Add Warning in WindowsDesktop SDK for UseWinForms

See https://github.com/dotnet/wpf/pull/1027#issuecomment-505092719

The comment from @drewnoakes suggests that there is occasional confusion that leads to the use of UseWinForms in Microsoft.NET.Sdk.WindowsDesktop based projects instead of using the correct property UseWindowsForms.

The current behavior will be warning NETSDK1106 (“Windows Desktop SDK requires UseWpf or UseWindowsForms”). This should help the vast majority of WinForms projects that guess the property name wrong.

The small number of projects that try to set UseWpf and UseWinForms (instead of the correct property UseWindowsForms) will not receive any warnings today.

We should evaluate whether a new warning is needed to help those who set UseWinForms and add it to the WindowsDesktop SDK.

/cc @merriemcgaw, @OliaG

[vatsan-madhavan]

/cc @RussKie, @davkean, @dsplaisted, @drewnoakes , @stevenbrix , @rladuca , @ryalanms , @nguerrera @davidwengier

This is my attempt at gently moving over the discussion about WindowsForms vs. WinForms from https://github.com/dotnet/wpf/pull/1027 over to here. I think I’ve tagged everyone who was a participant there except bot-accounts 😉

[davkean]

@vatsan-madhavan The assumption there is that SDK targets files are evaluated before anything else that consumes UseWindowsForms, which might not always be the case. The other assumption that is folks are going to know that they need to check UseWindowsForms in targets and not UseWinForms which is not the canonical value. I’m convinced that having multiple ways to represent the same thing, will result in the reverse confusion “which one do we set and which one do I use in conditions and targets?”

If we were to do something about this, I think a warning or error would be enough, they’ll immediately see the mistake and fix it. Do we have any existing confusion already on this during the previews @dsplaisted @nguerrera that would lead towards this?