[Breaking change]: Windows Forms SDK support for implicit namespaces in C# projects

[RussKie]

Description

Related to #25066

Windows Forms SDK has enabled global usings in .NET 6.0 RC1 in line with the rest of .NET SDK with all guards and requirements outlined in #25066.

Implicit namespace will be enabled by default for C# projects targeting the net6.0 TFM or higher and using Microsoft.NET.Sdk, Microsoft.NET.Sdk.Web or Microsoft.NET.Sdk.Worker SDK. A generated file containing the default namespaces will be included in the set of files passed to the compiler. Projects using a C# version less than 10 need to explicitly disable this feature.

There are three ways to control the namespaces that are included in the project:

1. Disable the feature completely by setting <DisableImplicitNamespaceImports> to true or <DisableImplicitNamespaceImports_DotNet> to true in the project file. Refer to SDK support for implicit namespaces in C# projects #25066.
2. Disable a set of implicit namespaces added by the Windows Forms SDK by setting <DisableImplicitNamespaceImports_WindowsForms> to true in the project file.
3. Add or remove individual namespaces in the project file by modifying the <Import> item group in the project file. For example:

<ItemGroup>
  <Import Remove="System.Linq" />
  <Import Include="MyCool.Project" />
</ItemGroup>

These are the implicit namespaces added to projects using by the Windows Forms SDKs:

System
System.Collections.Generic
System.IO
System.Linq
System.Drawing
System.Windows.Forms

Version

.NET 6 RC1

Previous behavior

No implicit namespaces are added for applicable C# projects.

New behavior

Implicit namespaces are added via global usings in a generated file in the obj directory. See how to control the list of included implicit namespaces above.

Type of breaking change

• Binary incompatible: Existing binaries may encounter a breaking change in behavior, such as failure to load/execute or different run-time behavior.
• Source incompatible: Source code may encounter a breaking change in behavior when targeting the new runtime/component/SDK, such as compile errors or different run-time behavior.

Reason for change

These changes in the SDK are made to reduce the amount of boilerplate using statements that appear at the top of most C# projects.

Recommended action

This feature will be enabled by default and for most users, no action will need to be taken. However, the introduction of this mechanism can cause type name conflicts with the namespaces listed above. In those cases, action need to be take to modify the list of implicit namespaces.

Feature area

C#, Windows Forms

Affected APIs

None.

[gewarren]

@RussKie Does it make sense to you to add this info to the existing article at https://docs.microsoft.com/en-us/dotnet/core/compatibility/sdk/6.0/implicit-namespaces? Or create a new article under WinForms?

[RussKie]

I don't have a strong preference. If it makes sense and easier - let's add it to the existing article. Would be good to have a link from the Windows Forms area to the article though.
I expect WPF submitting a similar breaking change notification, as they have made similar changes too. /cc: @ryalanms

[gewarren]

@RussKie Why was this closed?

[RussKie]

Sorry, I thought I have posted a comment.

The whole feature is being re-designed, see dotnet/sdk#19521. So the current description is no longer relevant.