# Breaking changes in Windows Forms

Important

The following is a list of breaking changes in Windows Forms by .NET Core version.

## .NET Core 3.0 Preview 9

### Switch.System.Windows.Forms.AllowUpdateChildControlIndexForTabControls compatibility switch not supported

The Switch.System.Windows.Forms.AllowUpdateChildControlIndexForTabControls compatibility switch is supported in Windows Forms on .NET Framework 4.6 and later versions but is not supported in Windows Forms starting with .NET Core 3.0.

#### Change description

In .NET Framework 4.6 and later versions, selecting a tab reorders its control collection. The Switch.System.Windows.Forms.AllowUpdateChildControlIndexForTabControls compatibility switch allows an application to skip this reordering when this behavior is undesirable.

In .NET Core, the Switch.System.Windows.Forms.AllowUpdateChildControlIndexForTabControls switch is not supported.

#### Version introduced

3.0 Preview 9

Remove the switch. The switch is not supported, and no alternative functionality is available.

Windows Forms

• None

### Switch.System.Windows.Forms.DoNotLoadLatestRichEditControl compatibility switch not supported

The Switch.System.Windows.Forms.UseLegacyImages compatibility switch, which was introduced in .NET Framework 4.7.1, is not supported in Windows Forms on .NET Core 3.0.

#### Change description

In the .NET Framework 4.6.2 and previous versions, the RichTextBox control would instantiate the Win32 RichEdit control v3.0, and for applications that target .NET Framework 4.7.1, the RichTextBox control would instantiate RichEdit v4.1 (in msftedit.dll). The Switch.System.Windows.Forms.DoNotLoadLatestRichEditControl compatibility switch was introduced to allow applications that target .NET Framework 4.7.1 and later versions to opt-out of the new RichEdit v4.1 control and use the old RichEdit v3 control instead.

In .NET Core, the Switch.System.Windows.Forms.DoNotLoadLatestRichEditControl switch is not supported. Only new versions of the RichTextBox control are supported.

#### Version introduced

3.0 Preview 9

Remove the switch. The switch is not supported, and no alternative functionality is available.

Windows Forms

### Switch.System.Windows.Forms.DoNotSupportSelectAllShortcutInMultilineTextBox compatibility switch not supported

The Switch.System.Windows.Forms.DoNotSupportSelectAllShortcutInMultilineTextBox compatibility switch, which was introduced in .NET Framework 4.6.1, is not supported in Windows Forms on .NET Core 3.0.

#### Change description

Starting with .NET Framework 4.6.1, selecting the Ctrl + A shortcut key in a TextBox control selected all text. In .NET Framework 4.6 and previous versions, selecting the Ctrl + A shortcut key failed to select all text if the Textbox.ShortcutsEnabled and TextBox.Multiline properties were both set to true. The Switch.System.Windows.Forms.DoNotSupportSelectAllShortcutInMultilineTextBox compatibility switch was introduced in .NET Framework 4.6.1 to retain the original behavior. For more information see TextBox.ProcessCmdKey.

In .NET Core, the Switch.System.Windows.Forms.DoNotSupportSelectAllShortcutInMultilineTextBox switch is not supported.

#### Version introduced

3.0 Preview 9

Remove the switch. The switch is not supported, and no alternative functionality is available.

Windows Forms

• None

### Switch.System.Windows.Forms.DontSupportReentrantFilterMessage compatibility switch not supported

The Switch.System.Windows.Forms.DontSupportReentrantFilterMessage compatibility switch, which was introduced in .NET Framework 4.6.1, is not supported in Windows Forms on .NET Core 3.0.

#### Change description

Starting with the .NET Framework 4.6.1, the Switch.System.Windows.Forms.DontSupportReentrantFilterMessage compatibility switch addresses possible IndexOutOfRangeException exceptions when the Application.FilterMessage message is called with a custom IMessageFilter.PreFilterMessage implementation. For more information, see Mitigation: Custom IMessageFilter.PreFilterMessage Implementations.

In .NET Core, the Switch.System.Windows.Forms.DontSupportReentrantFilterMessage switch is not supported.

