diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index 6a9f72039a..b70e0bf77f 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -35,16 +35,16 @@ jobs:
     vmImage: 'macOS-10.14'
   steps:
   - task: UseDotNet@2
-    displayName: 'Use .NET Core SDK 3.1.x'
+    displayName: 'Use .NET Core SDK 3.1.101'
     inputs:
       packageType: sdk
-      version: 3.1.x
+      version: 3.1.101
 
   - task: UseDotNet@2
-    displayName: 'Use .NET Core Runtime 3.1.x'
+    displayName: 'Use .NET Core Runtime 3.1.1'
     inputs:
       packageType: runtime
-      version: 3.1.x
+      version: 3.1.1
 
   - task: CmdLine@2
     displayName: 'Install Mono 5.18'
diff --git a/src/Avalonia.Styling/Styling/Activators/StyleClassActivator.cs b/src/Avalonia.Styling/Styling/Activators/StyleClassActivator.cs
index c884b6ac43..7906a29cb5 100644
--- a/src/Avalonia.Styling/Styling/Activators/StyleClassActivator.cs
+++ b/src/Avalonia.Styling/Styling/Activators/StyleClassActivator.cs
@@ -14,6 +14,7 @@ namespace Avalonia.Styling.Activators
     {
         private readonly IList<string> _match;
         private readonly IAvaloniaReadOnlyList<string> _classes;
+        private NotifyCollectionChangedEventHandler? _classesChangedHandler;
 
         public StyleClassActivator(IAvaloniaReadOnlyList<string> classes, IList<string> match)
         {
@@ -21,6 +22,9 @@ namespace Avalonia.Styling.Activators
             _match = match;
         }
 
+        private NotifyCollectionChangedEventHandler ClassesChangedHandler =>
+            _classesChangedHandler ??= ClassesChanged;
+
         public static bool AreClassesMatching(IReadOnlyList<string> classes, IList<string> toMatch)
         {
             int remainingMatches = toMatch.Count;
@@ -51,16 +55,15 @@ namespace Avalonia.Styling.Activators
             return remainingMatches == 0;
         }
 
-
         protected override void Initialize()
         {
             PublishNext(IsMatching());
-            _classes.CollectionChanged += ClassesChanged;
+            _classes.CollectionChanged += ClassesChangedHandler;
         }
 
         protected override void Deinitialize()
         {
-            _classes.CollectionChanged -= ClassesChanged;
+            _classes.CollectionChanged -= ClassesChangedHandler;
         }
 
         private void ClassesChanged(object sender, NotifyCollectionChangedEventArgs e)