Change RecurrencePattern.Until to Nullable<DateTime>

Ical.Net.Tests/RecurrenceTests.cs

@@ -3570,7 +3570,7 @@ public void UntilTimeZoneSerializationTests(string tzId, DateTimeKind expectedKi
 
         Assert.That(serialized.Contains(expectedContains), Is.True);
 
-        var deserializedKind = Calendar.Load(serialized).Events.First().RecurrenceRules.First().Until.Kind;
+        var deserializedKind = Calendar.Load(serialized).Events.First().RecurrenceRules.First().Until?.Kind;
 
         Assert.That(deserializedKind, Is.EqualTo(expectedKind));
     }

Ical.Net/DataTypes/RecurrencePattern.cs

@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.IO;
@@ -26,20 +27,7 @@ public class RecurrencePattern : EncodableDataType
 #pragma warning restore 0618
     public FrequencyType Frequency { get; set; }
 
-    private DateTime _until = DateTime.MinValue;
-    public DateTime Until
-    {
-        get => _until;
-        set
-        {
-            if (_until == value && _until.Kind == value.Kind)
-            {
-                return;
-            }
-
-            _until = value;
-        }
-    }
+    public DateTime? Until { get; set; }
 
     public int Count { get; set; } = int.MinValue;
 
@@ -146,7 +134,8 @@ public RecurrencePattern(string value) : this()
             return;
         }
         var serializer = new RecurrencePatternSerializer();
-        CopyFrom(serializer.Deserialize(new StringReader(value)) as ICopyable);
+        if (serializer.Deserialize(new StringReader(value)) is ICopyable deserialized)
+            CopyFrom(deserialized);
     }
 
     public override string ToString()
@@ -174,7 +163,7 @@ protected bool Equals(RecurrencePattern other) => (Interval == other.Interval)
                                                       && CollectionEquals(ByMonth, other.ByMonth)
                                                       && CollectionEquals(BySetPosition, other.BySetPosition);
 
-    public override bool Equals(object obj)
+    public override bool Equals(object? obj)
     {
         if (ReferenceEquals(null, obj)) return false;
         if (ReferenceEquals(this, obj)) return true;
@@ -237,4 +226,4 @@ public override void CopyFrom(ICopyable obj)
     }
 
     private static bool CollectionEquals<T>(IEnumerable<T> c1, IEnumerable<T> c2) => c1.SequenceEqual(c2);
-}
+}

Ical.Net/Evaluation/RecurrencePatternEvaluator.cs

@@ -29,9 +29,9 @@ private RecurrencePattern ProcessRecurrencePattern(IDateTime referenceDate)
         r.CopyFrom(Pattern);
 
         // Convert the UNTIL value to one that matches the same time information as the reference date
-        if (r.Until != DateTime.MinValue)
+        if (r.Until is not null)
         {
-            r.Until = MatchTimeZone(referenceDate, r.Until);
+            r.Until = MatchTimeZone(referenceDate, r.Until.Value);
         }
 
         if (referenceDate.HasTime)
@@ -243,10 +243,10 @@ private IEnumerable<DateTime> EnumerateDates(DateTime originalDate, DateTime see
 
         var noCandidateIncrementCount = 0;
         var candidate = DateTime.MinValue;
-        int dateCount = 0;
+        var dateCount = 0;
         while (maxCount < 0 || dateCount < maxCount)
         {
-            if (pattern.Until != DateTime.MinValue && candidate != DateTime.MinValue && candidate > pattern.Until)
+            if (pattern.Until is not null && candidate != DateTime.MinValue && candidate > pattern.Until)
             {
                 break;
             }
@@ -291,7 +291,7 @@ private IEnumerable<DateTime> EnumerateDates(DateTime originalDate, DateTime see
                         continue;
                     }
 
-                    if (pattern.Until == DateTime.MinValue || candidate <= pattern.Until)
+                    if (pattern.Until is null || candidate <= pattern.Until)
                     {
                         yield return candidate;
                         dateCount++;

Ical.Net/Serialization/DataTypes/RecurrencePatternSerializer.cs

@@ -83,9 +83,10 @@ public virtual void CheckMutuallyExclusive<T, TU>(string name1, string name2, T
         // that to be unassigned.
 
         var t1 = obj1.GetType();
+        var t2 = obj2.GetType();
 
         var fi1 = t1.GetField("MinValue");
-        var fi2 = t1.GetField("MinValue");
+        var fi2 = t2.GetField("MinValue");
 
         var isMin1 = fi1 != null && obj1.Equals(fi1.GetValue(null));
         var isMin2 = fi2 != null && obj2.Equals(fi2.GetValue(null));
@@ -142,13 +143,13 @@ public override string SerializeToString(object obj)
             values.Add("INTERVAL=" + interval);
         }
 
-        if (recur.Until != DateTime.MinValue)
+        if (recur.Until is not null)
         {
             var serializer = factory.Build(typeof(IDateTime), SerializationContext) as IStringSerializer;
             if (serializer != null)
             {
-                var until = new CalDateTime(DateOnly.FromDateTime(recur.Until), TimeOnly.FromDateTime(recur.Until),
-                    recur.Until.Kind == DateTimeKind.Utc ? "UTC" : null);
+                var until = new CalDateTime(DateOnly.FromDateTime(recur.Until.Value), TimeOnly.FromDateTime(recur.Until.Value),
+                    recur.Until.Value.Kind == DateTimeKind.Utc ? "UTC" : null);
 
                 values.Add("UNTIL=" + serializer.SerializeToString(until));
             }