Disable WinFormsAvaloniaControlHost in design mode (#16296)

src/Windows/Avalonia.Win32.Interoperability/WinForms/WinFormsAvaloniaControlHost.cs

@@ -1,5 +1,7 @@
 using System;
 using System.ComponentModel;
+using System.Drawing;
+using System.Drawing.Drawing2D;
 using System.Windows.Forms;
 using Avalonia.Controls.Embedding;
 using Avalonia.Win32.Interop;
@@ -49,15 +51,20 @@ public AvControl? Content
     /// <inheritdoc />
     protected override void OnHandleCreated(EventArgs e)
     {
-        _root = new();
-        _root.Content = _content;
-        _root.Prepare();
-        _root.StartRendering();
-        _root.GotFocus += RootGotFocus;
+        // EmbeddableControlRoot requires Avalonia to be initialized which is not done by the Windows Forms designer.
+        if (!DesignMode)
+        {
+            _root = new();
+            _root.Content = _content;
+            _root.Prepare();
+            _root.StartRendering();
+            _root.GotFocus += RootGotFocus;
+
+            FixPosition();
+
+            UnmanagedMethods.SetParent(WindowHandle, Handle);
+        }
 
-        FixPosition();
-
-        UnmanagedMethods.SetParent(WindowHandle, Handle);
         base.OnHandleCreated(e);
     }
 
@@ -93,14 +100,14 @@ protected override void OnGotFocus(EventArgs e)
         if (handle != default)
             UnmanagedMethods.SetFocus(handle);
     }
-        
+
     private void FixPosition()
     {
         var handle = WindowHandle;
         if (handle != default && Width > 0 && Height > 0)
             UnmanagedMethods.MoveWindow(handle, 0, 0, Width, Height, true);
     }
-        
+
     /// <inheritdoc />
     protected override void OnResize(EventArgs e)
     {
@@ -111,6 +118,21 @@ protected override void OnResize(EventArgs e)
     /// <inheritdoc />
     protected override void OnPaint(PaintEventArgs e)
     {
+        if (DesignMode)
+        {
+            const string message = "Avalonia control is disabled in design mode.";
 
+            using var pen = new Pen(SystemBrushes.ControlDark);
+            var outline = ClientSize - new SizeF(pen.Width, pen.Width);
+            e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
+            e.Graphics.DrawRectangle(pen, 0, 0, outline.Width, outline.Height);
+            e.Graphics.DrawLine(pen, 0, 0, outline.Width, outline.Height);
+            e.Graphics.DrawLine(pen, 0, outline.Height, outline.Width, 0);
+
+            var messageSize = e.Graphics.MeasureString(message, Font, ClientSize);
+            var messageLocation = new PointF(ClientSize.Width / 2 - messageSize.Width / 2, ClientSize.Height / 2 - messageSize.Height / 2);
+            var messageArea = new RectangleF(messageLocation, messageSize);
+            e.Graphics.DrawString(message, Font, SystemBrushes.ControlText, messageArea);
+        }
     }
 }