[Preview 6 regression] `GenerateStaticWebAssetEndpointsManifest` throws `Sequence contains more than one element` when a MAUI Blazor Hybrid project references a Razor class library and the BlazorWebView package

[PureWeen]

Summary

In SDK 11.0.100-preview.6.26313.102, building or publishing a .NET MAUI app that uses BlazorWebView and references a Razor Class Library fails on multiple TFMs (Windows AND Apple) with:

error : InvalidOperationException: Sequence contains more than one element
   at System.Linq.ThrowHelper.ThrowMoreThanOneElementException()
   at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1 source)
   at Microsoft.AspNetCore.StaticWebAssets.Tasks.GenerateStaticWebAssetEndpointsManifest
       .ComputeManifestAssets(IEnumerable`1 assets, String kind)+MoveNext()
   at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
   at Microsoft.AspNetCore.StaticWebAssets.Tasks.GenerateStaticWebAssetEndpointsManifest.Execute()

The same project builds and publishes cleanly on SDK 11.0.100-preview.5.26256.105.

Environment

Failing SDK	11.0.100-preview.6.26313.102
Last good SDK	11.0.100-preview.5.26256.105
OS	Reproduces on both Windows and macOS CI hosted images
TFMs affected	net11.0-windows10.0.19041.0, net11.0-windows10.0.20348.0, net11.0-maccatalyst, net11.0-ios (any TFM where the BlazorWebView package targets are imported)
Failing CI jobs (PR #35364, build 1463118)	Build Windows (Debug), Build Windows (Release), Build macOS (Release), Samples windows (integration test)
Project SDK	Microsoft.NET.Sdk.Razor
Target	GenerateStaticWebAssetsPublishManifest
Task	GenerateStaticWebAssetEndpointsManifest
Targets file	.dotnet\sdk\11.0.100-preview.6.26313.102\Sdks\Microsoft.NET.Sdk.StaticWebAssets\targets\Microsoft.NET.Sdk.StaticWebAssets.Publish.targets(55,5)

Repro shape

A MAUI Hybrid app csproj (<Project Sdk="Microsoft.NET.Sdk.Razor">) that:

1. Has Razor components and a wwwroot/ folder with at least one Maui.Controls.Sample.lib.module.js plus a css/app.css.
2. ProjectReferences a Razor Class Library (MauiRazorClassLibrarySample.csproj) that also has wwwroot/ content and scoped CSS.
3. ProjectReferences the BlazorWebView package (Microsoft.AspNetCore.Components.WebView.Maui) — preview.6 SDK assemblies for that package include a _framework/blazor.modules.json static web asset.

The full failing project is publicly available: src/Controls/samples/Controls.Sample/Maui.Controls.Sample.csproj on the .NET MAUI net11.0 branch.

Root cause (from inspecting the binlog)

Three distinct StaticWebAssetEndpoint items collide on the same Route='_framework/blazor.modules.json':

#	Source kind	AssetFile (paths abbreviated)
1	Package (All)	.packages\microsoft.aspnetcore.components.webview\11.0.0-preview.6.26313.102\staticwebassets\blazor.modules.json
2	Build	artifacts\obj\Maui.Controls.Sample\Release\net11.0-windows10.0.20348.0\win-x64\jsmodules\jsmodules.build.manifest.json
3	Publish	artifacts\obj\Maui.Controls.Sample\Release\net11.0-windows10.0.20348.0\win-x64\jsmodules\jsmodules.publish.manifest.json

Asset #3 (note the .publish. segment in the path) appears to be new in preview.6 — preview.5 only produced jsmodules.build.manifest.json.

Looking at the failing code in src/StaticWebAssetsSdk/Tasks/GenerateStaticWebAssetEndpointsManifest.cs:

foreach (var group in assetsByTargetPath)
{
    var asset = StaticWebAsset.ChooseNearestAssetKind(group, kind).SingleOrDefault(); // 💥 throws here
    ...
}

ChooseNearestAssetKind(group, "Publish") is returning >1 asset for the _framework/blazor.modules.json group, and SingleOrDefault() rejects it. The same pattern repeats for several _content/MauiRazorClassLibrarySample/* routes where the raw asset, the .gz compressed asset, and the .br publish-compressed asset collide under the same target path.

Likely culprit

The behavior change in preview.6 is consistent with #54353 — Add support for materializing framework static web assets at project-to-project reference boundaries (merged 2026-05-18). That PR changes how framework assets from package references flow across P2P boundaries, which is exactly the boundary being crossed in this scenario (Razor app → Razor class library → BlazorWebView package).

I haven't bisected to confirm — happy to do so if useful.

Reproduction

The simplest public repro is to run dotnet build on a checkout of the .NET MAUI net11.0 branch with SDK 11.0.100-preview.6.26313.102. The Build Windows jobs of any maui-pr build on dotnet/maui PR #35364 (which bumps SDK from preview.5 → preview.6) fail with this exception; the corresponding stage on the net11.0 branch (still on preview.5) is green.

Reference failing build: Azure DevOps 1463118, job Build Windows (Release). Binlog is from that build; happy to attach (or grab from the AzDO Windows_NT_Build Windows (Release)_Attempt1 artifact).

Expected behavior

The Publish manifest generation should disambiguate Build-kind, Publish-kind, and All-kind assets that map to the same target route, producing exactly one chosen asset per route per manifest kind — not throwing.

Workaround on consumer side?

Not aware of a clean opt-out. Removing the MauiRazorClassLibrarySample ProjectReference avoids the failure but is not a real solution. Suggestions welcome.

Related prior art

• #50183 — same SingleOrDefault exception in the sibling GenerateStaticWebAssetsDevelopmentManifest class; root cause was also two assets collapsing to the same target path.
• #52281 — [StaticWebAssets] Fix duplicate asset detection in GroupAssetsByTargetPath (Dec 2025).
• #53305 — Handle duplicate static web asset identities gracefully (Mar 2026).
• #54353 — likely culprit; introduces P2P framework asset materialization.

cc @javiercn @captainsafia (recent owners of this code)