.net9 Blazor app behaves differently when building and running with VS2026 vs. VS2022

[AdmiralSnyder]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

When building and running the application from VS 2026 Insider, i get lots of logging error messages

[MONO] /__w/1/s/src/mono/mono/metadata/mono-hash.c:439
Error
at Fc (http://localhost:5240/_framework/dotnet.runtime.st3wwc8rqy.js:3:168832)
at dotnet.native.wasm.wasm_trace_logger (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[32306]:0x90fb71)
at dotnet.native.wasm.eglib_log_adapter (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[2947]:0x1548df)
at dotnet.native.wasm.monoeg_g_logv_nofree (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[2843]:0x151ab0)
at dotnet.native.wasm.monoeg_g_log (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[2845]:0x151bc5)
at dotnet.native.wasm.monoeg_g_log_disabled (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[2846]:0x151c13)
at dotnet.native.wasm.mono_g_hash_table_insert_replace (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[5611]:0x203359)
at dotnet.native.wasm.mono_g_hash_table_insert_internal (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[5612]:0x2034c1)
at dotnet.native.wasm.get_objref (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[4232]:0x1a37f8)
at dotnet.native.wasm.buffer_add_objid (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[4240]:0x1a4e68)

and before those, one error saying

Invalid UTF-8 leading byte 0x000000fe encountered when deserializing a UTF-8 string in wasm memory to a JS string!

i did pin the SDK version in the global.json, so i would expect both projects to behave the same, runtime-asset wise.

sadly, this is a very big project, so i cannot easily share the code, but i'd be happy to e.g. meet in discord to figure this out. it's not pressing, for now, but i plan to get everything to VS2026 at release date :-)

here's a screenshot:

Expected Behavior

just like when compiling with VS2022, no errors (described above) should appear

Steps To Reproduce

our project is quite old, so i have no idea if this is reproducible with a freshly-created blazor app, i'll try that tomorrow. gotta go sleep now.

Exceptions (if any)

[MONO] /__w/1/s/src/mono/mono/metadata/mono-hash.c:439
Error
at Fc (http://localhost:5240/_framework/dotnet.runtime.st3wwc8rqy.js:3:168832)
at dotnet.native.wasm.wasm_trace_logger (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[32306]:0x90fb71)
at dotnet.native.wasm.eglib_log_adapter (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[2947]:0x1548df)
at dotnet.native.wasm.monoeg_g_logv_nofree (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[2843]:0x151ab0)
at dotnet.native.wasm.monoeg_g_log (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[2845]:0x151bc5)
at dotnet.native.wasm.monoeg_g_log_disabled (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[2846]:0x151c13)
at dotnet.native.wasm.mono_g_hash_table_insert_replace (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[5611]:0x203359)
at dotnet.native.wasm.mono_g_hash_table_insert_internal (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[5612]:0x2034c1)
at dotnet.native.wasm.get_objref (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[4232]:0x1a37f8)
at dotnet.native.wasm.buffer_add_objid (wasm://wasm/dotnet.native.wasm-060f784e:wasm-function[4240]:0x1a4e68)

.NET Version

9.0.305

Anything else?

it's working fine in VS2022 17.14.14 - Enterprise
it's erroring in VS2026 Insiders - Enterprise

[AdmiralSnyder]

i just compared the bin folders at the two different points-in-time - they are identical. so it's not an aspect of the build system, but rather, the debugger, i suppose.

[AdmiralSnyder]

i just confirmed it by running without the debugger attached. no errors.

[javiercn]

@AdmiralSnyder thanks for contacting us.

Can you file an issue through the visual studio report a problem dialog in Visual Studio? This repo is for runtime issues and that way it'll be routed property to the right team. Clearly state that the issue is with running under debugging and that running without debugging solves the problem.

I suspect this is the new webassembly debugger, you might be able to disable it in VS 2026 and fallback to the old debugger. If you want, link the user voice ticket and I'll try to route it internally.

[AdmiralSnyder]

@javiercn this is the ticket i just filed:
https://developercommunity.visualstudio.com/t/VS-2026:-Blazor-debugger-causes-lots-of/10963481?port=1027&fsid=47cbae91-4b51-4dbf-bdae-3979d676ef82

[javiercn]

@thaystg can you take a look?

[AdmiralSnyder]

i turned off the new mono debugger, and the bad behaviour went away

[javiercn]

@AdmiralSnyder thanks for the confirmation.

Let's follow up on the linked ticket, since this is a VS/Debugger issue. It'll be routed in the following days to the right folks and they'll take a look.

I'm going to close the issue for now as there's no action we can take on the framework side.