Fixed reserialization of empty MimeParts within multiparts

Fixes issue #213

MimeKit/Multipart.cs

@@ -423,10 +423,14 @@ public override void Prepare (EncodingConstraint constraint, int maxLineLength =
 
 			if (cancellable != null) {
 				for (int i = 0; i < children.Count; i++) {
+					var part = children[i] as MimePart;
+
 					cancellable.Write (boundary, 0, boundary.Length - 2, cancellationToken);
 					cancellable.Write (options.NewLineBytes, 0, options.NewLineBytes.Length, cancellationToken);
 					children[i].WriteTo (options, stream, false, cancellationToken);
-					cancellable.Write (options.NewLineBytes, 0, options.NewLineBytes.Length, cancellationToken);
+
+					if (part == null || (part.ContentObject != null && part.ContentObject.Stream.Length != 0))
+						cancellable.Write (options.NewLineBytes, 0, options.NewLineBytes.Length, cancellationToken);
 				}
 
 				cancellable.Write (boundary, 0, boundary.Length, cancellationToken);
@@ -435,11 +439,15 @@ public override void Prepare (EncodingConstraint constraint, int maxLineLength =
 					cancellable.Write (options.NewLineBytes, 0, options.NewLineBytes.Length, cancellationToken);
 			} else {
 				for (int i = 0; i < children.Count; i++) {
+					var part = children[i] as MimePart;
+
 					cancellationToken.ThrowIfCancellationRequested ();
 					stream.Write (boundary, 0, boundary.Length - 2);
 					stream.Write (options.NewLineBytes, 0, options.NewLineBytes.Length);
 					children[i].WriteTo (options, stream, false, cancellationToken);
-					stream.Write (options.NewLineBytes, 0, options.NewLineBytes.Length);
+
+					if (part == null || (part.ContentObject != null && part.ContentObject.Stream.Length != 0))
+						stream.Write (options.NewLineBytes, 0, options.NewLineBytes.Length);
 				}
 
 				cancellationToken.ThrowIfCancellationRequested ();

UnitTests/MimeMessageTests.cs

@@ -103,6 +103,51 @@ [An Andrew ToolKit view (mailobjv) was included here, but could not be
 
 --Alternative.Boundary.IeCBvV20M2Yt4oU=wd--
 
+--Interpart.Boundary.IeCBvV20M2YtEoUA0A--
+";
+			string result;
+
+			using (var source = new MemoryStream (Encoding.UTF8.GetBytes (rawMessageText.Replace ("\r\n", "\n")))) {
+				var parser = new MimeParser (source, MimeFormat.Default);
+				var message = parser.ParseMessage ();
+
+				using (var serialized = new MemoryStream ()) {
+					var options = FormatOptions.Default.Clone ();
+					options.NewLineFormat = NewLineFormat.Unix;
+
+					message.WriteTo (options, serialized);
+
+					result = Encoding.UTF8.GetString (serialized.ToArray ());
+				}
+			}
+
+			Assert.AreEqual (rawMessageText, result, "Reserialized message is not identical to the original.");
+		}
+
+		[Test]
+		public void TestReserializationEmptyPart ()
+		{
+			const string rawMessageText = @"Date: Fri, 12 Jun 1992 13:29:05 -0400 (EDT)
+From: Nathaniel Borenstein <nsb>
+X-Andrew-Message-Size: 152+1
+MIME-Version: 1.0
+Content-Type: multipart/mixed; 
+	boundary=""Interpart.Boundary.IeCBvV20M2YtEoUA0A""
+To: Ned Freed <ned@innosoft.com>,
+    ysato@etl.go.jp (Yutaka Sato =?ISO-2022-JP?B?GyRAOjRGI0stGyhK?= )
+Subject: MIME & int'l mail
+
+> THIS IS A MESSAGE IN 'MIME' FORMAT.  Your mail reader does not support MIME.
+> Please read the first section, which is plain text, and ignore the rest.
+
+--Interpart.Boundary.IeCBvV20M2YtEoUA0A
+Content-type: text/plain; charset=US-ASCII
+
+This is the body.
+
+--Interpart.Boundary.IeCBvV20M2YtEoUA0A
+Content-type: text/plain; charset=US-ASCII; name=empty.txt
+
 --Interpart.Boundary.IeCBvV20M2YtEoUA0A--
 ";
 			string result;