Allow peers to dynamically expose implemented interfaces.

src/Avalonia.Controls/Automation/Peers/AutomationPeer.cs

@@ -160,6 +160,13 @@ namespace Avalonia.Automation.Peers
         /// <returns>true if a context menu is present for the element; otherwise false.</returns>
         public bool ShowContextMenu() => ShowContextMenuCore();
 
+        /// <summary>
+        /// Tries to get a provider of the specified type from the peer.
+        /// </summary>
+        /// <typeparam name="T">The provider type.</typeparam>
+        /// <returns>The provider, or null if not implemented on this peer.</returns>
+        public T? GetProvider<T>() => (T?)GetProviderCore(typeof(T));
+
         public event EventHandler<AutomationPropertyChangedEventArgs>? PropertyChanged;
 
         /// <summary>
@@ -223,6 +230,12 @@ namespace Avalonia.Automation.Peers
         protected abstract bool IsKeyboardFocusableCore();
         protected abstract void SetFocusCore();
         protected abstract bool ShowContextMenuCore();
+        
+        protected virtual object? GetProviderCore(Type providerType)
+        {
+            return providerType.IsAssignableFrom(this.GetType()) ? this : null;
+        }
+
         protected internal abstract bool TrySetParent(AutomationPeer? parent);
 
         protected void EnsureEnabled()

src/Windows/Avalonia.Win32/Automation/AutomationNode.cs

@@ -99,17 +99,19 @@ namespace Avalonia.Win32.Automation
         [return: MarshalAs(UnmanagedType.IUnknown)]
         public virtual object? GetPatternProvider(int patternId)
         {
+            AutomationNode? ThisIfPeerImplementsProvider<T>() => Peer.GetProvider<T>() is object ? this : null;
+
             return (UiaPatternId)patternId switch
             {
-                UiaPatternId.ExpandCollapse => Peer is IExpandCollapseProvider ? this : null,
-                UiaPatternId.Invoke => Peer is AAP.IInvokeProvider ? this : null,
-                UiaPatternId.RangeValue => Peer is AAP.IRangeValueProvider ? this : null,
-                UiaPatternId.Scroll => Peer is AAP.IScrollProvider ? this : null,
+                UiaPatternId.ExpandCollapse => ThisIfPeerImplementsProvider<IExpandCollapseProvider>(),
+                UiaPatternId.Invoke => ThisIfPeerImplementsProvider<AAP.IInvokeProvider>(),
+                UiaPatternId.RangeValue => ThisIfPeerImplementsProvider<AAP.IRangeValueProvider>(),
+                UiaPatternId.Scroll => ThisIfPeerImplementsProvider<AAP.IScrollProvider>(),
                 UiaPatternId.ScrollItem => this,
-                UiaPatternId.Selection => Peer is AAP.ISelectionProvider ? this : null,
-                UiaPatternId.SelectionItem => Peer is AAP.ISelectionItemProvider ? this : null,
-                UiaPatternId.Toggle => Peer is AAP.IToggleProvider ? this : null,
-                UiaPatternId.Value => Peer is AAP.IValueProvider ? this : null,
+                UiaPatternId.Selection => ThisIfPeerImplementsProvider<ISelectionProvider>(),
+                UiaPatternId.SelectionItem => ThisIfPeerImplementsProvider<AAP.ISelectionItemProvider>(),
+                UiaPatternId.Toggle => ThisIfPeerImplementsProvider<AAP.IToggleProvider>(),
+                UiaPatternId.Value => ThisIfPeerImplementsProvider<AAP.IValueProvider>(),
                 _ => null,
             };
         }
@@ -232,7 +234,7 @@ namespace Avalonia.Win32.Automation
 
         protected void InvokeSync<TInterface>(Action<TInterface> action)
         {
-            if (Peer is TInterface i)
+            if (Peer.GetProvider<TInterface>() is TInterface i)
             {
                 try
                 {
@@ -248,7 +250,7 @@ namespace Avalonia.Win32.Automation
         [return: MaybeNull]
         protected TResult InvokeSync<TInterface, TResult>(Func<TInterface, TResult> func)
         {
-            if (Peer is TInterface i)
+            if (Peer.GetProvider<TInterface>() is TInterface i)
             {
                 try
                 {
@@ -280,7 +282,7 @@ namespace Avalonia.Win32.Automation
             var peer = Peer;
             var parent = peer.GetParent();
 
-            while (!(peer is AAP.IRootProvider) && parent is object)
+            while (peer.GetProvider<AAP.IRootProvider>() is null && parent is object)
             {
                 peer = parent;
                 parent = peer.GetParent();
@@ -291,7 +293,9 @@ namespace Avalonia.Win32.Automation
 
         private static AutomationNode Create(AutomationPeer peer)
         {
-            return peer is AAP.IRootProvider ? new RootAutomationNode(peer) : new AutomationNode(peer);
+            return peer.GetProvider<AAP.IRootProvider>() is object ?
+                new RootAutomationNode(peer) :
+                new AutomationNode(peer);
         }
 
         private static UiaControlTypeId ToUiaControlType(AutomationControlType role)

src/Windows/Avalonia.Win32/Automation/RootAutomationNode.cs

@@ -14,11 +14,13 @@ namespace Avalonia.Win32.Automation
         public RootAutomationNode(AutomationPeer peer)
             : base(peer)
         {
-            ((IRootProvider)peer).FocusChanged += FocusChanged;
+            Peer = base.Peer.GetProvider<IRootProvider>() ?? throw new AvaloniaInternalException(
+                "Attempt to create RootAutomationNode from peer which does not implement IRootProvider.");
+            Peer.FocusChanged += FocusChanged;
         }
 
         public override IRawElementProviderFragmentRoot? FragmentRoot => this;
-        public new IRootProvider Peer => (IRootProvider)base.Peer;
+        public new IRootProvider Peer { get; }
         public WindowImpl? WindowImpl => Peer.PlatformImpl as WindowImpl;
         
         public IRawElementProviderFragment? ElementProviderFromPoint(double x, double y)