Use UTF-8 Encoding Without BOM by Default in All Serializers

Ical.Net.Tests/SerializationTests.cs

@@ -7,6 +7,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Globalization;
+using System.IO;
 using System.Linq;
 using System.Text;
 using System.Text.RegularExpressions;
@@ -611,4 +612,35 @@ public void TestConvertToInt32WithNegativeNumberInDifferentCultures(string cultu
         }
     }
 
+    [Test]
+    public void CalendarSerialization_ShouldDefaultTo_Utf8NoBom()
+    {
+        var calendar = new Calendar();
+        calendar.Events.Add(new CalendarEvent
+        {
+            Summary = "Sample Event",
+            DtStart = new CalDateTime(2025, 6, 10, 9, 0, 0),
+            DtEnd = new CalDateTime(2025, 6, 10, 10, 0, 0)
+        });
+
+        // Serialize with default encoding (should be UTF-8 without BOM)
+        using var msNoBom = new MemoryStream();
+        new CalendarSerializer().Serialize(calendar, msNoBom);
+        var noBomBytes = msNoBom.ToArray();
+
+        // Serialize with explicit UTF-8 BOM
+        using var msWithBom = new MemoryStream();
+        new CalendarSerializer().Serialize(calendar, msWithBom, new UTF8Encoding(true));
+        var withBomBytes = msWithBom.ToArray();
+
+        // UTF-8 BOM is 0xEF,0xBB,0xBF
+        bool HasBom(byte[] bytes) =>
+            bytes.Length >= 3 && bytes[0] == 0xEF && bytes[1] == 0xBB && bytes[2] == 0xBF;
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(HasBom(noBomBytes), Is.False, "Stream should not contain a UTF-8 BOM");
+            Assert.That(HasBom(withBomBytes), Is.True, "Stream should contain a UTF-8 BOM");
+        });
+    }
 }

Ical.Net/Serialization/ISerializer.cs

@@ -14,6 +14,6 @@ public interface ISerializer : IServiceProvider
     SerializationContext SerializationContext { get; set; }
 
     Type TargetType { get; }
-    void Serialize(object obj, Stream stream, Encoding encoding);
+    void Serialize(object obj, Stream stream, Encoding? encoding = null);
     object? Deserialize(Stream stream, Encoding encoding);
 }

Ical.Net/Serialization/SerializerBase.cs

@@ -43,13 +43,28 @@ protected SerializerBase(SerializationContext ctx)
         return obj;
     }
 
-    public void Serialize(object obj, Stream stream, Encoding encoding)
+    /// <summary>
+    /// Serializes the specified object to the provided stream using the specified encoding.
+    /// </summary>
+    /// <remarks>This method writes the serialized representation of the object to the stream without closing
+    /// the stream, allowing the caller to continue using it.
+    /// bytes.</remarks>
+    /// <param name="obj">The object to serialize. Must not be null.</param>
+    /// <param name="stream">The stream to which the serialized data will be written. Must be writable.</param>
+    /// <param name="encoding">
+    /// The character encoding to use for serialization.
+    /// If <see langword="null"/> or missing, UTF-8 encoding
+    /// without a byte order mark (BOM) is used.
+    /// A BOM is incompatible with many iCalendar apps.
+    /// </param>
+    public void Serialize(object obj, Stream stream, Encoding? encoding = null)
     {
-        // NOTE: we don't use a 'using' statement here because
-        // we don't want the stream to be closed by this serialization.
-        // Fixes bug #3177278 - Serialize closes stream
+        // Ensure that no BOM is written to the stream
+        encoding ??= new UTF8Encoding(false);
 
-        const int defaultBuffer = 1024;     //This is StreamWriter's built-in default buffer size
+        //This is StreamWriter's built-in default buffer size
+        const int defaultBuffer = 1024;
+        // Important: leave the stream open so that the caller can continue to use it
         using var sw = new StreamWriter(stream, encoding, defaultBuffer, leaveOpen: true);
 
         // Push the current object onto the serialization stack