[mono][wasm] Fix AOT + BlazorWebAssemblyLazyLoad startup crash by lewing · Pull Request #131020 · dotnet/runtime

lewing · 2026-07-18T22:21:40Z

Summary

Fixes #125794. AOT-published Blazor WebAssembly apps that use BlazorWebAssemblyLazyLoad crash during Mono runtime startup — reported as appdomain.c assertion (condition '<disabled>' not met) and, in other configurations, interp.c NIY … should not be reached.

Root cause

An AOT image hard-binds to every assembly it references when the image is loaded. load_aot_module in src/mono/mono/mini/aot-runtime.c eagerly calls load_image for all referenced images unless the aot-lazy-assembly-load runtime option is set:

/* … we have to load all referenced assemblies non-lazily … */
if (!mono_opt_aot_lazy_assembly_load) {
    for (guint32 i = 0; i < amodule->image_table_len; ++i)
        load_image (amodule, i, load_error);
}

A BlazorWebAssemblyLazyLoad assembly is not present at runtime startup (it is downloaded on demand). So when the main app's AOT image is loaded, resolving its lazy dependency fails and the image is marked "unusable because dependency <asm> is not found". Every method in that image then falls back to the interpreter, which hits an unsupported construct and aborts:

[MONO] AOT: module WasmBasicTestApp is unusable because dependency Json is not found.
MONO interpreter: NIY encountered in method <Module>:.cctor ()
[MONO] * Assertion: should not be reached at .../mono/mini/interp/interp.c:4135

Fix

Automatically enable the runtime's existing aot-lazy-assembly-load option from the WebAssembly SDK when AOT is combined with BlazorWebAssemblyLazyLoad. This defers binding of a referenced lazy assembly until it is actually loaded, so the referencing AOT image stays usable. The option flows through runtimeOptions in the boot config → mono_wasm_parse_runtime_options → mono_options_parse_options.

The change is scoped to RunAOTCompilation=true + at least one BlazorWebAssemblyLazyLoad item, so non-AOT and non-lazy builds are unaffected.

Test

LazyLoadingTests previously only covered the interpreter (Debug) configuration. Added LoadLazyAssemblyWithAOT (native-mono, Release + AOT + lazy load) which reproduces the crash without the fix and passes with it.

Verification

Built the browser (Mono + CoreCLR runtime packs) and workload locally and ran the new test.

Without the fix: module WasmBasicTestApp is unusable because dependency Json is not found → interp.c:4135 assertion → timed out waiting for WASM EXIT. ❌
With the fix: AOT: image 'WasmBasicTestApp' found., firstJsonLoad=true, {"FirstName":"John","LastName":"Doe"}, WASM EXIT 0. ✅

Note

This pull request was authored with GitHub Copilot.

When an app is published with AOT and uses BlazorWebAssemblyLazyLoad, the Mono runtime crashes during startup (appdomain.c assertion / interp.c NIY 'should not be reached'). Root cause: an AOT image hard-binds to every assembly it references when the image is loaded (see load_aot_module in aot-runtime.c, which eagerly calls load_image for all referenced images unless aot-lazy-assembly-load is set). A lazy-loaded assembly is not present at runtime startup, so any AOT image that references it is marked 'unusable because dependency <asm> is not found' and its methods fall back to the interpreter, which then hits an unsupported construct and aborts. Enable the runtime's 'aot-lazy-assembly-load' option automatically from the WebAssembly SDK when AOT is combined with BlazorWebAssemblyLazyLoad, so referenced lazy assemblies are only bound when they are actually loaded. Also add AOT + lazy-load regression coverage to LazyLoadingTests, which previously only exercised the interpreter (Debug) configuration. Fixes #125794

azure-pipelines · 2026-07-18T22:22:16Z

Azure Pipelines: Successfully started running 3 pipeline(s). 13 pipeline(s) were filtered out due to trigger conditions. There may be pipelines that require an authorized user to comment /azp run to run.

dotnet-policy-service · 2026-07-18T22:22:53Z

Tagging subscribers to 'arch-wasm': @lewing, @pavelsavara
See info in area-owners.md if you want to be subscribed.

lewing · 2026-07-18T22:23:55Z

/backport to release/10.0

github-actions · 2026-07-18T22:24:05Z

Started backporting to release/10.0 (link to workflow run)

github-actions · 2026-07-18T22:26:09Z

@lewing backporting to release/10.0 was not run because the source pull request has not been merged. Please merge this pull request before requesting a backport.

