dotnet/runtime · error · Error

No assemblies have been marked as lazy-loadable. Use the…

Error message

No assemblies have been marked as lazy-loadable. Use the 'BlazorWebAssemblyLazyLoad' item group in your project file to enable lazy loading an assembly.

What it means

Thrown by fetchLazyAssembly() (src/native/libs/Common/JavaScript/loader/assets.ts:268). When managed code calls AssemblyLoadContext to lazily load an assembly at runtime, the loader looks up loaderConfig.resources.lazyAssembly; if that collection does not exist, no assembly was ever marked lazy-loadable, so it throws and tells you to use the BlazorWebAssemblyLazyLoad item group.

Solutions

1. Add a <BlazorWebAssemblyLazyLoad Include="MyLazyAssembly.dll" /> item to the Blazor app project for each assembly you load lazily.
2. If you no longer intend to lazy-load, remove the code path that calls LoadAssembliesAsync instead of leaving it dangling.
3. Rebuild/re-publish after editing the item group so the boot config includes the lazyAssembly resource list.

Example fix

<!-- before: LoadAssembliesAsync called, no item group -> No assemblies have been marked as lazy-loadable -->

<!-- after: add to the .csproj of the Blazor app -->
<ItemGroup>
  <BlazorWebAssemblyLazyLoad Include="MyLazyAssembly.dll" />
</ItemGroup>

function hasLazyAssemblies(resources: any): boolean {
  return Array.isArray(resources?.lazyAssembly) && resources.lazyAssembly.length > 0;
}
if (!hasLazyAssemblies(loaderConfig.resources)) {
  // do not call LazyAssemblyLoader.LoadAssembliesAsync, or add the item group
}

function hasLazyLoadables(r: any): r is { lazyAssembly: { virtualPath: string }[] } {
  return Array.isArray(r?.lazyAssembly) && r.lazyAssembly.length > 0;
}

When it happens

Trigger: Managed code triggers a lazy assembly load (e.g. NavigationManager for a page whose component assembly is lazy, or an explicit AssemblyLoadContext.Load). fetchLazyAssembly is invoked; resources.lazyAssembly is undefined, so line 270 throws.

Common situations: Calling LazyAssemblyLoader.LoadAssembliesAsync without having marked any assembly with BlazorWebAssemblyLazyLoad in the project; removing the item group but keeping the lazy-load call site; a route to a lazy component without registering its assembly as lazy-loadable.

Related errors

• must be marked with 'BlazorWebAssemblyLazyLoad' item group…
• must be marked with 'BlazorWebAssemblyLazyLoad' item group…
• No assemblies have been marked as lazy-loadable. Use the…
• Can't use moduleFactory callback of createDotnetRuntime…
• Cannot call synchronous C# methods.

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/25d1ef81839d3450. Report an issue: GitHub.

Appendix: source

Thrown at src/native/libs/Common/JavaScript/loader/assets.ts:271

            continue;
        }
        for (const asset of satelliteResources[culture]) {
            const assetInternal = asset as AssetEntryInternal;
            assetInternal.culture = culture;
            promises.push(fetchAssembly(asset));
        }
    }
    await Promise.all(promises);
}

function lazyAssetFileName(virtualPath: string): string {
    return virtualPath.substring(virtualPath.lastIndexOf("/") + 1);
}

export async function fetchLazyAssembly(assemblyNameToLoad: string): Promise<boolean> {
    const lazyAssemblies = loaderConfig.resources?.lazyAssembly;
    if (!lazyAssemblies) {
        throw new Error("No assemblies have been marked as lazy-loadable. Use the 'BlazorWebAssemblyLazyLoad' item group in your project file to enable lazy loading an assembly.");
    }

    let assemblyNameWithoutExtension = assemblyNameToLoad;
    if (assemblyNameToLoad.endsWith(".dll"))
        assemblyNameWithoutExtension = assemblyNameToLoad.substring(0, assemblyNameToLoad.length - 4);
    else if (assemblyNameToLoad.endsWith(".wasm"))
        assemblyNameWithoutExtension = assemblyNameToLoad.substring(0, assemblyNameToLoad.length - 5);

    if (loadedLazyAssemblies.has(assemblyNameWithoutExtension)) {
        return false;
    }

    const assemblyNameToLoadDll = assemblyNameWithoutExtension + ".dll";
    const assemblyNameToLoadWasm = assemblyNameWithoutExtension + ".wasm";

    let dllAsset: AssemblyAsset | null = null;
    for (const asset of lazyAssemblies) {
        const fileName = lazyAssetFileName(asset.virtualPath);

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