fix: Prevent unexpected ContentDialog behavior at design time (#1543)

Co-authored-by: pomian <[email protected]>

Author: apachezy

src/Wpf.Ui/Controls/ContentDialog/ContentDialog.cs

@@ -470,11 +470,16 @@ public ContentDialog()
     {
         SetValue(TemplateButtonCommandProperty, new RelayCommand<ContentDialogButton>(OnButtonClick));
 
-        Loaded += static (sender, _) =>
+        // Avoid registering runtime code that triggers designer behavior or throws exceptions
+        // at design time (to reduce the possibility of designer crashes/rendering failures).
+        if (!Wpf.Ui.Designer.DesignerHelper.IsInDesignMode)
         {
-            var self = (ContentDialog)sender;
-            self.OnLoaded();
-        };
+            Loaded += static (sender, _) =>
+            {
+                var self = (ContentDialog)sender;
+                self.OnLoaded();
+            };
+        }
     }
 
     /// <summary>
@@ -492,11 +497,16 @@ public ContentDialog(ContentPresenter? dialogHost)
 
         SetValue(TemplateButtonCommandProperty, new RelayCommand<ContentDialogButton>(OnButtonClick));
 
-        Loaded += static (sender, _) =>
+        // Avoid registering runtime code that triggers designer behavior or throws exceptions
+        // at design time (to reduce the possibility of designer crashes/rendering failures).
+        if (!Wpf.Ui.Designer.DesignerHelper.IsInDesignMode)
         {
-            var self = (ContentDialog)sender;
-            self.OnLoaded();
-        };
+            Loaded += static (sender, _) =>
+            {
+                var self = (ContentDialog)sender;
+                self.OnLoaded();
+            };
+        }
     }
 
     /// <summary>
@@ -601,6 +611,12 @@ protected virtual void OnButtonClick(ContentDialogButton button)
 
     protected override Size MeasureOverride(Size availableSize)
     {
+        // Avoid throwing exceptions when visual child elements cannot be obtained (designer or template not applied).
+        if (VisualChildrenCount == 0)
+        {
+            return base.MeasureOverride(availableSize);
+        }
+
         var rootElement = (UIElement)GetVisualChild(0)!;
 
         rootElement.Measure(availableSize);
@@ -622,6 +638,7 @@ protected override Size MeasureOverride(Size availableSize)
     /// </summary>
     protected virtual void OnLoaded()
     {
+        // Focus is only needed at runtime.
         _ = Focus();
 
         RaiseEvent(new RoutedEventArgs(OpenedEvent));