could not load file or assembly exception NET 5/6 of NuGet package for NET Standard

Question

I'm at the end of my rope trying to figure out why my working DLL cannot load a NuGet package that it could always load before (like for a whole year). My system has 70 NET 6 projects and half a dozen of them use the same NuGet package to read XML files. Recently (a few months ago) I upgraded everything to NET 6, and even after that upgrade, everything worked fine.

But now, one of my C# class libraries generates an exception in a constructor because it can't find the NuGet package that sits right beside it in the filesystem app folder where everything is loaded from. The error message was:

.ctor Could not load file or assembly xxx blah blah. The system could not find the file specified.

For what it is worth, here is a trace of the build options in the batch file that I have been using for months.

Deleting hscore\hscore\bin Deleting hscore\hscore\obj Deleting hscore\hscore\packages "C:\Program Files\Microsoft Visual Studio\2022\Community\MSBuild\Current\Bin\msbuild.exe" -nologo -m -V:minimal -t:clean;restore;publish /p:Platform=AnyCPU /p:PlatformTarget=AnyCPU /p:Configuration=Debug /p:TargetFramework=net6.0-windows7.0 /p:RuntimeIdentifier=win-x64 /p:SelfContained=false /p:PublishProtcol=FileSystem /p:DeleteExistingFiles=true /p:PublishDir=c:\dev\holding\core.plt -nowarn:MSB3305 c:\dev\products\hscore\hscore\hscore.csproj

Determining projects to restore... Restored c:\dev\products\hscore\hscore\hscore.csproj (in 874 ms). hscore -> c:\dev\products\hscore\hscore\bin\Debug\net6.0-windows7.0\win-x64\hscore.dll hscore -> c:\dev\holding\core.plt\

I have done the following to debug it, without success:

• deleted the bin/obj folders of the failing class library

• checked the target framework of all projects (= net6.0-windows7.0)

• checked the runtime identifier of all projects (= win-x64)

• checked the build configuration (Debug, AnyCPU)

• removed and re-added the NuGet package (with a specific version)

• the Nuget package depends on NET Standard, compatible with NET 6

• and I use the same version in other NET 6 programs that are working fine

• restored and rebuilt the DLL

• published it to the destination folder where it is run from

• checked that the expected Nuget package is there (it was)

• and yet it still fails to find the Nuget package

• I tried to trace DLL loads of the process with the SysInternals Process Monitor, but I am not good enough to do that (if it is even possible). I can filter events to see the parent app process load, but the failing DLL is called by the parent process and I don't know how to see the search paths it is using to find the Nuget package.

I want to believe that the problem is a version mismatch between NET 5 and NET 6 and the version of the Nuget package sitting in the folder, because I once had a problem like that. For example, if I had a NET Framework version of the Nuget package, it would not be "found" by a NET 5 DLL that wanted to load it. (I am NOT saying that is my problem, because I have no NET Framework projects anymore.)

The most recent thing I have been working on is the build system and options. I can build the whole system with batch files or with parallel builds (when the system is working). I use the same msbuild options in both cases, and the batch method has been reliable and shows no build errors, no publish errors, all the correct build options, and no warnings or errors whatsoever.

The Nuget package pulls in 10 or 20 dependencies, but that is all automatic, and the package works with other console programs and apps that are working fine. So, I don't think there is anything wrong with Nuget package. All the working apps use the same Nuget package from the same runtime folder. I don't know why this one DLL is having a problem. AND I have not changed anything in the failing DLL class library for months (and it has been working fine).

Does anyone have suggestions on what else I might try to debug the problem and get the system working again? Thank you.

UPDATE - New version and build, but still a runtime failure.

Just to be sure, I upgraded the Nuget package that could not be found to the latest version and replaced all references in my 70 projects to the new version. Then I rebuilt the system (no warnings, no errors). But that did not make a difference. Now the new version cannot be found.

All unit tests of the code in question work flawlessly in VStudio, presumably because VS loads the Nuget package properly. I am mystified and would appreciate thoughts on how to proceed. Thank you.

UPDATE 2 - installed 'dotnet-trace' and traced DLL loads

I found a page that described how to dotnet tool install --global dotnet-trace and then captured a trace of my app trying to find the elusive Nuget package that is sitting right beside the requesting DLL. Although I could my app loading various app DLLs (and tons of system DLLs) with found pathnames, the trace showed nothing useful about the Nuget package.

The trace entries show the system searching in stages FindInLoadContext, then ApplicationAssemblies, then AppDomainAssemblyResolveEvent. After that, the system throws an exception because it can't find the Nuget package.

