Don't use C#9 features.

Causing the build to fail on OSX/Linux

src/Avalonia.Controls/Automation/AutomationElementIdentifiers.cs

@@ -11,18 +11,18 @@ namespace Avalonia.Automation
         /// Identifies the bounding rectangle automation property. The bounding rectangle property
         /// value is returned by the <see cref="AutomationPeer.GetBoundingRectangle"/> method.
         /// </summary>
-        public static AutomationProperty BoundingRectangleProperty { get; } = new();
+        public static AutomationProperty BoundingRectangleProperty { get; } = new AutomationProperty();
 
         /// <summary>
         /// Identifies the class name automation property. The class name property value is returned
         /// by the <see cref="AutomationPeer.GetClassName"/> method.
         /// </summary>
-        public static AutomationProperty ClassNameProperty { get; } = new();
+        public static AutomationProperty ClassNameProperty { get; } = new AutomationProperty();
 
         /// <summary>
         /// Identifies the name automation property. The class name property value is returned
         /// by the <see cref="AutomationPeer.GetName"/> method.
         /// </summary>
-        public static AutomationProperty NameProperty { get; } = new();
+        public static AutomationProperty NameProperty { get; } = new AutomationProperty();
     }
 }

src/Avalonia.Controls/Automation/ExpandCollapsePatternIdentifiers.cs

@@ -10,6 +10,6 @@ namespace Avalonia.Automation
         /// <summary>
         /// Identifies <see cref="IExpandCollapseProvider.ExpandCollapseState"/> automation property.
         /// </summary>
-        public static AutomationProperty ExpandCollapseStateProperty { get; } = new();
+        public static AutomationProperty ExpandCollapseStateProperty { get; } = new AutomationProperty();
     }
 }

src/Avalonia.Controls/Automation/RangeValuePatternIdentifiers.cs

@@ -10,21 +10,21 @@ namespace Avalonia.Automation
         /// <summary>
         /// Identifies <see cref="IRangeValueProvider.IsReadOnly"/> automation property.
         /// </summary>
-        public static AutomationProperty IsReadOnlyProperty { get; } = new();
+        public static AutomationProperty IsReadOnlyProperty { get; } = new AutomationProperty();
 
         /// <summary>
         /// Identifies <see cref="IRangeValueProvider.Minimum"/> automation property.
         /// </summary>
-        public static AutomationProperty MinimumProperty { get; } = new();
+        public static AutomationProperty MinimumProperty { get; } = new AutomationProperty();
 
         /// <summary>
         /// Identifies <see cref="IRangeValueProvider.Maximum"/> automation property.
         /// </summary>
-        public static AutomationProperty MaximumProperty { get; } = new();
+        public static AutomationProperty MaximumProperty { get; } = new AutomationProperty();
 
         /// <summary>
         /// Identifies <see cref="IRangeValueProvider.Value"/> automation property.
         /// </summary>
-        public static AutomationProperty ValueProperty { get; } = new();
+        public static AutomationProperty ValueProperty { get; } = new AutomationProperty();
     }
 }

src/Avalonia.Controls/Automation/ScrollPatternIdentifiers.cs

@@ -15,31 +15,31 @@ namespace Avalonia.Automation
         /// <summary>
         /// Identifies <see cref="IScrollProvider.HorizontallyScrollable"/> automation property.
         /// </summary>
-        public static AutomationProperty HorizontallyScrollableProperty { get; } = new();
+        public static AutomationProperty HorizontallyScrollableProperty { get; } = new AutomationProperty();
 
         /// <summary>
         /// Identifies <see cref="IScrollProvider.HorizontalScrollPercent"/> automation property.
         /// </summary>
-        public static AutomationProperty HorizontalScrollPercentProperty { get; } = new();
+        public static AutomationProperty HorizontalScrollPercentProperty { get; } = new AutomationProperty();
 
         /// <summary>
         /// Identifies <see cref="IScrollProvider.HorizontalViewSize"/> automation property.
         /// </summary>
-        public static AutomationProperty HorizontalViewSizeProperty { get; } = new();
+        public static AutomationProperty HorizontalViewSizeProperty { get; } = new AutomationProperty();
 
         /// <summary>
         /// Identifies <see cref="IScrollProvider.VerticallyScrollable"/> automation property.
         /// </summary>
-        public static AutomationProperty VerticallyScrollableProperty { get; } = new();
+        public static AutomationProperty VerticallyScrollableProperty { get; } = new AutomationProperty();
 
         /// <summary>
         /// Identifies <see cref="IScrollProvider.VerticalScrollPercent"/> automation property.
         /// </summary>
-        public static AutomationProperty VerticalScrollPercentProperty { get; } = new();
+        public static AutomationProperty VerticalScrollPercentProperty { get; } = new AutomationProperty();
 
         /// <summary>
         /// Identifies <see cref="IScrollProvider.VerticalViewSize"/> automation property.
         /// </summary>
-        public static AutomationProperty VerticalViewSizeProperty { get; } = new();
+        public static AutomationProperty VerticalViewSizeProperty { get; } = new AutomationProperty();
     }
 }

src/Avalonia.Controls/Automation/SelectionPatternIdentifiers.cs

@@ -10,16 +10,16 @@ namespace Avalonia.Automation
         /// <summary>
         /// Identifies <see cref="ISelectionProvider.CanSelectMultiple"/> automation property.
         /// </summary>
-        public static AutomationProperty CanSelectMultipleProperty { get; } = new();
+        public static AutomationProperty CanSelectMultipleProperty { get; } = new AutomationProperty();
 
         /// <summary>
         /// Identifies <see cref="ISelectionProvider.IsSelectionRequired"/> automation property.
         /// </summary>
-        public static AutomationProperty IsSelectionRequiredProperty { get; } = new();
+        public static AutomationProperty IsSelectionRequiredProperty { get; } = new AutomationProperty();
 
         /// <summary>
         /// Identifies the property that gets the selected items in a container.
         /// </summary>
-        public static AutomationProperty SelectionProperty { get; } = new();
+        public static AutomationProperty SelectionProperty { get; } = new AutomationProperty();
     }
 }

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

@@ -25,7 +25,7 @@ namespace Avalonia.Win32.Automation
         IRawElementProviderAdviseEvents,
         IInvokeProvider
     {
-        private static Dictionary<AutomationProperty, UiaPropertyId> s_propertyMap = new()
+        private static Dictionary<AutomationProperty, UiaPropertyId> s_propertyMap = new Dictionary<AutomationProperty, UiaPropertyId>()
         {
             { AutomationElementIdentifiers.BoundingRectangleProperty, UiaPropertyId.BoundingRectangle },
             { AutomationElementIdentifiers.ClassNameProperty, UiaPropertyId.ClassName },
@@ -276,7 +276,7 @@ namespace Avalonia.Win32.Automation
             var peer = Peer;
             var parent = peer.GetParent();
 
-            while (peer is not AAP.IRootProvider && parent is object)
+            while (!(peer is AAP.IRootProvider) && parent is object)
             {
                 peer = parent;
                 parent = peer.GetParent();