RawProperties { get; } = new();
        public string ETag { get; set; }
        public string Href { get; set; }

        public DateTime? CalculatedEnd
        {
            get => DtEnd ?? DtStart + Duration;
            set
            {
                if (Duration != null)
                    Duration = DtStart - value;
                else
                    DtEnd = value;
            }
        }

        private static string _escape(string s)
        {
            if (s == null)
                return null;

            return s
                .Replace(@"\", @"\\")
                .Replace(",", @"\,")
                .Replace(";", @"\;")
                .Replace("\n", @"\n");
        }

        public string ToICal()
        {
            var sb = new StringBuilder();
            sb.Append("BEGIN:VCALENDAR\r\n");
            sb.Append("VERSION:2.0\r\n");
            sb.Append("PRODID:-//Sijmen//blazor-calendar//EN\r\n");
            sb.Append("BEGIN:VEVENT\r\n");

            var seenNames = new HashSet<string>();

            foreach (var (name, parameters, oldValue) in RawProperties)
            {
                var value = name switch
                {
                    "SUMMARY" => _escape(Summary),
                    "DESCRIPTION" => _escape(Description),
                    "DTSTART" => FormatDate(parameters, DtStart),
                    "DTEND" => FormatDate(parameters, DtEnd),
                    _ => oldValue
                };

                if (value != null)
                {
                    sb.Append(name);
                    if (parameters != null)
                        sb.Append($";{parameters}");
                    sb.Append($":{value}\r\n");
                }

                seenNames.Add(name);
            }

            // If one of the event's properties did not exist yet, add it to the end.
            if (!seenNames.Contains("SUMMARY") && !string.IsNullOrWhiteSpace(Summary))
                sb.Append($"SUMMARY:{_escape(Summary)}\r\n");
            if (!seenNames.Contains("DESCRIPTION") && !string.IsNullOrWhiteSpace(Description))
                sb.Append($"DESCRIPTION:{_escape(Description)}\r\n");
            if (!seenNames.Contains("DTSTART") && DtStart != null)
                sb.Append($"DTSTART:{DtStart?.ToUniversalTime().ToString(Calendar.DateTimeFormat)}\r\n");
            if (!seenNames.Contains("DTEND") && !seenNames.Contains("DURATION") && CalculatedEnd != null)
                sb.Append($"DTEND:{DtStart?.ToUniversalTime().ToString(Calendar.DateTimeFormat)}\r\n");

            sb.Append("END:VEVENT\r\n");
            sb.Append("END:VCALENDAR\r\n");
            return sb.ToString();
        }

        private static string FormatDate(string parameters, DateTime? dateTime)
        {
            if (dateTime == null)
                return null;

            DateTimeZone timezone;
            if (parameters != null)
            {
                // If there's parameters, see if we can find a timezone, or default to UTC.
                var parameterList = parameters.Split('=');
                var timezoneIana =
                    Array.Find(parameterList, s => s.StartsWith("TZID="))?[5..] ?? "UTC";
                timezone = DateTimeZoneProviders.Tzdb[timezoneIana];
            }
            else
            {
                // If there's no parameters, default to UTC.
                timezone = DateTimeZone.Utc;
            }

            // Convert the datetime to the used timezone, to avoid changing it.
            return Instant
                .FromDateTimeUtc(((DateTime) dateTime).ToUniversalTime())
                .InZone(timezone)
                .ToString(Calendar.DateTimeFormat, CultureInfo.InvariantCulture);
        }
    }
}