#### Version introduced

3.0 Preview 9

Remove the switch. The switch is not supported, and no alternative functionality is available.

Windows Forms

### Switch.System.Windows.Forms.DomainUpDown.UseLegacyScrolling compatibility switch not supported

The Switch.System.Windows.Forms.DomainUpDown.UseLegacyScrolling compatibility switch, which was introduced in .NET Framework 4.7.1, is not supported in Windows Forms on .NET Core 3.0.

#### Change description

Starting with the .NET Framework 4.7.1, the Switch.System.Windows.Forms.DomainUpDown.UseLegacyScrolling compatibility switch allowed developers to opt-out of independent DomainUpDown.DownButton() and DomainUpDown.UpButton() actions. The switch restored the legacy behavior, in which the DomainUpDown.UpButton() is ignored if context text is present, and the developer is required to use DomainUpDown.DownButton() action on the control before the DomainUpDown.UpButton() action. For more information, see <AppContextSwitchOverrides> element.

In .NET Core, the Switch.System.Windows.Forms.DomainUpDown.UseLegacyScrolling switch is not supported.

#### Version introduced

3.0 Preview 9

Remove the switch. The switch is not supported, and no alternative functionality is available.

Windows Forms

### Switch.System.Windows.Forms.EnableVisualStyleValidation compatibility switch not supported

The Switch.System.Windows.Forms.EnableVisualStyleValidation compatibility switch is not supported in Windows Forms on .NET Core 3.0.

#### Change description

In .NET Framework, the Switch.System.Windows.Forms.EnableVisualStyleValidation compatibility switch allowed an application to opt out of validation of visual styles supplied in a numeric form.

In .NET Core, the Switch.System.Windows.Forms.EnableVisualStyleValidation switch is not supported.

#### Version introduced

3.0 Preview 9

Remove the switch. The switch is not supported, and no alternative functionality is available.

Windows Forms

• None

### Switch.System.Windows.Forms.UseLegacyContextMenuStripSourceControlValue compatibility switch not supported

The Switch.System.Windows.Forms.UseLegacyContextMenuStripSourceControlValue compatibility switch, which was introduced in .NET Framework 4.7.2, is not supported in Windows Forms on .NET Core 3.0.

#### Change description

Starting with the .NET Framework 4.7.2, the Switch.System.Windows.Forms.UseLegacyContextMenuStripSourceControlValue compatibility switch allows the developer to opt out of the new behavior of the ContextMenuStrip.SourceControl property, which now returns a reference to the source control. The previous behavior of the property was to return null. For more information, see <AppContextSwitchOverrides> element.

In .NET Core, the Switch.System.Windows.Forms.UseLegacyContextMenuStripSourceControlValue switch is not supported.

#### Version introduced

3.0 Preview 9

Remove the switch. The switch is not supported, and no alternative functionality is available.

Windows Forms

### Switch.System.Windows.Forms.UseLegacyImages compatibility switch not supported

The Switch.System.Windows.Forms.UseLegacyImages compatibility switch, which was introduced in .NET Framework 4.8, is not supported in Windows Forms on .NET Core 3.0.

#### Change description

Starting with the .NET Framework 4.8, the Switch.System.Windows.Forms.UseLegacyImages compatibility switch addressed possible image scaling issues in ClickOnce scenarios in high DPI environments. When set to true, the switch allows the user to restore legacy image scaling on high DPI displays whose scale is set to greater than 100%. For more information, see .NET Framework 4.8 Release Notes on GitHub.

In .NET Core, the Switch.System.Windows.Forms.UseLegacyImages switch is not supported.

#### Version introduced

3.0 Preview 9

Remove the switch. The switch is not supported, and no alternative functionality is available.

Windows Forms

• None

### SerializableAttribute removed from some Windows Forms types

The SerializableAttribute has been removed from some Windows Forms classes that have no known binary serialization scenarios.

