[Windows] Inherit default Window icon from the Process resources

src/Windows/Avalonia.Win32/IconImpl.cs

@@ -12,6 +12,12 @@ namespace Avalonia.Win32
 
         private static readonly int s_taskbarIconSize = Win32Platform.WindowsVersion < PlatformConstants.Windows10 ? 32 : 24;
 
+        public IconImpl(Win32Icon smallIcon, Win32Icon bigIcon)
+        {
+            _smallIcon = smallIcon;
+            _bigIcon = bigIcon;
+        }
+
         public IconImpl(Stream smallIcon, Stream bigIcon)
         {
             _smallIcon = CreateIconImpl(smallIcon);
@@ -23,6 +29,9 @@ namespace Avalonia.Win32
             _smallIcon = _bigIcon = CreateIconImpl(icon);
         }
 
+        public Win32Icon SmallIcon => _smallIcon;
+        public Win32Icon BigIcon => _bigIcon;
+
         private static Win32Icon CreateIconImpl(Stream stream)
         {
             if (stream.CanSeek)

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -1376,6 +1376,10 @@ namespace Avalonia.Win32.Interop
         [DllImport("user32.dll")]
         public static extern bool DestroyIcon(IntPtr hIcon);
 
+        [DllImport("shell32.dll", CharSet = CharSet.Unicode)]
+        public static extern int ExtractIconEx(string szExeFileName, int nIconIndex, out IntPtr phiconLarge,
+            out IntPtr phiconSmall, int nIcons);
+
         [DllImport("user32.dll", EntryPoint = "PeekMessageW", ExactSpelling = true)]
         public static extern bool PeekMessage(out MSG lpMsg, IntPtr hWnd, uint wMsgFilterMin, uint wMsgFilterMax, uint wRemoveMsg);
 
@@ -1824,6 +1828,28 @@ namespace Avalonia.Win32.Interop
             }
         }
 
+        [DllImport("kernel32", EntryPoint = "GetModuleFileName", CharSet=CharSet.Unicode, SetLastError = true)]
+        private static extern uint IntGetModuleFileName(IntPtr hModule, StringBuilder buffer, uint length);
+
+        internal static string? GetModuleFileName(IntPtr hModule)
+        {
+            // Environment.ProcessPath is not avaialable on netstandard.
+            uint length;
+            var buffer = new StringBuilder(260);
+            while ((length = IntGetModuleFileName(hModule, buffer, (uint)buffer.Capacity)) >= buffer.Capacity)
+            {
+                buffer.EnsureCapacity((int)length);
+            }
+
+            if (length == 0)
+            {
+                return null;
+            }
+
+            buffer.Length = (int)length;
+            return buffer.ToString();
+        }
+
         [DllImport("kernel32", EntryPoint = "WaitForMultipleObjectsEx", SetLastError = true, CharSet = CharSet.Auto)]
         private static extern int IntWaitForMultipleObjectsEx(int nCount, IntPtr[] pHandles, bool bWaitAll, int dwMilliseconds, bool bAlertable);
 

src/Windows/Avalonia.Win32/Interop/Win32Icon.cs

@@ -11,11 +11,16 @@ namespace Avalonia.Win32.Interop;
 
 internal class Win32Icon : IDisposable
 {
+    public Win32Icon(IntPtr handle)
+    {
+        Handle = handle;
+    }
+
     public Win32Icon(Bitmap bitmap, PixelPoint hotSpot = default)
     {
         Handle = CreateIcon(bitmap, hotSpot);
     }
-    
+
     public Win32Icon(IBitmapImpl bitmap, PixelPoint hotSpot = default)
     {
         using var memoryStream = new MemoryStream();

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -55,6 +55,8 @@ namespace Avalonia.Win32
                 { WindowEdge.West, HitTestValues.HTLEFT }
             };
 
+        private static readonly Lazy<IconImpl?> s_defaultIconImpl = new(LoadDefaultIcon);
+
         /// <summary>
         /// The Windows DPI which equates to a <see cref="RenderScaling"/> of 1.0.
         /// </summary>
@@ -782,14 +784,24 @@ namespace Avalonia.Win32
 
         private Win32Icon? LoadIcon(Icons type, uint dpi)
         {
-            if (_iconImpl == null)
+            if (type == Icons.ICON_SMALL2)
             {
-                return null;
+                type = Icons.ICON_SMALL;
             }
 
-            if (type == Icons.ICON_SMALL2)
+            if (_iconImpl == null)
             {
-                type = Icons.ICON_SMALL;
+                if (s_defaultIconImpl.Value is { } defaultIcon)
+                {
+                    return type switch
+                    {
+                        Icons.ICON_SMALL => defaultIcon.SmallIcon,
+                        Icons.ICON_BIG => defaultIcon.BigIcon,
+                        _ => throw new NotImplementedException(),
+                    };
+                }
+
+                return null;
             }
 
             var iconKey = (type, dpi);
@@ -807,6 +819,20 @@ namespace Avalonia.Win32
             return icon;
         }
 
+        private static IconImpl? LoadDefaultIcon()
+        {
+            if (GetModuleFileName(default) is { } moduleFileName)
+            {
+                var iconsCount = ExtractIconEx(moduleFileName, 0, out var largeIcon, out var smallIcon, 1);
+                if (iconsCount >= 1)
+                {
+                    return new IconImpl(new Win32Icon(smallIcon), new Win32Icon(largeIcon));
+                }
+            }
+
+            return null;
+        }
+
         private void RefreshIcon()
         {
             SendMessage(_hwnd, (int)WindowsMessage.WM_SETICON, (nint)Icons.ICON_SMALL, LoadIcon(Icons.ICON_SMALL, _dpi)?.Handle ?? default);