How to use [RequiresPreviewFeaturesAttribute] with Razor .cshtml file

[imalkevich]

Hi Team,

I'm having some preview features that are being used besides other places inside Razor pages.

While adding new attribute to .cs files works as it should, I'm having issues with adding [RequiresPreviewFeatures] attribute to Razor .cshtml page - actually I can add this attribute, but during the validation I'm having the following error:

**Statistics.cshtml**(39,27): Error CA2252: Using 'CpuStatistics' requires opting into preview features. See https://aka.ms/dotnet-warnings/preview-features for more information. 

**Pages_Statistics_cshtml.g.cs**(218,16): Error CA2252: 'get_Model' returns the preview type 'StatisticsModel' and therefore needs to opt into preview features. See https://aka.ms/dotnet-warnings/preview-features for more information. 

And here is how my recent attempt to add the attribute to Razor page looks like:

Would please somebody suggest a way how to add the needed attribute to the Razor page?

Thanks

[danmoseley]

@buyaa-n maybe you can help here?

[imalkevich]

Hi Team,

Just wanted to share my feedback on this feature:

1. In my .DLL's .csproj file I added the following line <EnablePreviewFeatures>true</EnablePreviewFeatures> - but the compiler still asked me to add [RequiresPreviewFeatures] attribute
2. In my consuming project .csproj I added <EnablePreviewFeatures>true</EnablePreviewFeatures> - but now I'm blocked by the fact that I can't add this [RequiresPreviewFeatures] attribute to Razor's autogenerated file

I followed two documents https://github.com/dotnet/designs/blob/main/accepted/2021/preview-features/preview-features.md and https://docs.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca2252, but my understanding that putting <EnablePreviewFeatures>true</EnablePreviewFeatures> inside consuming .csproj doesn't work...

Would be nice to get some better understanding how this should work.

Thanks

[imalkevich]

Team,

Quick update:

1. I removed this experimental feature in favor of a simpler yet not as elegant API with INumber I was supposed to use
2. Last thing I tried was - in library .csproj file set <EnablePreviewFeatures>True</EnablePreviewFeatures> and <GenerateRequiresPreviewFeaturesAttribute>False</GenerateRequiresPreviewFeaturesAttribute>. For consuming project I configured <EnablePreviewFeatures>True</EnablePreviewFeatures> and it didn't work.

Sad I had to revert this feature, and even sadder that it worked this way for me...

Thanks

[danmoseley]

@tannergooding

[tannergooding]

There is probably a disconnect between what https://github.com/dotnet/designs/blob/main/accepted/2021/preview-features/preview-features.md currently says and what it actually does. The doc says:

Note: The analyzer doesn't track the <EnablePreviewFeatures> property itself, it tracks the attribute [RequiresPreviewFeatures]. By default, setting <EnablePreviewFeatures> to True will mark the entire assembly with [RequiresPreviewFeatures], but developers can turn this off which means that they now need to manually annotate parts of their code with [RequiresPreviewFeatures].

But, I believe you need to additionally annotate GenerateRequiresPreviewFeaturesAttribute=true.

From what I can tell, that matches what the SDK currently has logic for:

• https://github.com/dotnet/sdk/blob/2a515cdbd8f6be1b019ae2c8d7f21952592f0697/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.GenerateAssemblyInfo.targets#L40
• https://github.com/dotnet/sdk/blob/2a515cdbd8f6be1b019ae2c8d7f21952592f0697/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.GenerateAssemblyInfo.targets#L118-L119
• https://github.com/dotnet/sdk/blob/aac033b9324f687eda51d9d7291537bb46cf3a55/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.Common.targets#L51-L53

Perhaps @terrajobst can confirm what the intent here was.

[terrajobst]

Let's step back. Why would you want to add that attribute to a Razor page in the first place?

The 90% usage scenario is: you enable preview features for your projects, which makes them available everywhere in your project, including Razor pages.

1. In my .DLL's .csproj file I added the following line <EnablePreviewFeatures>true</EnablePreviewFeatures> - but the compiler still asked me to add [RequiresPreviewFeatures] attribute

That would be a bug IMHO, unless you explicitly set GenerateRequiresPreviewFeaturesAttribute to false.

2. Last thing I tried was - in library .csproj file set <EnablePreviewFeatures>True</EnablePreviewFeatures> and <GenerateRequiresPreviewFeaturesAttribute>False</GenerateRequiresPreviewFeaturesAttribute>. For consuming project I configured <EnablePreviewFeatures>True</EnablePreviewFeatures> and it didn't work.

This combination of flags is for super advanced scenarios where you want to ship a library that itself uses preview features but you don't want to force your consumers having to enable preview features. That's what we do internally in order to build, for example, System.Runtime itself. Application projects should never need that.

Let me take a look.

[buyaa-n]

I followed two documents https://github.com/dotnet/designs/blob/main/accepted/2021/preview-features/preview-features.md and https://docs.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca2252, but my understanding that putting <EnablePreviewFeatures>true</EnablePreviewFeatures> inside consuming .csproj doesn't work...

That should work, but it would not work if <GenerateAssemblyInfo/> property was set to false, because MSBuild would add assembly-level PreviewFeaturesAttribute into the generated AssemblyInfo.cs file, but if the project is disabled generating this file the attribute could not be added. The workaround would be adding assembly-level [assembly: System.Runtime.Versioning.RequiresPreviewFeaturesAttribute()] attribute in your custom AssemblyInfo.cs file or anywhere in your project. Note that this will make your entire project/assembly Preview assembly and any usage from outside would cause the error

Last thing I tried was - in library .csproj file set <EnablePreviewFeatures>True</EnablePreviewFeatures> and <GenerateRequiresPreviewFeaturesAttribute>False</GenerateRequiresPreviewFeaturesAttribute>. For consuming project I configured <EnablePreviewFeatures>True</EnablePreviewFeatures> and it didn't work.

EDIT Apparently, we added <GenerateRequiresPreviewFeaturesAttribute>False</GenerateRequiresPreviewFeaturesAttribute> property to avoid adding the attribute in the assembly and also to avoid the analyzer warning within the project. But i don't see this is implemented in the analyzers, Filed a bug in the analyzers repo This is not a bug, it was by design