Fix for TextBlock.TextDecorations not inheriting down to inlines.

src/Avalonia.Controls/Documents/Inline.cs

@@ -13,9 +13,10 @@ public abstract class Inline : TextElement
         /// <summary>
         /// AvaloniaProperty for <see cref="TextDecorations" /> property.
         /// </summary>
-        public static readonly StyledProperty<TextDecorationCollection?> TextDecorationsProperty =
-            AvaloniaProperty.Register<Inline, TextDecorationCollection?>(
-                nameof(TextDecorations));
+        public static readonly AttachedProperty<TextDecorationCollection?> TextDecorationsProperty =
+            AvaloniaProperty.RegisterAttached<Inline, Inline, TextDecorationCollection?>(
+                nameof(TextDecorations),
+                inherits: true);
 
         /// <summary>
         /// AvaloniaProperty for <see cref="BaselineAlignment" /> property.
@@ -43,7 +44,27 @@ public BaselineAlignment BaselineAlignment
             get { return GetValue(BaselineAlignmentProperty); }
             set { SetValue(BaselineAlignmentProperty, value); }
         }
+
+        /// <summary>
+        /// Gets the value of the attached <see cref="TextDecorationsProperty"/> on a control.
+        /// </summary>
+        /// <param name="control">The control.</param>
+        /// <returns>The font style.</returns>
+        public static TextDecorationCollection? GetTextDecorations(Control control)
+        {
+            return control.GetValue(TextDecorationsProperty);
+        }
 
+        /// <summary>
+        /// Sets the value of the attached <see cref="TextDecorationsProperty"/> on a control.
+        /// </summary>
+        /// <param name="control">The control.</param>
+        /// <param name="value">The property value to set.</param>
+        public static void SetTextDecorations(Control control, TextDecorationCollection? value)
+        {
+            control.SetValue(TextDecorationsProperty, value);
+        }
+
         internal abstract void BuildTextRun(IList<TextRun> textRuns);
 
         internal abstract void AppendText(StringBuilder stringBuilder);

src/Avalonia.Controls/TextBlock.cs

@@ -135,7 +135,7 @@ public class TextBlock : Control, IInlineHost
         /// Defines the <see cref="TextDecorations"/> property.
         /// </summary>
         public static readonly StyledProperty<TextDecorationCollection?> TextDecorationsProperty =
-            AvaloniaProperty.Register<TextBlock, TextDecorationCollection?>(nameof(TextDecorations));
+            Inline.TextDecorationsProperty.AddOwner<TextBlock>();
 
         /// <summary>
         /// Defines the <see cref="Inlines"/> property.

tests/Avalonia.Controls.UnitTests/TextBlockTests.cs

@@ -202,5 +202,26 @@ public void Setting_Text_Should_Reset_Inlines()
                 Assert.Equal(0, target.Inlines.Count);
             }
         }
+
+        [Fact]
+        public void Setting_TextDecorations_Should_Update_Inlines()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var target = new TextBlock();
+
+                target.Inlines.Add(new Run("Hello World"));
+
+                Assert.Equal(1, target.Inlines.Count);
+
+                Assert.Null(target.Inlines[0].TextDecorations);
+
+                var underline = TextDecorations.Underline;
+
+                target.TextDecorations = underline;
+
+                Assert.Equal(underline, target.Inlines[0].TextDecorations);
+            }
+        }
     }
 }