MapStaticAssets is breaking a background video from playing

Question

MapStaticAssets is breaking a background video from playing

Daniel Cooper 30

I have a rather large Blazor WASM application running on Dot Net 9. The login page has a background video. Everything works well. We are making some significant changes and want to include an upgrade to Dot Net 10.

The upgrade results in a Blazor.web.js 404 error at runtime which (from research) is fixed by adding MapStaticAssets to the program.cs. Even though UseStaticFiles is still there, the video (mp4) doesn't play - or is included in wwwroot. jpg and svg files appear. it's just the video. It doesn't matter where I place the MapStaticAssets.

I cannot find any official guidance on how to address this issue. There are plenty of ideas on the various forums but most don't work for me.

So, is there a way of avoiding the Blazor.web.js 404 error or what is the correct configuration for MapStaticAssets to play the video?

I've lost 2 days on this already and any help would be very much appreciated.

Daniel Cooper 30 Reputation points

2025-11-24T16:43:25.1433333+00:00

Further information - it appears that if you publish the app to azure it works as expected so this could be related to how the app is served locally. It also doesn't seem to matter if I run via IIS or https

Answer accepted by question author

2 additional answers

Your answer

Daniel Cooper 30 Reputation points

2025-11-24T16:43:25.1433333+00:00

Further information - it appears that if you publish the app to azure it works as expected so this could be related to how the app is served locally. It also doesn't seem to matter if I run via IIS or https

Answer 1

Hi Daniel Cooper,

Thanks for clarifying the sequence.

The fact the video works in production (Azure) but fails locally suggests that there's a difference in how static files, especially videos which use byte-range requests, are processed by MapStaticAssets() versus your local hosting pipeline (Kestrel). There's a known issue reported in the ASP.NET Core repo titled "MapStaticAssets returns incorrect Content-Length for custom Range". The issue link is:

https://github.com/dotnet/aspnetcore/issues/63320
(Disclaimer: Some links are non-Microsoft website. The pages appear to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classifies as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it. )

In that issue, a video file served via MapStaticAssets() caused a mismatch between Content-Range and Content-Length, which breaks playback in some browsers.

Here's a more targeted workaround you can test locally:

In your server project's .csproj, exclude the .mp4 video file(s) from static-web-asset processing:

<ItemGroup>
<Content Update="wwwroot/videos/**/*.mp4"
              CopyToOutputDirectory="PreserveNewest">
<ExcludeFromSingleFile>true</ExcludeFromSingleFile>
</Content>
</ItemGroup>
<ItemGroup>
<StaticWebAsset Remove="wwwroot/videos/**/*.mp4" />
</ItemGroup>

Run locally and check whether the video plays. If yes, then you've effectively sidestepped the issue locally by letting UseStaticFiles() serve the video.
For confirmation, open your browser's Developer Tools → Network tab, request the video and inspect the headers:
- Check if the request has a Range header
- Check if the response has Accept-Ranges: bytes
- Check if Content-Range is present and if Content-Length matches the range length (rather than full file size). If Content-Length equals full file size despite a partial request.

Hope this helps!

Daniel Cooper 30 Reputation points

2025-11-26T14:10:09.0133333+00:00

Hi Raymond,

Thanks for taking the time to reply so comprehensively. I tried the workaround but it hasn't made a difference for me. I'll play around with it some more. The solution that the video file sits in is a Blazor hybrid so the wwwroot resources are in the shared project. I tried changing wwwroot/... to _content/[project].Shared/... which didn't work. Tried copying the video into the clients wwwroot also and that didn't work. Will continue to play. Knowing it works in Azure is good enough for now though as no one sees my local environment except me.

Hopefully Microsoft will patch this properly in an upcoming release or issue some formal guidance on how to migrate from UseStaticFiles to MapStaticAssets as it seems to be a mandatory requirement in Blazor Net 10.

Thanks again for taking the time to reply. Much appreciated.
Raymond Huynh (WICLOUD CORPORATION) 3,645 Reputation points Microsoft External Staff Moderator

2025-11-28T10:34:13.6666667+00:00

Hello Daniel Cooper,
Thank you for your update. I hope the incoming patches will resolve this issue for you. If you find my answer helpful, please mark it as the final answer.
Warm thanks.

Answer 2

To address the issue with the background video not playing after implementing MapStaticAssets, ensure that the video file is correctly placed in the wwwroot directory and that the file path used in your application is accurate. Here are some steps you can follow:

Verify Video Path: Ensure that the path to the video file in your code is correct. It should point to the location within the wwwroot directory.
Check MIME Types: Ensure that the server is configured to serve the video file type (e.g., .mp4). You can check the server's configuration to ensure that the MIME type for .mp4 is set correctly.
Order of Middleware: The order in which you call UseStaticFiles and MapStaticAssets in your Program.cs file matters. Typically, you should call UseStaticFiles() before MapStaticAssets(). Here’s an example:
```
   app.UseStaticFiles();
   app.MapStaticAssets();
```
Caching Issues: Sometimes, caching can cause issues with loading updated files. Clear your browser cache or test in an incognito window to see if that resolves the issue.
Check for Errors: Use the browser's developer tools to check for any errors in the console or network tab that might give you more insight into why the video is not loading.

If you continue to experience issues, consider reviewing the official documentation for any updates or changes related to static file handling in Blazor applications.

References:

Answer 3

Bruce (SqlWork.com) 81,971 Volunteer Moderator

With blazor WASM, static files are just server files cached by manifest preload. When using the separate server, client projects, the static are hosted by the server. When it creates the manifest it should include them, but from the client they are just root urls.

Daniel Cooper 30 Reputation points

2025-11-24T23:50:13.2733333+00:00

Thanks Bruce but this doesn't move me forward. Can I avoid the 404 error in Dot Net 10 another way (i.e. not using MapStaticAssets) or is there a proper way to configure the program.cs file so it behaves as it does in Dot Net 9. As I have followed up on the original question, if you publish to azure it works so its a development runtime issue rather than a core issue.

Share via

MapStaticAssets is breaking a background video from playing

2 additional answers

Your answer