Include ApplicationIcon as a resource for default Window icon (#15298)

* [Windows] Inherit default Window icon from the Process resources

* Include ApplicationIcon as a AvaloniaResource and use it as a default icon

* Revert "[Windows] Inherit default Window icon from the Process resources"

This reverts commit b08f5d52cc.

* Safer LoadDefaultIcon

* Rename !AppIcon to !__AvaloniaDefaultWindowIcon

* Even safer LoadDefaultIcon (for tests)

packages/Avalonia/AvaloniaBuildTasks.targets

@@ -3,9 +3,8 @@
     <AvaloniaXamlReportImportance Condition="'$(AvaloniaXamlReportImportance)' == ''">low</AvaloniaXamlReportImportance>
     <_AvaloniaSkipXamlCompilation Condition="'$(_AvaloniaSkipXamlCompilation)' == ''">false</_AvaloniaSkipXamlCompilation>
     <AvaloniaUseCompiledBindingsByDefault Condition="'$(AvaloniaUseCompiledBindingsByDefault)' == ''">false</AvaloniaUseCompiledBindingsByDefault>
+    <AvaloniaIncludeApplicationIconAsWindowIcon Condition="'$(AvaloniaIncludeApplicationIconAsWindowIcon)' == ''">true</AvaloniaIncludeApplicationIconAsWindowIcon>
   </PropertyGroup>
-  
-  
 
   <PropertyGroup>
     <!-- Enable property page if VisualStudioVersion is great or equal 17.0 and Language is'not VB.NET -->
@@ -31,7 +30,14 @@
     <None Remove="**\*.axaml" />
     <None Remove="**\*.paml" />
   </ItemGroup>
-  
+
+  <!-- Include App icon, so we can use it as a default Window icon. -->
+  <ItemGroup Condition="'$(AvaloniaIncludeApplicationIconAsWindowIcon)' == 'true'">
+    <AvaloniaResource Include="$(ApplicationIcon)"
+                      Link="!__AvaloniaDefaultWindowIcon"
+                      Condition="'$(ApplicationIcon)' != ''" />
+  </ItemGroup>
+
   <UsingTask TaskName="GenerateAvaloniaResourcesTask"
              AssemblyFile="$(AvaloniaBuildTasksLocation)"
              />

src/Avalonia.Controls/Window.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Reflection;
 using System.Threading.Tasks;
 using Avalonia.Automation.Peers;
 using Avalonia.Controls.Platform;
@@ -87,6 +88,7 @@ namespace Avalonia.Controls
     /// </summary>
     public class Window : WindowBase, IFocusScope, ILayoutRoot
     {
+        private static readonly Lazy<WindowIcon?> s_defaultIcon = new(LoadDefaultIcon);
         private readonly List<(Window child, bool isDialog)> _children = new List<(Window, bool)>();
         private bool _isExtendedIntoWindowDecorations;
         private Thickness _windowDecorationMargin;
@@ -228,7 +230,7 @@ namespace Avalonia.Controls
             this.GetObservable(ClientSizeProperty).Skip(1).Subscribe(x => PlatformImpl?.Resize(x, WindowResizeReason.Application));
 
             CreatePlatformImplBinding(TitleProperty, title => PlatformImpl!.SetTitle(title));
-            CreatePlatformImplBinding(IconProperty, icon => PlatformImpl!.SetIcon(icon?.PlatformImpl));
+            CreatePlatformImplBinding(IconProperty, icon => PlatformImpl!.SetIcon((icon ?? s_defaultIcon.Value)?.PlatformImpl));
             CreatePlatformImplBinding(CanResizeProperty, canResize => PlatformImpl!.CanResize(canResize));
             CreatePlatformImplBinding(ShowInTaskbarProperty, show => PlatformImpl!.ShowTaskbarIcon(show));
 
@@ -1100,5 +1102,19 @@ namespace Avalonia.Controls
         {
             return new WindowAutomationPeer(this);
         }
+
+        private static WindowIcon? LoadDefaultIcon()
+        {
+            // Use AvaloniaLocator instead of static AssetLoader, so it won't fail on Unit Tests without any asset loader. 
+            if (AvaloniaLocator.Current.GetService<IAssetLoader>() is { } assetLoader
+                && Assembly.GetEntryAssembly()?.GetName()?.Name is { } assemblyName
+                && Uri.TryCreate($"avares://{assemblyName}/!__AvaloniaDefaultWindowIcon", UriKind.Absolute, out var path)
+                && assetLoader.Exists(path))
+            {
+                using var stream = assetLoader.Open(path);
+                return new WindowIcon(stream);
+            }
+            return null;
+        }
     }
 }