# Breaking changes for migration from .NET Framework to .NET Core

If you're migrating an app from .NET Framework to .NET Core, the breaking changes listed in this article may affect you. Breaking changes are grouped by category, and within those categories, by the version of .NET Core in which they were introduced.

Note

## Core .NET libraries

### Change in default value of UseShellExecute

ProcessStartInfo.UseShellExecute has a default value of false on .NET Core. On .NET Framework, its default value is true.

#### Change description

Process.Start lets you launch an application directly, for example, with code such as Process.Start("mspaint.exe") that launches Paint. It also lets you indirectly launch an associated application if ProcessStartInfo.UseShellExecute is set to true. On .NET Framework, the default value for ProcessStartInfo.UseShellExecute is true, meaning that code such as Process.Start("mytextfile.txt") would launch Notepad, if you've associated .txt files with that editor. To prevent indirectly launching an app on .NET Framework, you must explicitly set ProcessStartInfo.UseShellExecute to false. On .NET Core, the default value for ProcessStartInfo.UseShellExecute is false. This means that, by default, associated applications are not launched when you call Process.Start.

This change was introduced in .NET Core for performance reasons. Typically, Process.Start is used to launch an application directly. Launching an app directly does not need to involve the Windows shell and incur the associated performance cost. To make this default case faster, .NET Core changes the default value of ProcessStartInfo.UseShellExecute to false. You can opt in to the slower path if you need it.

#### Version introduced

2.1

Note

In earlier versions of .NET Core, UseShellExecute was not implemented for Windows.

If your app relies on the old behavior, call Process.Start(ProcessStartInfo) with UseShellExecute set to true on the ProcessStartInfo object.

#### Category

Core .NET libraries

### UnauthorizedAccessException thrown by FileSystemInfo.Attributes

In .NET Core, an UnauthorizedAccessException is thrown when the caller attempts to set a file attribute value but doesn't have write permission.

#### Change description

In .NET Framework, an ArgumentException is thrown when the caller attempts to set a file attribute value in FileSystemInfo.Attributes but doesn't have write permission. In .NET Core, an UnauthorizedAccessException is thrown instead. (In .NET Core, an ArgumentException is still thrown if the caller attempts to set an invalid file attribute.)

#### Version introduced

1.0

Modify any catch statements to catch an UnauthorizedAccessException instead of, or in addition to, an ArgumentException, as necessary.

#### Category

Core .NET libraries

### Handling corrupted state exceptions is not supported

Handling corrupted-process-state exceptions in .NET Core is not supported.

#### Change description

Previously, corrupted-process-state exceptions could be caught and handled by managed code exception handlers, for example, by using a try-catch statement in C#.

Starting in .NET Core 1.0, corrupted-process-state exceptions cannot be handled by managed code. The common language runtime doesn't deliver corrupted-process-state exceptions to managed code.

#### Version introduced

1.0

Avoid the need to handle corrupted-process-state exceptions by addressing the situations that lead to them instead. If it's absolutely necessary to handle corrupted-process-state exceptions, write the exception handler in C or C++ code.

#### Category

Core .NET libraries

### UriBuilder properties no longer prepend leading characters

UriBuilder.Fragment no longer prepends a leading # character and UriBuilder.Query no longer prepends a leading ? character when one is already present.

#### Change description

In .NET Framework, the UriBuilder.Fragment and UriBuilder.Query properties always prepend a # or ? character, respectively, to the value being stored. This behavior can result in multiple # or ? characters in the stored value if the string already contains one of these leading characters. For example, the value of UriBuilder.Fragment might become ##main.

Starting in .NET Core 1.0, these properties no longer prepend the # or ? characters to the stored value if one is already present at the beginning of the string.

#### Version introduced

1.0

You no longer need to explicitly remove any of these leading characters when setting the property values. This is especially useful when you're appending values, because you no longer have to remove the leading # or ? each time you append.

For example, the following code snippet shows the behavior difference between .NET Framework and .NET Core.

var builder = new UriBuilder();
builder.Query = "one=1";
builder.Query += "&two=2";
builder.Query += "&three=3";
builder.Query += "&four=4";

Console.WriteLine(builder.Query);

• In .NET Framework, the output is ????one=1&two=2&three=3&four=4.
• In .NET Core, the output is ?one=1&two=2&three=3&four=4.

#### Category

Core .NET libraries

### Process.StartInfo throws InvalidOperationException for processes you didn't start

Reading the Process.StartInfo property for processes that your code didn't start throws an InvalidOperationException.

#### Change description

In .NET Framework, accessing the Process.StartInfo property for processes that your code didn't start returns a dummy ProcessStartInfo object. The dummy object contains default values for all of its properties except EnvironmentVariables.

