Roslyn: Documentation for consuming feature flags is unclear

[WreckedAvent]

Version Used:
VS "15" Preview

Steps to Reproduce:

1. Create a new C# project
2. Add the feature flag /features:patterns to the command line arguments in the properties page
3. Use any pattern matching syntax
4. Error telling you to do step 2

Expected Behavior:

No error at step 4.

Actual Behavior:

Notes:

Also tried adding as an argument to the shortcut to launch "15", but that just triggered a help window describing what valid flags were available.

Apologies if this is not where you enable feature flags, but I can't appear to find any documentation _at all_ talking about them. Kind of weird, considering it's advertised on the download page.

[Pilchie]

That page is for specifying options to the program you want to debug, not to the compiler. Also, devenv.exe (the shortcut) is the IDE, not the compiler. To do this, you'll need to do it by editing the .csproj file directly. Tagging @jasonmalinowski for an example property to add to the .csproj file.

[jasonmalinowski]

Yep, if you want to test a language feature, add:

<Features>patterns</Features>

To your project file in the first <PropertyGroup> section.

[WreckedAvent]

Adding that in the csproj got it. Is that documented anywhere other than this page?

[jasonmalinowski]

Not that I know of. @jaredpar or @kuhlenh might know?

[WreckedAvent]

Let me put the error down for sake of google then:

error CS8058: Feature 'pattern matching' is experimental and unsupported; use '/features:patterns' to enable.

[TheGrandUser]

Putting the features in the first property group (nor in any of the others) is not working for me, in both the enterprise and light weight installs of VS 15, so is there something else that needs to be done?

[WreckedAvent]

@TheGrandUser Hmm, I was able to make it work just by adding to the csproj as @jasonmalinowski indicated. Here's a screenshot of what's working for me:

(The IDE will complain it's not a valid element, but it will load just file)

[WreckedAvent]

Also, I forgot to reply to @Pilchie

Yes, I know what the page is for, and I know that the shortcut goes to devenv, but there was no documentation I could find about where to apply a feature toggle. Usually, "feature toggles" would be switches to applications you'd run (ie something you'd want to add to your shortcut). Since devenv gives you a cold shoulder to this one, there's no more information scent about where you could possibly add one.

I can't say editing the csproj is particularly intuitive. I'm certainly not the only one to then try the debug page out of desperation. Some people gave up trying to find feature toggles all together. Even if I _had_ the idea to try and play with the csproj, the IDE says that Features isn't a valid element.

I'm definitely not going to be the only one getting stuck on this. Some documentation or a page in properties somewhere would be perfect. Currently, the debug page is the only one that gives even remotely the correct information scent about accepting feature toggles, and it's not correct either.

Hopefully that gives you some insight as to how I ended up there, and ultimately here.

[jaredpar]

The feature flags need to be separated by a ;

<Features>patterns;patternsExperimental</Features>

[WreckedAvent]

@jaredpar either ; or a new line works for me

[MgSam]

Can someone update the docs at https://www.visualstudio.com/news/vs15-preview-vs#CsharpAndVB to add this documentation?

[TheGrandUser]

Ok, I can confirm that fixes it. The problem seems to have been I put

<Features>patterns</Features>
<Features>localFunctions</Features>

which I think was causing it to overwrite the earlier one.

[jasonmalinowski]

@TheGrandUser: correct, that just overwrites. @MgSam, we'll look at getting that updated.

[Eyas]

For my PCL library, I added <Features>patterns</Features> and it worked wonderfully. Yet, for a UWP app project that I have, adding the same flag to the .csproj file doesn't seem to help, I still get:

error CS8058: Feature 'pattern matching' is experimental and unsupported; use '/features:patterns' to enable.

[jasonmalinowski]

@Eyas, same NuGet package for each? I guess it's possible we may have a bug where projects consuming the compiler via project.json might be broken...

[jasonmalinowski]

@kuhlenh, can I give this to you to get the release notes updated? (cc @DustinCampbell, @Pilchie)

[kuhlenh]

Updated the documentation here: https://www.visualstudio.com/news/releasenotes/vs15-relnotes#cshappvb.

Hope that helps!

[WreckedAvent]

@kuhlenh that's much better!