Update PRODID and VERSION property handling

Issue: `Calendar` has setters for `ProductId` and `Version` which are overridden with fixed values when serializing.

- When creating a new `Calendar` instance, `ProductId` and `Version` contain default values
- When Deserializing an iCalendar, `ProductId` and `Version` will be taken from the input
- `ProductId` and `Version` can be overridden by user code. An attempt to set as an empty string will throw.
- Update the default `PRODID` property `LibraryMetadata.ProdId` to include the ical.net assembly version. Example: "PRODID:-//github.com/ical-org/ical.net//NONSGML ical.net 5.4.3//EN"
- Modified `CalendarSerializer.SerializeToString` so that the `ProdId` or `Version` set by users do not get overridden
- Add an xmldoc description about the purpose of `ProdId` and `Version`, and about the risks when modified
- Add unit tests

Resolves #531

Ical.Net.Tests/DeserializationTests.cs

@@ -588,4 +588,28 @@ public void KeepApartDtEndAndDuration_Tests(bool useDtEnd)
             Assert.That(calendar.Events.Single().Duration != null, Is.EqualTo(!useDtEnd));
         });
     }
+
+    [Test]
+    public void CalendarWithMissingProdIdOrVersion_ShouldLeavePropertiesInvalid()
+    {
+        var ics = """
+                      BEGIN:VCALENDAR
+                      BEGIN:VEVENT
+                      DTSTART:20070406T230000Z
+                      DTEND:20070407T010000Z
+                      END:VEVENT
+                      END:VCALENDAR
+                      """;
+
+        var calendar = Calendar.Load(ics);
+        var deserialized = new CalendarSerializer(calendar).SerializeToString();
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(calendar.ProductId, Is.EqualTo(null));
+            Assert.That(calendar.Version, Is.EqualTo(null));
+            // The serialized calendar should not contain the PRODID or VERSION properties, which are required
+            Assert.That(deserialized, Does.Not.Contain("PRODID:").And.Not.Contains("VERSION:"));
+        });
+    }
 }

Ical.Net.Tests/SerializationTests.cs

@@ -525,20 +525,44 @@ public void TestRRuleUntilSerialization()
         Assert.That(!until.EndsWith("Z"), Is.True);
     }
 
-    [Test(Description = "PRODID and VERSION should use ical.net values instead of preserving deserialized values")]
-    public void LibraryMetadataTests()
+    [Test]
+    public void ProductId_and_Version_CanBeChanged()
     {
         var c = new Calendar
         {
             ProductId = "FOO",
-            Version = "BAR"
+            Version = "BAR",
         };
+
         var serialized = new CalendarSerializer().SerializeToString(c);
-        var expectedProdid = $"PRODID:{LibraryMetadata.ProdId}";
-        Assert.That(serialized.Contains(expectedProdid, StringComparison.Ordinal), Is.True);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(serialized, Does.Contain($"PRODID:{c.ProductId}"));
+            Assert.That(serialized, Does.Contain($"VERSION:{c.Version}"));
+        });
+    }
+
+    [Test]
+    public void ProductId_and_Version_CannotBeSetAsEmpty()
+    {
+        var c = new Calendar();
+        Assert.Multiple(() =>
+        {
+            Assert.That(() => c.ProductId = string.Empty, Throws.ArgumentException);
+            Assert.That(() => c.Version = string.Empty, Throws.ArgumentException);
+        });
+    }
 
-        var expectedVersion = $"VERSION:{LibraryMetadata.Version}";
-        Assert.That(serialized.Contains(expectedVersion, StringComparison.Ordinal), Is.True);
+    [Test]
+    public void ProductId_and_Version_HaveDefaultValues()
+    {
+        var c = new Calendar();
+        Assert.Multiple(() =>
+        {
+            Assert.That(c.ProductId, Is.EqualTo(LibraryMetadata.ProdId));
+            Assert.That(c.Version, Is.EqualTo(LibraryMetadata.Version));
+        });
     }
 
     [Test]

Ical.Net/Calendar.cs

@@ -60,12 +60,16 @@ public static IList<T> Load<T>(string ical)
     /// </summary>
     public Calendar()
     {
-        Name = Components.Calendar;
+        // Note: ProductId and Version Property values will be empty before _deserialization_
+        ProductId = LibraryMetadata.ProdId;
+        Version = LibraryMetadata.Version;
+
         Initialize();
     }
 
     private void Initialize()
     {
+        Name = Components.Calendar;
         _mUniqueComponents = new UniqueComponentListProxy<IUniqueComponent>(Children);
         _mEvents = new UniqueComponentListProxy<CalendarEvent>(Children);
         _mTodos = new UniqueComponentListProxy<Todo>(Children);
@@ -143,20 +147,54 @@ public override int GetHashCode()
     public virtual ICalendarObjectList<VTimeZone> TimeZones => _mTimeZones;
 
     /// <summary>
-    /// A collection of <see cref="Todo"/> components in the iCalendar.
+    /// A collection of <see cref="CalendarComponents.Todo"/> components in the iCalendar.
     /// </summary>
     public virtual IUniqueComponentList<Todo> Todos => _mTodos;
 