Starting in .NET Core 1.0, if you read the Process.StartInfo property for a process that you didn't start (that is, by calling Process.Start), an InvalidOperationException is thrown.

#### Version introduced

1.0

Do not access the Process.StartInfo property for processes that your code didn't start. For example, don't read this property for processes returned by Process.GetProcesses.

#### Category

Core .NET libraries

## Cryptography

### Boolean parameter of SignedCms.ComputeSignature is respected

In .NET Core, the Boolean silent parameter of the SignedCms.ComputeSignature(CmsSigner, Boolean) method is respected. A PIN prompt is not shown if this parameter is set to true.

#### Change description

In .NET Framework, the silent parameter of the SignedCms.ComputeSignature(CmsSigner, Boolean) method is ignored, and a PIN prompt is always shown if required by the provider. In .NET Core, the silent parameter is respected, and if set to true, a PIN prompt is never shown, even if it's required by the provider.

Support for CMS/PKCS #7 messages was introduced into .NET Core in version 2.1.

#### Version introduced

2.1

To ensure a PIN prompt appears if required, desktop applications should call SignedCms.ComputeSignature(CmsSigner, Boolean) and set the Boolean parameter to false. The resulting behavior is the same as on .NET Framework regardless of whether the silent context is disabled there.

Cryptography

## MSBuild

### Resource manifest file name change

Starting in .NET Core 3.0, in the default case, MSBuild generates a different manifest file name for resource files.

3.0

#### Change description

Prior to .NET Core 3.0, if no LogicalName, ManifestResourceName, or DependentUpon metadata was specified for an EmbeddedResource item in the project file, MSBuild generated a manifest file name in the pattern <RootNamespace>.<ResourceFilePathFromProjectRoot>.resources. If RootNamespace is not defined in the project file, it defaults to the project name. For example, the generated manifest name for a resource file named Form1.resx in the root project directory was MyProject.Form1.resources.

Starting in .NET Core 3.0, if a resource file is colocated with a source file of the same name (for example, Form1.resx and Form1.cs), MSBuild uses type information from the source file to generate the manifest file name in the pattern <Namespace>.<ClassName>.resources. The namespace and class name are extracted from the first type in the colocated source file. For example, the generated manifest name for a resource file named Form1.resx that's colocated with a source file named Form1.cs is MyNamespace.Form1.resources. The key thing to note is that the first part of the file name is different to prior versions of .NET Core (MyNamespace instead of MyProject).

Note

If you have LogicalName, ManifestResourceName, or DependentUpon metadata specified on an EmbeddedResource item in the project file, then this change does not affect that resource file.

This breaking change was introduced with the addition of the EmbeddedResourceUseDependentUponConvention property to .NET Core projects. By default, resource files aren't explicitly listed in a .NET Core project file, so they have no DependentUpon metadata to specify how to name the generated .resources file. When EmbeddedResourceUseDependentUponConvention is set to true, which is the default, MSBuild looks for a colocated source file and extracts a namespace and class name from that file. If you set EmbeddedResourceUseDependentUponConvention to false, MSBuild generates the manifest name according to the previous behavior, which combines RootNamespace and the relative file path.

In most cases, no action is required on the part of the developer, and your app should continue to work. However, if this change breaks your app, you can either:

• Change your code to expect the new manifest name.

• Opt out of the new naming convention by setting EmbeddedResourceUseDependentUponConvention to false in your project file.

<PropertyGroup>
<EmbeddedResourceUseDependentUponConvention>false</EmbeddedResourceUseDependentUponConvention>
</PropertyGroup>


MSBuild

N/A

## Networking

### WebClient.CancelAsync doesn't always cancel immediately

Starting in .NET Core 2.0, calling WebClient.CancelAsync() doesn't cancel the request immediately if the response has started to fetch.

#### Change description

Previously, calling WebClient.CancelAsync() canceled the request immediately. Starting in .NET Core 2.0, calling WebClient.CancelAsync() cancels the request immediately only if the response hasn't started fetching. If the response has started to fetch, the request is cancelled only after a complete response is read.

This change was implemented because the WebClient API is deprecated in favor of HttpClient.

#### Version introduced

2.0

Use the System.Net.Http.HttpClient class instead of System.Net.WebClient, which is deprecated.

Networking

## Windows Forms

Windows Forms support was added to .NET Core in version 3.0. If you're migrating a Windows Forms app from .NET Framework to .NET Core, the breaking changes listed here may affect your app.

### Removed controls

Starting in .NET Core 3.1, some Windows Forms controls are no longer available.

#### Change description

Starting with .NET Core 3.1, various Windows Forms controls are no longer available. Replacement controls that have better design and support were introduced in .NET Framework 2.0. The deprecated controls were previously removed from designer toolboxes but were still available to be used.

