Winforms: Cannot add ElementHost control in project because there’s no Integration namespace in System.Windows.Forms

[John-Qiao]

• .NET Core Version: 3.0.100-preview7-012434 from Master branch
• Have you experienced this same bug with .NET Framework?: No

OS: RS4 X64
VS: Dev16 16.2.0 Preview 3.0 [29014.106.d16.2]
.NET Core: 3.0.100-preview7-012434 from Master branch

More Info:
This issue can repro in 3.0.100-preview7-012361 from Master branch too. But it’s not existed in latest 3.0.100-preview6-012266 from Release branch. It's a regression issue.

Steps to reproduce:

1. Install the .NET Core 3.0 SDK from https://github.com/dotnet/core-sdk for Master branch.
2. Create a template of Winforms .NET Core app in VS.
3. Open From1.cs file and type System.Windows.Forms.Integration.

Actual:
There’s no Integration namespace exists in System.Windows.Forms, and cannot add ElementHost control in .NET Core project.

Expected:
There should has the Integration namespace in System.Windows.Forms and we can create ElementHost object.

[weltkante]

This namespace is not part of WinForms, its part of WPF. The Assembly is WindowsFormsIntegration.dll and in Desktop Framework you have to add the reference explicitely.

It's a regression issue

Not sure if this is a regression, could be a bugfix. I wouldn't expect it to be on by default, because that would mean all WinForms projects would have to load WPF "just in case". You probably have to hit some switch in the project file to make the project support both WPF and WinForms and get this feature enabled.

[merriemcgaw]

@vatsan-madhavan what are your thoughts on how we ought to get this enabled for the developers using the ElementHost control in their apps, and not have WPF load for all apps?

[merriemcgaw]

@John-Qiao can you help narrow down which build it changed in?

[vatsan-madhavan]

It's not going to _load_ WPF.

Setting both <UseWindowsForms>true</UseWindowsForms> and <UseWpf>true</UseWpf> will switch from Microsoft.WindowsDesktop.App.WindowsForms FrameworkReference profile to Microsoft.WindowsDesktop.App profile.

The _full_ profile will add references to all assemblies in Microsoft.WindowsDesktop.App - yes - but what the app _loads_ is only what intends to _use_. In this instance, it will really only _load_ WindowsFormsIntegration. (That said, if the app intends to use ElementHost in WinForms, I'd imagine that a WPF control may be hosted within...)

I realize that this design could given the _impression_ of a regression. In .NET Framework, the extra reference was WindowsFromsIntegration (no words like 'wpf' in there), whereas in .NET Core, the change being requested to the project looks like <UseWpf>true</UseWpf>, suggesting (perhaps in the extreme) that "all of WPF may be used".

Do we think the problem is sufficiently bad that we have to consider adding a synonym for _use everything in WindowsDesktop.App_ that doesn't involve 'wpf' in the wording? something like UseWindowsFormsIntegration - which will really be a synonym for UseWpf (but it could be less jarring to use in a Windows Forms project) ?

/cc @nguerrera, @dsplaisted

[vatsan-madhavan]

BTW this would have changed recently (<2 weeks)

FrameworkReference profile refactoring was done in sdk ~month ago (https://github.com/dotnet/sdk/pull/3259), and support for it was just added in WindowsDesktop SDK ~2wks ago:https://github.com/dotnet/wpf/pull/868

Also see https://github.com/dotnet/cli/issues/10536

[dsplaisted]

I think for now having to set UseWPF is OK, and once we have the Reference support, you should also be able to specifically reference WindowsFormsIntegration without the other WPF references.

[vatsan-madhavan]

I think for now having to set UseWPF is OK

+1

[merriemcgaw]

@John-Qiao can you confirm adding the UseWPF flag to the project file will allow you to add an ElementHost control.

[John-Qiao]

@merriemcgaw I retested this scenario in .NET Core 3.0.100-preview7-012578 app, after added true flag in project file, the ElementHost control works well in form now.

[RussKie]

Can we close it now?

[merriemcgaw]

Yes we can