fix: handle null inline style declarations in style attribute comparers

CHANGELOG.md

@@ -1,6 +1,10 @@
+# 1.1.2
+
+- Fixed `StyleAttributeComparer` and `OrderingStyleAttributeComparer` throwing a `NullReferenceException` when comparing a `style` attribute on an element that has no inline CSS style declaration — e.g. a non-HTML (SVG/MathML) element, or any element when the browsing context has no CSS parser registered. Such `style` attributes are now compared by their raw value.
+
 # 1.1.1
 
-- Fixed marking comaprison results in the pipeline so that strategies down the line see if there is a diff. By [@egil](https://github.com/egil) and [@linkdotnet](https://github.com/linkdotnet).
+- Fixed marking comparison results in the pipeline so that strategies down the line see if there is a diff. By [@egil](https://github.com/egil) and [@linkdotnet](https://github.com/linkdotnet).
 
 # 1.1.0
 

src/AngleSharp.Diffing.Tests/Strategies/AttributeStrategies/CssLessComparisonFactory.cs

@@ -0,0 +1,38 @@
+using AngleSharp.Html.Parser;
+
+namespace AngleSharp.Diffing.Strategies.AttributeStrategies;
+
+/// <summary>
+/// Builds style-attribute comparisons whose elements are parsed in a browsing context without CSS support.
+/// With no <see cref="AngleSharp.Css.Parser.ICssParser"/> registered, <c>IElement.GetStyle()</c> returns
+/// <c>null</c> — the situation CSS-less consumers such as bUnit hit when diffing markup that contains inline
+/// SVG. The shared <see cref="DiffingTestFixture"/> enables CSS, so it cannot reproduce this on its own.
+/// </summary>
+internal static class CssLessComparisonFactory
+{
+    public static AttributeComparison ToStyleAttributeComparison(string controlHtml, string testHtml)
+    {
+        // Same parser setup as DiffingTestFixture, but deliberately without .WithCss().
+        var config = Configuration.Default
+            .With<IHtmlParser>(_ =>
+                new HtmlParser(
+                    new()
+                    {
+                        IsKeepingSourceReferences = true
+                    },
+                    _));
+        var context = BrowsingContext.New(config);
+        var parser = context.GetService<IHtmlParser>()!;
+        var document = context.OpenNewAsync().Result;
+
+        return new AttributeComparison(
+            ToStyleSource(ComparisonSourceType.Control, controlHtml),
+            ToStyleSource(ComparisonSourceType.Test, testHtml));
+
+        AttributeComparisonSource ToStyleSource(ComparisonSourceType sourceType, string html)
+        {
+            var element = parser.ParseFragment(html, document.Body!)[0];
+            return new("style", element.ToComparisonSource(0, sourceType));
+        }
+    }
+}

src/AngleSharp.Diffing.Tests/Strategies/AttributeStrategies/OrderingStyleAttributeComparerTest.cs

@@ -71,4 +71,19 @@ public void Test006(string control, string test)
         var comparison = ToAttributeComparison(control, "style", test, "style");
         OrderingStyleAttributeComparer.Compare(comparison, CompareResult.Unknown).ShouldBe(CompareResult.Same);
     }
+
+    [Theory(DisplayName = "Style comparison falls back to the raw value when an element has no CSS style declaration (e.g. inline SVG)")]
+    [InlineData(@"<svg style=""color:red"">", @"<svg style=""color:red"">", true)]
+    [InlineData(@"<svg style=""color:red"">", @"<svg style=""color:blue"">", false)]
+    public void Test007(string control, string test, bool expectedSame)
+    {
+        // GetStyle() returns null when the browsing context has no CSS parser; the comparer must not throw.
+        var comparison = CssLessComparisonFactory.ToStyleAttributeComparison(control, test);
+
+        var expected = expectedSame
+            ? CompareResult.Same
+            : CompareResult.FromDiff(new AttrDiff(comparison, AttrDiffKind.Value));
+
+        OrderingStyleAttributeComparer.Compare(comparison, CompareResult.Unknown).ShouldBe(expected);
+    }
 }

src/AngleSharp.Diffing.Tests/Strategies/AttributeStrategies/StyleAttributeComparerTest.cs

