From a6dc6b1c887c8a5139be7bf1abba1315c26af0d7 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Wed, 1 Jun 2022 21:45:45 +0200
Subject: [PATCH] Prevent ControlTheme as a nested style.

---
 src/Avalonia.Base/Styling/ControlTheme.cs     | 16 +++++++++++
 .../Styling/ControlThemeTests.cs              | 28 +++++++++++++++++++
 2 files changed, 44 insertions(+)
 create mode 100644 tests/Avalonia.Base.UnitTests/Styling/ControlThemeTests.cs

diff --git a/src/Avalonia.Base/Styling/ControlTheme.cs b/src/Avalonia.Base/Styling/ControlTheme.cs
index 54fc972c31..9dcbd7d2c4 100644
--- a/src/Avalonia.Base/Styling/ControlTheme.cs
+++ b/src/Avalonia.Base/Styling/ControlTheme.cs
@@ -7,6 +7,17 @@ namespace Avalonia.Styling
     /// </summary>
     public class ControlTheme : StyleBase
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ControlTheme"/> class.
+        /// </summary>
+        public ControlTheme() { }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ControlTheme"/> class.
+        /// </summary>
+        /// <param name="targetType">The value for <see cref="TargetType"/>.</param>
+        public ControlTheme(Type targetType) => TargetType = targetType;
+
         /// <summary>
         /// Gets or sets the type for which this control theme is intended.
         /// </summary>
@@ -23,5 +34,10 @@ namespace Avalonia.Styling
                 SelectorMatch.AlwaysThisType :
                 SelectorMatch.NeverThisType;
         }
+
+        internal override void SetParent(StyleBase? parent)
+        {
+            throw new InvalidOperationException("ControlThemes cannot be added as a nested style.");
+        }
     }
 }
diff --git a/tests/Avalonia.Base.UnitTests/Styling/ControlThemeTests.cs b/tests/Avalonia.Base.UnitTests/Styling/ControlThemeTests.cs
new file mode 100644
index 0000000000..93a0e6c2fd
--- /dev/null
+++ b/tests/Avalonia.Base.UnitTests/Styling/ControlThemeTests.cs
@@ -0,0 +1,28 @@
+using System;
+using Avalonia.Controls;
+using Avalonia.Styling;
+using Xunit;
+
+namespace Avalonia.Base.UnitTests.Styling
+{
+    public class ControlThemeTests
+    {
+        [Fact]
+        public void ControlTheme_Cannot_Be_Added_To_Style_Children()
+        {
+            var target = new ControlTheme(typeof(Button));
+            var style = new Style();
+
+            Assert.Throws<InvalidOperationException>(() => style.Children.Add(target));
+        }
+
+        [Fact]
+        public void ControlTheme_Cannot_Be_Added_To_ControlTheme_Children()
+        {
+            var target = new ControlTheme(typeof(Button));
+            var other = new ControlTheme(typeof(CheckBox));
+
+            Assert.Throws<InvalidOperationException>(() => other.Children.Add(target));
+        }
+    }
+}