Fix NullReferenceException in FindIndexOfNextParaMark

CHANGELOG.md

@@ -18,6 +18,7 @@ All notable changes to this project will be documented in this file.
   - CSS-based label filtering enables responsive toggle without any re-rendering
 
 ### Fixed
+- **NullReferenceException in FindIndexOfNextParaMark when comparing documents with body-level bookmarks** - `FindIndexOfNextParaMark` assumed all elements in the comparison unit array were `ComparisonUnitWord`, but documents with `bookmarkStart`/`bookmarkEnd` as direct children of `w:body` produce other `ComparisonUnit` types. Now handles any `ComparisonUnit` with `Contents` (including `ComparisonUnitGroup`) and adds a null guard for the `LastOrDefault()` call.
 - **Paginated rendering: text clipped at page bottom + inconsistent paragraph spacing (Issue #114)**
   - Fixed `lineRule` default handling: when `w:lineRule` is absent but `w:line` is present, treat as "auto" per OOXML spec (ISO/IEC 29500). Previously the line value was ignored, causing accumulated line-height mismatches that clipped the last line on pages.
   - Fixed `contextualSpacing` handling: now suppresses both `spacingAfter` (margin-bottom) AND `spacingBefore` (margin-top) for consecutive same-style paragraphs. Previously only `spacingAfter` was suppressed, leaving inconsistent inter-paragraph gaps.

Docxodus.Tests/WmlComparerBodyLevelBookmarkTests.cs

@@ -0,0 +1,40 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+#nullable enable
+
+using System;
+using System.IO;
+using Docxodus;
+using Xunit;
+
+namespace OxPt
+{
+    public class WmlComparerBodyLevelBookmarkTests
+    {
+        [Fact]
+        public void Compare_WithBodyLevelBookmarks_DoesNotThrowNullReference()
+        {
+            DirectoryInfo sourceDir = new DirectoryInfo("../../../../TestFiles/");
+            var original = new WmlDocument(Path.Combine(sourceDir.FullName, "WC/WC-BodyBookmarks-Before.docx"));
+            var modified = new WmlDocument(Path.Combine(sourceDir.FullName, "WC/WC-BodyBookmarks-After.docx"));
+
+            var settings = new WmlComparerSettings();
+
+            // The modified document has bookmarkStart/bookmarkEnd as direct children
+            // of w:body (siblings of w:p). This caused a NullReferenceException in
+            // FindIndexOfNextParaMark because it assumed all ComparisonUnits were
+            // ComparisonUnitWord, but body-level bookmarks produce other types.
+            //
+            // This document pair also triggers a separate "Internal error" in
+            // ProcessFootnoteEndnote (the modified document converted endnotes to
+            // footnotes). That is a different bug — this test verifies only that the
+            // NullReferenceException in FindIndexOfNextParaMark is fixed.
+            var ex = Record.Exception(() => WmlComparer.Compare(original, modified, settings));
+
+            Assert.True(
+                ex is not NullReferenceException,
+                $"Expected no NullReferenceException but got: {ex}");
+        }
+    }
+}

Docxodus/WmlComparer.cs

@@ -7676,9 +7676,12 @@ private static int FindIndexOfNextParaMark(ComparisonUnit[] cul)
         {
             for (int i = 0; i < cul.Length; i++)
             {
-                var cuw = cul[i] as ComparisonUnitWord;
-                var lastAtom = cuw.DescendantContentAtoms().LastOrDefault();
-                if (lastAtom.ContentElement.Name == W.pPr)
+                var comparisonUnit = cul[i];
+                if (comparisonUnit == null || comparisonUnit.Contents == null)
+                    continue;
+
+                var lastAtom = comparisonUnit.DescendantContentAtoms().LastOrDefault();
+                if (lastAtom != null && lastAtom.ContentElement.Name == W.pPr)
                     return i;
             }
             return cul.Length;