SourceFileManager::PropagateOutOfDateStatus: report malformed XAML against the offending file

src/XamlCompiler/BuildTasks/CompileXamlInternal.cs

@@ -3087,9 +3087,19 @@ internal void LogError_XamlInternalError(Exception e, string file)
 
         internal void LogUnhandledException(string subcategory, ErrorCode code, Exception e, string file)
         {
-            XamlException xamlException = e as XamlException;
-            int line = xamlException != null ? xamlException.LineNumber : 0;
-            int column = xamlException != null ? xamlException.LinePosition : 0;
+            int line = 0;
+            int column = 0;
+            if (e is XamlException xamlException)
+            {
+                line = xamlException.LineNumber;
+                column = xamlException.LinePosition;
+            }
+            else if (e is XmlException xmlException)
+            {
+                // XmlException does not derive from XamlException, so extract its position separately.
+                line = xmlException.LineNumber;
+                column = xmlException.LinePosition;
+            }
             string message = e.Message;
 
             LogError(subcategory, code, null, file, line, column, 0, 0, message);

src/XamlCompiler/BuildTasks/Microsoft/Xaml/XamlCompiler/XamlNodeStreamReader.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License. See LICENSE in the project root for license information.
 
+using System;
 using System.IO;
 using System.Xaml;
 using System.Xml;
@@ -9,6 +10,22 @@ namespace Microsoft.UI.Xaml.Markup.Compiler.Utilities
 {
     class XamlNodeStreamHelper
     {
+        // Speculative x:Class read. Malformed markup is reported by the authoritative parse in
+        // CompileXamlInternal.LoadXamlDom, which names the offending file, so it must not fail here first.
+        public static bool TryReadXClassFromXamlFileStream(TextReader fileStream, XamlSchemaContext schemaContext, out string className)
+        {
+            try
+            {
+                className = ReadXClassFromXamlFileStream(fileStream, schemaContext);
+                return true;
+            }
+            catch (Exception e) when (e is XmlException || e is XamlException)
+            {
+                className = null;
+                return false;
+            }
+        }
+
         public static string ReadXClassFromXamlFileStream(TextReader fileStream, XamlSchemaContext schemaContext)
         {
             string className = null;

src/XamlCompiler/BuildTasks/SourceFileManager.cs

@@ -125,12 +125,13 @@ public void PropagateOutOfDateStatus(DirectUI.DirectUISchemaContext context)
                     // look to see if a new class was specified; if so, we guessed wrong before, undo that.
                     // Note that both the old and new classes are out of date.
                     string newClassFullName = null;
+                    bool readClassName;
                     using (var fileReader = TaskFileService.GetFileContents(tif.SourceXamlFullPath))
                     {
-                        newClassFullName = XamlNodeStreamHelper.ReadXClassFromXamlFileStream(fileReader, context);
+                        readClassName = XamlNodeStreamHelper.TryReadXClassFromXamlFileStream(fileReader, context, out newClassFullName);
                     }
 
-                    if (newClassFullName != tif.ClassFullName)
+                    if (readClassName && newClassFullName != tif.ClassFullName)
                     {
                         UnregisterClassOfTaskItem(tif);
                         tif.ClassFullName = newClassFullName;