BlazorWebView on Windows Forms locks CSS files while the app is running

[SteveSandersonMS]

Hot reload for CSS doesn't work for WinForms apps because the files are locked while the app is running, so you can't save changes to them.

Repro

1. Run a WinForms blazor app
2. Open wwwroot\css\app.css
3. Try to save the file. It will prompt to save as since the file is locked.

[javiercn]

@SteveSandersonMS do we know why/how the file is locked?

[TanayParikh]

Confirmed the bug. It's the running winforms app which is holding onto the lock:

Filing bug with VS (just need to update to latest first, the above was on 17.2 Preview 6). https://developercommunity.visualstudio.com/t/Resource-Files-Locked-in-Blazor-Hybrid-P/10055002?port=1025&fsid=6e855901-9b89-425f-b0ea-e6774f12f18b&entry=problem

[TanayParikh]

On second thought though, why is the running app using the wwwroot/app.css. Shouldn't it be making a copy in the bin folder? Thoughts @javiercn?

[javiercn]

Yep, not sure what or why it is being blocked, but during development it must stay in the original wwwroot folder.

[TanayParikh]

I believe this is likely a bug in Visual Studio, specifically related to how it handles file sharing for write (save) operations.

On the Blazor Hybrid WebView end, we request the file from the assembly here:

https://github.com/dotnet/aspnetcore/blob/4aa4ec39894cb9ca4bc5cf50460ded9c46846e23/src/Components/WebView/WebView/src/StaticContentProvider.cs#L75

This in turn goes through the file provider which uses the PhysicalFileInfo to open a read stream to the asset with FileShare.ReadWrite here:

https://github.com/dotnet/runtime/blob/757a1918c7f65183c4e727561dd9962a3afc0566/src/libraries/Microsoft.Extensions.FileProviders.Physical/src/PhysicalFileInfo.cs#L51-L57

So VS shouldn't have any trouble saving the file, unless it's trying to write to the file without FileShare.ReadWrite. If I try to save the file using notepad / notepad++ then I'm able to save it successfully. It is only VS which is having trouble saving the file. This makes me suspect this is a VS file sharing issue during the write/save stage.

Bug has been reported to VS/WinForms team here: https://developercommunity.visualstudio.com/t/Resource-Files-Locked-in-Blazor-Hybrid-P/10055002?viewtype=all / https://dev.azure.com/devdiv/DevDiv/_workitems/edit/1546460

For reference; https://stackoverflow.com/a/3448307

[TanayParikh]

Just an update, closed out the AZDO issue given this is likely not a VS issue, but rather an issue with how the CoreWebView2 handles content streams. MicrosoftEdge/WebView2Feedback#2513

[SteveSandersonMS]

@TanayParikh Does this mean we need to work around it by loading the content into memory and then passing a different stream into WebView2? (Or more efficiently, creating some stream that maps the file data into WebView2 without WebView2 knowing that it comes from a file)

[TanayParikh]

The CoreWebView2 team mentions this issue should be resolved in Edge 104. I'm attempting to validate, and hopefully we're able to close this issue out shortly. 😄

The additional stream specifically for the WebView is definitely an option if the issue isn't resolved in CoreWebView2. I've confirmed it does resolve the issue, ideally however we can avoid it, and the additional associated overhead.

[TanayParikh]

Looks like the CoreWebView2 issue hasn't been resolved unfortunately. They're planning on investigating appropriately.

[TanayParikh]

Just an update on this issue; we've engaged with CoreWebView2, and this does seem to be a bug on their end in which they're not able to close the stream after consumption. Discussing next steps...

[TanayParikh]

Just an update for those tuning into this issue; continuing to engage with the CoreWebView2 team. They currently have a (internal) PR which attempts to resolve this issue.

[TanayParikh]

This has been confirmed to be resolved via #9645.

Backporting to 6.0: #9787