Copilot

Pull request overview

This PR updates the WebAssembly SDK’s browser targets to automatically enable Mono’s --aot-lazy-assembly-load runtime option when AOT is used together with BlazorWebAssemblyLazyLoad, and adds an integration test intended to prevent regressions in that AOT + lazy-load startup path.

Changes:

Append --aot-lazy-assembly-load to runtime options when RunAOTCompilation=true and at least one BlazorWebAssemblyLazyLoad item exists.
Add a new native-mono test that publishes with AOT and exercises lazy-loading at runtime.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/mono/wasm/Wasm.Build.Tests/LazyLoadingTests.cs	Adds an AOT + publish + lazy-load integration test.
src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets	Automatically injects `--aot-lazy-assembly-load` into runtime options under AOT + lazy-load conditions.

lewing · 2026-07-19T01:20:06Z

/backport to release/10.0

github-actions · 2026-07-19T01:20:14Z

Started backporting to release/10.0 (link to workflow run)

github-actions · 2026-07-19T01:22:13Z

@lewing backporting to release/10.0 failed, the patch most likely resulted in conflicts. Please backport manually!

git am output

$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patch

Applying: [wasm] Fix AOT + BlazorWebAssemblyLazyLoad startup crash
Using index info to reconstruct a base tree...
M	src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets
M	src/mono/wasm/Wasm.Build.Tests/LazyLoadingTests.cs
Falling back to patching base and 3-way merge...
Auto-merging src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets
CONFLICT (content): Merge conflict in src/mono/nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets
Auto-merging src/mono/wasm/Wasm.Build.Tests/LazyLoadingTests.cs
CONFLICT (content): Merge conflict in src/mono/wasm/Wasm.Build.Tests/LazyLoadingTests.cs
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Patch failed at 0001 [wasm] Fix AOT + BlazorWebAssemblyLazyLoad startup crash
Error: The process '/usr/bin/git' failed with exit code 128

Link to workflow output

@lewing

…up crash (#131038) Backport of #131020 to release/10.0 /cc @lewing 1. Fixes Issue #125794 main PR: #131020 # Description AOT-published Blazor WebAssembly apps that use `BlazorWebAssemblyLazyLoad` crash during Mono runtime startup — reported as an `appdomain.c` assertion (`condition '<disabled>' not met`) and, in other configurations, an `interp.c` `NIY … should not be reached` abort. An AOT image hard-binds to every assembly it references when the image is loaded (`load_aot_module` in `aot-runtime.c` eagerly calls `load_image` for all referenced images unless the `aot-lazy-assembly-load` runtime option is set). A `BlazorWebAssemblyLazyLoad` assembly is not present at runtime startup (it is downloaded on demand), so when the main app's AOT image loads, resolving the lazy dependency fails, the image is marked unusable, and every method in it falls back to the interpreter, which then hits an unsupported construct and aborts. The fix automatically enables the runtime's existing `aot-lazy-assembly-load` option from the WebAssembly SDK when AOT is combined with `BlazorWebAssemblyLazyLoad`. This defers binding of a referenced lazy assembly until it is actually loaded, keeping the referencing AOT image usable. The change is scoped to `RunAOTCompilation=true` + at least one `BlazorWebAssemblyLazyLoad` item, so non-AOT and non-lazy builds are unaffected. # Customer Impact Any Blazor WebAssembly app that combines AOT compilation with `BlazorWebAssemblyLazyLoad` crashes on startup and is completely unusable. Lazy loading is a common technique to reduce initial download size, and AOT is a common way to improve runtime performance, so customers who adopt both features together are blocked with no workaround other than disabling one of them. # Regression No. This is a long-standing interaction between AOT and lazy loading, not a regression introduced in the most recent release. # Testing Added `LoadLazyAssemblyWithAOT` to `LazyLoadingTests` (`native-mono`, Release + AOT + lazy load), which previously only covered the interpreter (Debug) configuration. The new test reproduces the crash without the fix and passes with it. Verified locally by building the browser (Mono + CoreCLR runtime packs) and workload and running the new test: - Without the fix: `module WasmBasicTestApp is unusable because dependency Json is not found` → `interp.c:4135` assertion → timed out waiting for `WASM EXIT`. - With the fix: `AOT: image 'WasmBasicTestApp' found.`, `firstJsonLoad=true`, `{"FirstName":"John","LastName":"Doe"}`, `WASM EXIT 0`. # Risk Low. The change is a single, narrowly-scoped MSBuild property that only appends `--aot-lazy-assembly-load` when `RunAOTCompilation=true` and at least one `BlazorWebAssemblyLazyLoad` item is present. Builds that do not combine AOT with lazy loading are unaffected, and the option simply defers assembly binding that would otherwise fail. > [!NOTE] > This backport pull request was authored with GitHub Copilot.