The following types are no longer available:

#### Version introduced

3.1

Each removed control has a recommended replacement control. Refer to the following table:

Removed control (API) Recommended replacement Associated APIs that are removed
DataGrid DataGridView DataGridCell, DataGridRow, DataGridTableCollection, DataGridColumnCollection, DataGridTableStyle, DataGridColumnStyle, DataGridLineStyle, DataGridParentRowsLabel, DataGridParentRowsLabelStyle, DataGridBoolColumn, DataGridTextBox, GridColumnStylesCollection, GridTableStylesCollection, HitTestType
ToolBar ToolStrip ToolBarAppearance
ToolBarButton ToolStripButton ToolBarButtonClickEventArgs, ToolBarButtonClickEventHandler, ToolBarButtonStyle, ToolBarTextAlign

Windows Forms

### CellFormatting event not raised if tooltip is shown

A DataGridView now shows a cell's text and error tooltips when hovered by a mouse and when selected via the keyboard. If a tooltip is shown, the DataGridView.CellFormatting event is not raised.

#### Change description

Prior to .NET Core 3.1, a DataGridView that had the ShowCellToolTips property set to true showed a tooltip for a cell's text and errors when the cell was hovered by a mouse. Tooltips were not shown when a cell was selected via the keyboard (for example, by using the Tab key, shortcut keys, or arrow navigation). If the user edited a cell, and then, while the DataGridView was still in edit mode, hovered over a cell that did not have the ToolTipText property set, a CellFormatting event was raised to format the cell's text for display in the cell.

To meet accessibility standards, starting in .NET Core 3.1, a DataGridView that has the ShowCellToolTips property set to true shows tooltips for a cell's text and errors not only when the cell is hovered, but also when it's selected via the keyboard. As a consequence of this change, the CellFormatting event is not raised when cells that don't have the ToolTipText property set are hovered while the DataGridView is in edit mode. The event is not raised because the content of the hovered cell is shown as a tooltip instead of being displayed in the cell.

#### Version introduced

3.1

Refactor any code that depends on the CellFormatting event while the DataGridView is in edit mode.

Windows Forms

None

### Default control font changed to Segoe UI 9 pt

#### Change description

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

Starting in .NET Core 3.0, the default font is set to Segoe UI 9 pt (the same font as SystemFonts.MessageBoxFont). As a result of this change, forms and controls are 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 user experience (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 8 pt. 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();
dialog.ShowDialog();


Windows Forms

### 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

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

Windows Forms

• None

### 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 on .NET Core or .NET 5.0 and later.

#### 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 and .NET 5.0 and later, the Switch.System.Windows.Forms.AllowUpdateChildControlIndexForTabControls switch is not supported.

#### Version introduced

3.0

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

Windows Forms

• None

### 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 or .NET 5.0 and later.

#### Change description

Starting with .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 and .NET 5.0 and later, the Switch.System.Windows.Forms.DomainUpDown.UseLegacyScrolling switch is not supported.

#### Version introduced

3.0

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

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 or .NET 5.0 and later.

#### Change description

In .NET Framework 4.6.2 and previous versions, the RichTextBox control instantiates the Win32 RichEdit control v3.0, and for applications that target .NET Framework 4.7.1, the RichTextBox control instantiates 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 and .NET 5.0 and later versions, the Switch.System.Windows.Forms.DoNotLoadLatestRichEditControl switch is not supported. Only new versions of the RichTextBox control are supported.

#### Version introduced

3.0

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

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 and .NET 5.0 and later.

#### 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 and .NET 5.0 and later versions, the Switch.System.Windows.Forms.DoNotSupportSelectAllShortcutInMultilineTextBox switch is not supported.

#### Version introduced

3.0

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

Windows Forms

• None

### 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 and .NET 5.0 and later.

#### 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 and .NET 5.0 and later, the Switch.System.Windows.Forms.DontSupportReentrantFilterMessage switch is not supported.

#### Version introduced

3.0

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

Windows Forms

### EnableVisualStyleValidation compatibility switch not supported

The Switch.System.Windows.Forms.EnableVisualStyleValidation compatibility switch is not supported in Windows Forms on .NET Core or .NET 5.0 and later.

#### 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 and .NET 5.0 and later, the Switch.System.Windows.Forms.EnableVisualStyleValidation switch is not supported.

#### Version introduced

3.0

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

Windows Forms

• None

### 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 or .NET 5.0 and later.

#### Change description

Starting with .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 and .NET 5.0 and later, the Switch.System.Windows.Forms.UseLegacyContextMenuStripSourceControlValue switch is not supported.

#### Version introduced

3.0

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

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 or .NET 5.0 and later.

#### Change description

Starting with .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 and .NET 5.0 and later, the Switch.System.Windows.Forms.UseLegacyImages switch is not supported.

#### Version introduced

3.0

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

Windows Forms

• None