MapStaticAssets doesn't work with WebApplication.CreateSlimBuilder

[NoahStahl]

Describe the bug

In an app built with WebApplication.CreateSlimBuilder() and using the new MapStaticAssets in .NET 9, responses for static files are empty with 200 status code. To reproduce, create a new ASPNET Core project from template, then change WebApplication.CreateBuilder() to WebApplication.CreateSlimBuilder() in Program.cs.

It appears that calling UseStaticWebAssets allows this to work, but this isn't documented as far as I can tell.

// Program.cs
var builder = WebApplication.CreateSlimBuilder(args);
builder.WebHost.UseStaticWebAssets();
var app = builder.Build();
...
app.MapStaticAssets();

Would like to understand what is expected and what the correct setup should be. It seems that if CreateSlimBuilder alone isn't sufficient, there should be some kind of warning visible to developer. At the very least, this aspect should be documented.

Expected Behavior

Either MapStaticAssets truly works as a "drop-in replacelement" of UseWebStaticFiles, or incompatbilities are documented and/or warned to developer in tooling/build.

.NET Version

9.0.100

[captainsafia]

cc: @javiercn

There's a chance that the MapStaticAssets API might take a dependency on some services/middleware that aren't registered when you call CreateSlimBuilder.

[javiercn]

@captainsafia I'm trying to think what that would be.

@NoahStahl are you seeing any error on the command line logs? Is there any exception being thrown?

[NoahStahl]

@javiercn The reproduction is pretty straightforward, are you able to see this behavior?

1. Create a new project from .NET 9 template: dotnet new mvc
2. On line 1 of Program.cs, change CreateBuilder to CreateSlimBuilder
3. Launch debug for http. Observe that all the bootstrap script file requests respond with empty 200.

[VictorioBerra]

Getting the same thing, content length is 0. Im not using CreateSlimBuilder() but I do have a pretty custom Program+Startup situation... what middleware am I missing??

For the record I am using this method:

            app.UseEndpoints(endpoints =>
            {
                // TODO https://github.com/dotnet/aspnetcore/issues/58986
                endpoints
                    .MapStaticAssets();

Since I am using a startup.cs

EDIT: Some logs... not sure if related??

[10:37:58 DBG] Request matched endpoint 'null'
[10:37:58 DBG] Request matched endpoint 'null'
[10:37:58 INF] Executing endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint'
[10:37:58 INF] Executing endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint'
[10:37:58 INF] Executed endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint'
[10:37:58 INF] Executed endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint'
[10:37:58 INF] HTTP GET /js/DataTables Plugins/jQuery.Mask.Plugin.js responded 200 in 145.0318 ms
[10:37:58 INF] HTTP GET /js/DataTables Plugins/Display.Plugin.js responded 200 in 143.9094 ms
[10:37:58 INF] Request finished HTTP/2 GET https://localhost:7018/js/DataTables Plugins/jQuery.Mask.Plugin.js?v=EcE4skD5Yjj5T1vtdumYSHbvcyRuO3W99bcl-MPCRKg - 200 0 null 2602.1226ms
[10:37:58 INF] Request finished HTTP/2 GET https://localhost:7018/js/DataTables Plugins/Display.Plugin.js?v=WJEJeFsNGZa5MrWr0dPI7b0GuEYi-Kw1ZeGTlVWDILE - 200 0 null 2587.445ms
[10:37:58 INF] Request starting HTTP/2 GET https://localhost:7018/favicon.ico?v=2 - null null
[10:37:58 DBG] 3 candidate(s) found for the request path '/favicon.ico'
[10:37:58 DBG] Endpoint 'null' with route pattern 'favicon.ico' is valid for the request path '/favicon.ico'
[10:37:58 DBG] Endpoint 'null' with route pattern 'favicon.ico' is valid for the request path '/favicon.ico'
[10:37:58 DBG] Endpoint 'Fallback {**path:file}' with route pattern '{**path:file}' is valid for the request path '/favicon.ico'
[10:37:58 DBG] Request matched endpoint 'null'
[10:37:58 INF] Executing endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint'
[10:37:58 INF] Executed endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint'
[10:37:58 INF] HTTP GET /favicon.ico responded 200 in 33.8560 ms
[10:37:58 INF] Request finished HTTP/2 GET https://localhost:7018/favicon.ico?v=2 - 200 0 null 43.8586ms
[10:37:58 INF] Request starting HTTP/2 GET https://localhost:7018/favicon.ico?v=2 - null null
[10:37:58 DBG] 3 candidate(s) found for the request path '/favicon.ico'
[10:37:58 DBG] Endpoint 'null' with route pattern 'favicon.ico' is valid for the request path '/favicon.ico'
[10:37:58 DBG] Endpoint 'null' with route pattern 'favicon.ico' is valid for the request path '/favicon.ico'
[10:37:58 DBG] Endpoint 'Fallback {**path:file}' with route pattern '{**path:file}' is valid for the request path '/favicon.ico'
[10:37:58 DBG] Request matched endpoint 'null'
[10:37:58 INF] Executing endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint'
[10:37:58 INF] Executed endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint'
[10:37:58 INF] HTTP GET /favicon.ico responded 200 in 39.0186 ms
[10:37:58 INF] Request finished HTTP/2 GET https://localhost:7018/favicon.ico?v=2 - 200 0 null 49.4181ms
[10:37:58 INF] Request starting HTTP/2 GET https://localhost:7018/favicon-32.png?v=2 - null null
[10:37:58 DBG] 2 candidate(s) found for the request path '/favicon-32.png'
[10:37:58 DBG] Endpoint 'null' with route pattern 'favicon-32.png' is valid for the request path '/favicon-32.png'
[10:37:58 DBG] Endpoint 'Fallback {**path:file}' with route pattern '{**path:file}' is valid for the request path '/favicon-32.png'
[10:37:58 DBG] Request matched endpoint 'null'
[10:37:58 INF] Executing endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint'
[10:37:58 INF] Sending file. Request path: 'favicon-32.png'. Physical path: 'C:\Source\REDACTED\wwwroot\favicon-32.png'
[10:37:58 INF] Executed endpoint 'Microsoft.AspNetCore.Routing.RouteEndpoint'
[10:37:58 INF] HTTP GET /favicon-32.png responded 200 in 42.5940 ms
[10:37:58 INF] Request finished HTTP/2 GET https://localhost:7018/favicon-32.png?v=2 - 200 1878 image/png 54.5157ms

[captainsafia]

@NoahStahl Dug into this a little bit more with help from @eerhardt.

It appears that the crux of the issue is related to the fact that when you use CreateSlimBuilder, we don't execute the code paths that register the ManifestStaticWebAssetFileProvider provider into the application. Ultimately, this is the IFileProvider implementation that handles resolving static assets using the manifest-based strategy vs. resolving them by searching directly on disk.

To resolve this issue, you'll want to wire up the correct file providers manually:

var builder = WebApplication.CreateSlimBuilder(args);

builder.WebHost.UseStaticWebAssets();

This issue affects both the MapStaticAssets and UseStaticFiles strategies in .NET 9 so workarounds will be needed for both.

I'll put this in the backlog for now to evaluate urgency for a fix while the workaround is out.