#### Change description

The following types are decorated with the SerializableAttribute in .NET Framework, but the attribute has been removed in .NET Core:

Historically, this serialization mechanism has had serious maintenance and security concerns. Maintaining SerializableAttribute on types means those types must be tested for version-to-version serialization changes and potentially framework-to-framework serialization changes. This makes it harder to evolve those types and can be costly to maintain. These types have no known binary serialization scenarios, which minimizes the impact of removing the attribute.

#### Version introduced

3.0 Preview 9

Update any code that may depend on these types being marked as serializable.

Windows Forms

• None

## .NET Core 3.0 RC1

### Change of access for AccessibleObject.RuntimeIDFirstItem

Starting in .NET Core 3.0 RC1, the accessibility of AccessibleObject.RuntimeIDFirstItem has changed from protected to internal.

#### Change description

Starting with .NET Core 3.0 Preview 4, the AccessibleObject.RuntimeIDFirstItem field was protected. Starting with .NET Core 3.0 RC1, it has changed from protected to internal to align with the accessibility of the field in the .NET Framework.

#### Version introduced

3.0 RC1

The change can affect you if you've developed a .NET Core app with a type that derives from AccessibleObject and accesses the RuntimeIDFirstItem field. If this is the case, you can define a local constant as follows:

const int RuntimeIDFirstItem = 0x2a;


Windows Forms

#### Affected APIs

• Not detectable via API analysis.

### Duplicated APIs removed from Windows Forms

A number of APIs accidentally duplicated in the System.Windows.Forms namespace starting in .NET Core 3.0 Preview 4 have been removed in .NET Core 3.0 RC1.

#### Change description

.NET Core 3.0 Preview 4 inadvertently duplicated a number of types in the System.Windows.Forms namespace that already existed in the System.ComponentModel.Design namespace. Starting with .NET Core 3.0 RC1, these duplicated types are no longer available. The following table shows lists the original type and its duplicated type:

Original type Duplicated type
System.ComponentModel.Design.DesignerActionListsChangedEventArgs System.Windows.Forms.DesignerActionListsChangedEventArgs
System.ComponentModel.Design.DesignerActionListsChangedEventHandler System.Windows.Forms.DesignerActionListsChangedEventHandler
System.ComponentModel.Design.DesignerActionListsChangedType System.Windows.Forms.DesignerActionListsChangedType
System.ComponentModel.Design.DesignerActionUIService System.Windows.Forms.DesignerActionUIService
System.ComponentModel.Design.DesignerCommandSet System.Windows.Forms.DesignerCommandSet

#### Version introduced

3.0 RC1

Update the code to reference the original type, as shown in the Original type column of the table.

Windows Forms

#### Affected APIs

• Not detectable via API analysis.

## .NET Core 3.0

### Control.DefaultFont changed to Segoe UI 9pt

#### Change description

In the .NET Framework, the Control.DefaultFont property was set to Microsoft Sans Serif 8pt. The following figure shows a window that uses the default font.

In .NET Core starting with .NET Core 3.0, it is set to Segoe UI 9pt (the same font as SystemFonts.MessageBoxFont). As a result of this change, forms and controls will be sized about 27% larger to account for the larger size of the new default font. For example:

This change was made to align with Windows UX guidelines.

#### Version introduced

3.0

Because of the change in the size of forms and controls, ensure that your application renders correctly.

To retain the original font, set your form's default font to Microsoft Sans Serif 8pt. For example:

public MyForm()
{
InitializeComponent();
Font = new Font(new FontFamily("Microsoft Sans Serif"), 8f);
}


#### Category

• Windows Forms

None.

### Modernization of the FolderBrowserDialog

The FolderBrowserDialog control has changed in Windows Forms applications for .NET Core.

#### Change description

In the .NET Framework, Windows forms uses the following dialog for the FolderBrowserDialog control:

In .NET Core 3.0, Windows Forms users a newer COM-based control that was introduced in Windows Vista:

#### Version introduced

3.0

The dialog will be upgraded automatically.

If you desire to retain the original dialog, set the FolderBrowserDialog.AutoUpgradeEnabled property to false before showing the dialog, as illustrated by the following code fragment:

var dialog = new FolderBrowserDialog();