Misleading error message when referenced assembly is built for different CPU architecture than project

[XVincentX]

Description

If you are running an application in an architecture, say arm64 and you are referencing an assembly for a different architecture, say x64 - the error message that appears is misleading:

Unhandled exception. System.IO.FileLoadException: Could not load file or assembly 'src, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null

Problem is, the file is definitely there. What is happening is an architecture mismatch - not the file not being present on the disk.

Reproduction Steps

1. Create a project for an architecture that is not compatible with your current computer. In my case I have a M3 MacBook, so x86 and x64 would not work:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <PlatformTarget>x86</PlatformTarget>
    <Nullable>enable</Nullable>
  </PropertyGroup>
</Project>

2. Run the project:

dotnet run

Unhandled exception. System.IO.FileLoadException: Could not load file or assembly 'src, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.

3. Yet, the assembly is there:

ls bin/Debug/net8.0/ | grep src                                                                         (master)test
src
src.deps.json
src.dll
src.pdb
src.runtimeconfig.json

Expected behavior

The message should clearly say that there is an architecture mismatch for the assembly

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[jkotas]

Duplicate of #85691, fixed by #91289

[jkotas]

Thank you for reporting this issue. The fix will be included in .NET 9

[elinor-fung]

@XVincentX reported that this is still happening in .NET 9

#91289 fixed the case for explicit loads or running a mismatched app dll itself (dotnet <mismached_arch_dll>), but it looks like it does not cover the app directly referencing the mismatched architecture dll.