+    /// <summary>
+    /// Gets or sets the version of the iCalendar definition. The default is <see cref="LibraryMetadata.Version"/>
+    /// as per RFC 5545 Section 3.7.4 and must be specified.
+    /// <para/>
+    /// It specifies the identifier corresponding to the highest version number of the iCalendar specification
+    /// that is required in order to interpret the iCalendar object.
+    /// <para/>
+    /// <b>Do not change unless you are sure about the consequences.</b>
+    /// <para/>
+    /// The default value does not apply to deserialized objects.
+    /// </summary>
     public virtual string Version
     {
         get => Properties.Get<string>("VERSION");
-        set => Properties.Set("VERSION", value);
+        set
+        {
+            if (string.IsNullOrEmpty(value))
+            {
+                throw new ArgumentException("Version to set must not be null or empty");
+            }
+            Properties.Set("VERSION", value);
+        }
     }
 
+    /// <summary>
+    /// Gets or sets the product ID of the iCalendar, which typically contains the name of the software
+    /// that created the iCalendar. The default is <see cref="LibraryMetadata.ProdId"/>.
+    /// <para/>
+    /// <b>Be careful when setting a custom value</b>, as it is free-form text that must conform to the iCalendar specification
+    /// (RFC 5545 Section 3.7.3). The product ID must be specified.
+    /// <para/>
+    /// The default value does not apply to deserialized objects.
+    /// </summary>
     public virtual string ProductId
     {
         get => Properties.Get<string>("PRODID");
-        set => Properties.Set("PRODID", value);
+        set
+        {
+            if (string.IsNullOrEmpty(value))
+            {
+                throw new ArgumentException("Product ID to set must not be null or empty");
+            }
+            Properties.Set("PRODID", value);
+        }
     }
 
     public virtual string Scale

Ical.Net/Constants.cs

@@ -3,7 +3,9 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
+using System.Diagnostics;
 
 namespace Ical.Net;
 
@@ -124,7 +126,7 @@ public class SerializationConstants
 }
 
 /// <summary>
-/// Status codes available to an <see cref="Components.Event"/> item
+/// Status codes available to an <see cref="CalendarComponents.CalendarEvent"/> item
 /// </summary>
 public static class EventStatus
 {
@@ -137,7 +139,7 @@ public static class EventStatus
 }
 
 /// <summary>
-/// Status codes available to a <see cref="Todo"/> item.
+/// Status codes available to a <see cref="CalendarComponents.Todo"/> item.
 /// </summary>
 public static class TodoStatus
 {
@@ -152,7 +154,7 @@ public static class TodoStatus
 }
 
 /// <summary>
-/// Status codes available to a <see cref="Journal"/> entry.
+/// Status codes available to a <see cref="CalendarComponents.Journal"/> entry.
 /// </summary>
 public static class JournalStatus
 {
@@ -235,8 +237,32 @@ public static class TransparencyType
 
 public static class LibraryMetadata
 {
+    private static readonly string _assemblyVersion = GetAssemblyVersion();
+
+    /// <summary>
+    /// The <c>VERSION</c> property for iCalendar objects generated by this library (RFC 5545 Section 3.7.4),
+    /// unless overridden by user code.
+    /// </summary>
     public const string Version = "2.0";
-    public static readonly string ProdId = "-//github.com/ical-org/ical.net//NONSGML ical.net 4.0//EN";
+
+    /// <summary>
+    /// The default <c>PRODID</c> property for iCalendar objects generated by this library (RFC 5545 Section 3.7.3),
+    /// unless overridden by user code.
+    /// <remarks>
+    /// The text between the double slashes represents the organization or software that created the iCalendar object.
+    /// </remarks>
+    /// </summary>
+    public static string ProdId => $"-//github.com/ical-org/ical.net//NONSGML ical.net {_assemblyVersion}//EN";
+
+    private static string GetAssemblyVersion()
+    {
+        var assembly = typeof(LibraryMetadata).Assembly;
+        var fileVersionInfo = FileVersionInfo.GetVersionInfo(assembly.Location);
+        // Prefer the file version, but fall back to the assembly version if it's not available.
+        return fileVersionInfo.FileVersion
+               ?? assembly.GetName().Version?.ToString() // will only change for major versions
+               ?? "1.0.0.0";
+    }
 }
 
 public static class CalendarScales

Ical.Net/Serialization/CalendarSerializer.cs

@@ -27,20 +27,6 @@ public CalendarSerializer(SerializationContext ctx) : base(ctx) { }
 
     protected override IComparer<ICalendarProperty> PropertySorter => new CalendarPropertySorter();
 
-    public override string SerializeToString(object obj)
-    {
-        if (obj is Calendar)
-        {
-            // If we're serializing a calendar, we should indicate that we're using ical.net to do the work
-            var calendar = (Calendar) obj;
-            calendar.Version = LibraryMetadata.Version;
-            calendar.ProductId = LibraryMetadata.ProdId;
-
-            return base.SerializeToString(calendar);
-        }
-
-        return base.SerializeToString(obj);
-    }
 
     public override object Deserialize(TextReader tr) => null;
 
@@ -70,4 +56,4 @@ public int Compare(ICalendarProperty x, ICalendarProperty y)
                 : string.Compare(x.Name, y.Name, StringComparison.OrdinalIgnoreCase);
         }
     }
-}
+}