lewing requested a review from maraf as a code owner July 18, 2026 22:21

Copilot AI review requested due to automatic review settings July 18, 2026 22:21

lewing requested review from akoeplinger and ilonatommy as code owners July 18, 2026 22:21

lewing temporarily deployed to copilot-pat-pool July 18, 2026 22:21 — with GitHub Actions Inactive

github-actions Bot added the area-Build-mono label Jul 18, 2026

lewing temporarily deployed to copilot-pat-pool July 18, 2026 22:22 — with GitHub Actions Inactive

dotnet-policy-service Bot assigned lewing Jul 18, 2026

Copilot started reviewing on behalf of lewing July 18, 2026 22:22 View session

lewing added the arch-wasm WebAssembly architecture label Jul 18, 2026

lewing had a problem deploying to copilot-pat-pool July 18, 2026 22:22 — with GitHub Actions Failure

lewing mentioned this pull request Jul 18, 2026

[wasm][AOT] Assertion at appdomain.c:188 — g_assert(string_empty_fld) fails on any AOT-compiled Blazor WASM project (.NET 10) #125794

Closed

lewing temporarily deployed to copilot-pat-pool July 18, 2026 22:23 — with GitHub Actions Inactive

lewing temporarily deployed to copilot-pat-pool July 18, 2026 22:24 — with GitHub Actions Inactive

Copilot AI reviewed Jul 18, 2026

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Comment thread src/mono/wasm/Wasm.Build.Tests/LazyLoadingTests.cs

Comment thread ...nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets

maraf approved these changes Jul 18, 2026

View reviewed changes

lewing enabled auto-merge (squash) July 18, 2026 23:15

lewing changed the title ~~[wasm] Fix AOT + BlazorWebAssemblyLazyLoad startup crash~~ [mono][wasm] Fix AOT + BlazorWebAssemblyLazyLoad startup crash Jul 18, 2026

lewing added this to the 10.0.x milestone Jul 19, 2026

lewing merged commit 7b783ab into main Jul 19, 2026
60 of 63 checks passed

lewing deleted the lewing-wasm-aot-appdomain-assert branch July 19, 2026 00:47

build-analysis Bot mentioned this pull request Jul 19, 2026

Unable to pull image from mcr.microsoft.com #117164

Open

dotnet-maestro Bot mentioned this pull request Jul 19, 2026

[main] Source code updates from dotnet/runtime dotnet/dotnet#7842

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lewing mentioned this pull request Jul 19, 2026

[release/10.0] [mono][wasm] Fix AOT + BlazorWebAssemblyLazyLoad startup crash #131038

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dotnet-milestone-bot Bot modified the milestones: 10.0.x, 11.0-preview7 Jul 19, 2026

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[mono][wasm] Fix AOT + BlazorWebAssemblyLazyLoad startup crash#131020

[mono][wasm] Fix AOT + BlazorWebAssemblyLazyLoad startup crash#131020
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lewing commented Jul 18, 2026

Summary

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Test

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