@@ -63,4 +63,19 @@ public void Test005(string control, string test)
         var comparison = ToAttributeComparison(control, "style", test, "style");
         StyleAttributeComparer.Compare(comparison, CompareResult.Unknown).ShouldBe(CompareResult.Same);
     }
+
+    [Theory(DisplayName = "Style comparison falls back to the raw value when an element has no CSS style declaration (e.g. inline SVG)")]
+    [InlineData(@"<svg style=""color:red"">", @"<svg style=""color:red"">", true)]
+    [InlineData(@"<svg style=""color:red"">", @"<svg style=""color:blue"">", false)]
+    public void Test006(string control, string test, bool expectedSame)
+    {
+        // GetStyle() returns null when the browsing context has no CSS parser; the comparer must not throw.
+        var comparison = CssLessComparisonFactory.ToStyleAttributeComparison(control, test);
+
+        var expected = expectedSame
+            ? CompareResult.Same
+            : CompareResult.FromDiff(new AttrDiff(comparison, AttrDiffKind.Value));
+
+        StyleAttributeComparer.Compare(comparison, CompareResult.Unknown).ShouldBe(expected);
+    }
 }

src/AngleSharp.Diffing/Strategies/AttributeStrategies/OrderingStyleAttributeComparer.cs

@@ -29,7 +29,16 @@ private static CompareResult CompareElementStyle(in AttributeComparison comparis
         var (ctrlElm, testElm) = comparison.AttributeElements;
         var ctrlStyle = ctrlElm.GetStyle();
         var testStyle = testElm.GetStyle();
-        return CompareCssStyleDeclarations(ctrlStyle, testStyle)
+
+        // GetStyle() returns null when an element exposes no inline CSS style declaration — e.g. a non-HTML
+        // (SVG/MathML) element, or any element when the browsing context has no CSS parser registered. Fall
+        // back to comparing the raw style attribute values in that case, so such elements are compared by
+        // value instead of throwing a NullReferenceException.
+        var areEqual = ctrlStyle is not null && testStyle is not null
+            ? CompareCssStyleDeclarations(ctrlStyle, testStyle)
+            : string.Equals(comparison.Control.Attribute.Value, comparison.Test.Attribute.Value, StringComparison.Ordinal);
+
+        return areEqual
             ? CompareResult.Same
             : CompareResult.FromDiff(new AttrDiff(comparison, AttrDiffKind.Value));
     }

src/AngleSharp.Diffing/Strategies/AttributeStrategies/StyleAttributeComparer.cs

@@ -29,7 +29,16 @@ private static CompareResult CompareElementStyle(in AttributeComparison comparis
         var (ctrlElm, testElm) = comparison.AttributeElements;
         var ctrlStyle = ctrlElm.GetStyle();
         var testStyle = testElm.GetStyle();
-        return CompareCssStyleDeclarations(ctrlStyle, testStyle)
+
+        // GetStyle() returns null when an element exposes no inline CSS style declaration — e.g. a non-HTML
+        // (SVG/MathML) element, or any element when the browsing context has no CSS parser registered. Fall
+        // back to comparing the raw style attribute values in that case, so such elements are compared by
+        // value instead of throwing a NullReferenceException.
+        var areEqual = ctrlStyle is not null && testStyle is not null
+            ? CompareCssStyleDeclarations(ctrlStyle, testStyle)
+            : string.Equals(comparison.Control.Attribute.Value, comparison.Test.Attribute.Value, StringComparison.Ordinal);
+
+        return areEqual
             ? CompareResult.Same
             : CompareResult.FromDiff(new AttrDiff(comparison, AttrDiffKind.Value));
     }