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 loadLazyAssembly in lazyLoading.ts when loaderHelpers.config.resources.lazyAssembly is null/undefined. Blazor lazy loading requires that assemblies be explicitly opted in via the BlazorWebAssemblyLazyLoad MSBuild item group; without it, no lazy assemblies are listed in the boot resources and the loader has nothing to fetch.

Solutions

  1. Add the assembly to the BlazorWebAssemblyLazyLoad item group in the .csproj: <BlazorWebAssemblyLazyLoad Include="MyLazyAssembly.dll" />.
  2. Rebuild so blazor.boot.json includes a lazyAssembly section.
  3. If you did not intend to lazy-load, remove the call to loadLazyAssembly / lazy-load router config.

Example fix

<!-- before: no lazy items -->
<!-- after: in .csproj -->
<ItemGroup>
  <BlazorWebAssemblyLazyLoad Include="MyLazyAssembly.dll" />
</ItemGroup>
Defensive patterns

Strategy: validation

Validate before calling

// before calling loadLazyAssembly
if (!config.resources?.lazyAssembly?.length)
    throw new Error('Add <BlazorWebAssemblyLazyLoad Include="..." /> to .csproj first');

Prevention

When it happens

Trigger: Calling JSRuntime's lazy-load API (or OnNavigateAsync with RouterConfig with lazy assemblies) in a Blazor WebAssembly project that has not added any <BlazorWebAssemblyLazyLoad Include="..." /> items.

Common situations: Adopting lazy loading but forgetting the MSBuild item; renaming an assembly so the Include no longer matches; deploying with a stale blazor.boot.json that lacks a lazyAssembly section.

Related errors

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/fff0a6822b83d1a6. Report an issue: GitHub.

Appendix: source

Thrown at src/mono/browser/runtime/lazyLoading.ts:13

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

import { loaderHelpers } from "./globals";
import { load_lazy_assembly } from "./managed-exports";
import { type AssemblyAsset, type PdbAsset } from "./types";
import { type AssetEntryInternal } from "./types/internal";

export async function loadLazyAssembly (assemblyNameToLoad: string): Promise<boolean> {
    const resources = loaderHelpers.config.resources!;
    const lazyAssemblies = 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);

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

    let dllAsset: (AssemblyAsset & AssetEntryInternal) | null = null;
    for (let i = 0; i < lazyAssemblies.length; i++) {
        const asset = lazyAssemblies[i];
        if (asset.virtualPath === assemblyNameToLoadDll || asset.virtualPath === assemblyNameToLoadWasm) {
            dllAsset = asset as AssemblyAsset & AssetEntryInternal;
            dllAsset.behavior = "assembly";
            break;

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