I would have thought the system FindInLoadContext would find it in the same folder as the executing and requesting assemblies, but it seems not so.

Ideas? I'm lost. And everything used to work fine.

UPDATE 3 - A small console app calls the DLL and it finds the Nuget package

The problem scenario in this question is: WindowsFormsApp -> loads MyCore.dll and calls new MyCore.MyObject() , which in the constructor tries to read some XML files using MyUtils.dll, which tries to reference the mystery Nuget package for reading XML files. MyUtils.dll cannot find the package, no matter what I do.

I wrote a small console program to load MyUtils.dll and called the exact same MyUtils.ReadMyXmlFiles API to read the files. The console app (actually, the system assembly loader) correctly found the Nuget package and read the XML files properly. This all occurred in the same folder that contains the Forms app and the Nuget package.

The only difference now is that the WindowsForms app calls an intermediate DLL that calls a constructor that references MyUtils.dll.

Could the intermediate DLL + constructor call be changing the Assembly Load Context (FindInLoadContext search rules) for the Nuget package load operation? Very strange.

Answer 1

The overall problem was that my Windows Forms app failed during boot (in the Form_Load event) because a Nuget package could not be found to read some XML configuration files.

Four assemblies were involved in the problem. Assembly1 (the app) called Assembly2 (.. new Assembly2Object()), whose constructor called an Assembly3 method (utilities.ReadXMLFile) which called Assembly4 (NugetPackage.XMLReader).

You can see that only Assembly4 references the Nugetpackage which could not be found. Normally, there should be no reason for Assemblies 1 or 2 to know about the existence of the NugetXmlReader used by Assembly3. But, the exceptions and assembly load traces of dotnet-trace and procmon clearly showed that Assembly3 (utilities.ReadXmlFile) could not find the NugetPackage at runtime, even though the right package was in the folder beside the executing assemblies (1->3).

The "solution" for my case was to add a Nuget package dependency to Assembly1 (the WinFormsApp). As soon as I did that, Assembly3 (utilities.ReadXmlFile) could find Assembly4 (Nugetpackage).

My working theory is that during a WinForms boot sequence (including the Form_Shown event), the AssemblyLoadContext from Assembly1 is used to look up all assemblies in any call chain (like Assembly2 calling Assembly3 calling NugetPackage4).

Because the load context of Assembly1 has no reference to NugetPackage, and because the .deps.json file for Assembly1 is present (the doc says it is used to form the load context), Assembly3 - using the assembly load context for Assembly1 - could not find the NugetPackage.

When I added a dependency to Assembly4 (Nugetpackage) to Assembly1, then the assembly load context from Assembly1 was used by the System AssemblyLoader FindInLoadContext phase to search for (and find) the NugetPackage4 wanted by (the referencing assembly) Assembly3.

I used to think that each assembly in a calling chain would have its own assembly load context used by AssemblyLoad/FindInLoadContext. But I think that no longer. At least for my case of a Windows Forms app booting up, the assembly load context from the top-level app is being used to look up Nuget packages far down the calling chain.

It's worth repeating that my little test console apps always found the Nuget package without adding the package as a dependency to the top-level console program. I wrote console programs to test the calling chain from the bottom up: Test1) console calls Nuget directly - found; Test2) console (w/o Nuget dependency) calls Assembly3 - Nuget found; (Test3) console (w/o Nuget dependency) calls Assembly2 - Nuget found.

Then I wrote a skeleton WindowsFormsTestApp to call Assembly2 (just like the console program did) - Nuget NOT found. When I added a Nuget dependency to the WindowsFormsTestApp - Nuget was found by Assembly3.

The final step was to add a Nuget package dependency to the WinFormsApp that started this whole mess. Presto! Assembly3 found the Nuget package immediately.

My (unproven) belief is that somewhere along the line of upgrades from NET 5 - NET 6 (several SDK versions of each one), the assembly load context rules changed somehow. I could be wrong, but I lean toward this belief because my code from App->Assembly2->Assembly3->Nuget did not change during several months of successful operation. And just a couple of weeks ago after another NET 6 SDK upgrade, things broke.

Hopefully, this record might help someone someday. It seems completely unintuitive to me to add a Nuget package dependency to the top-level Forms app to help Assembly3 find a Nuget package sitting in the same folder as all the other assemblies.

After all this, I am coming around to the idea that the top-level app must/should include dependencies on anything the app ever calls, including packages used by dependent assemblies. (Although I am still puzzled as to why the code worked for months before without the Nuget package dependency.)