manus-skills/.kortix/opencode/skills/coding/migrate-dotnet9-to-dotnet10/references/winforms-wpf-dotnet9to10.md

Windows Forms and WPF Breaking Changes (.NET 10)

These changes affect projects using Windows Forms (<UseWindowsForms>true</UseWindowsForms>) or WPF (<UseWPF>true</UseWPF>).

Windows Forms

Source-Incompatible Changes

API obsoletions

Several Windows Forms APIs have been marked obsolete with custom diagnostic IDs. Follow the guidance in the warning message for each.

Applications referencing both WPF and WinForms must disambiguate MenuItem and ContextMenu types

If a project references both WPF and WinForms, the MenuItem and ContextMenu types are ambiguous. Use fully qualified names:

// Before (ambiguous in .NET 10)
var item = new MenuItem("File");

// After
var item = new System.Windows.Forms.MenuItem("File");
// or
var item = new System.Windows.Controls.MenuItem();

Renamed parameter in HtmlElement.InsertAdjacentElement

The parameter name in HtmlElement.InsertAdjacentElement has changed from orientation. Calls that use named arguments with orientation will no longer compile; update them to use positional arguments:

// Before — named arguments with old parameter name
element.InsertAdjacentElement(orientation: HtmlElementInsertionOrientation.BeforeBegin, newElement: newElement);

// After — use positional arguments
element.InsertAdjacentElement(HtmlElementInsertionOrientation.BeforeBegin, newElement);

Behavioral Changes

TreeView checkbox image truncation

The checkbox rendering in TreeView controls has been adjusted, which changes text positioning. Visual appearance may differ slightly.

StatusStrip uses System RenderMode by default

StatusStrip now uses the system render mode by default instead of a custom renderer. The visual appearance may change. Set RenderMode explicitly to restore the previous look.

System.Drawing OutOfMemoryException changed to ExternalException

Important: Some System.Drawing operations that previously threw OutOfMemoryException now throw ExternalException (from System.Runtime.InteropServices — NOT ArgumentException). This reflects the actual GDI+ error code. Update catch blocks:

// Before — only catching OutOfMemoryException
try { /* drawing operation */ }
catch (OutOfMemoryException) { /* handle */ }

// After — catch ExternalException (the new exception type in .NET 10)
try { /* drawing operation */ }
catch (ExternalException) { /* handle */ }
catch (OutOfMemoryException) { /* handle — for older runtimes */ }

WPF

Source-Incompatible / Behavioral Changes

Empty ColumnDefinitions and RowDefinitions are disallowed

Empty <Grid.ColumnDefinitions/> and <Grid.RowDefinitions/> elements in XAML now cause errors. Remove them if they don't contain any definitions:

<!-- Before (now causes error) -->
<Grid>
    <Grid.ColumnDefinitions/>
    <Grid.RowDefinitions/>
</Grid>

<!-- After -->
<Grid>
    <!-- Only include definitions if you have columns/rows to define -->
</Grid>

Incorrect usage of DynamicResource causes application crash

Incorrect DynamicResource usage that was silently ignored now causes crashes at runtime. Common issues:

• Using DynamicResource where StaticResource is required (e.g., in non-dependency-property contexts)
• Referencing resources that don't exist

Audit all DynamicResource usage in XAML and ensure each reference points to a valid resource and is used in a context that supports dynamic resources.