What could be causing a System.TypeLoadException in a Visual Studio Unit Test?

Question

I've got a C# .NET class library MyClassLibrary that compiles fine. I'm trying to create a unit test project for it (using Visual Studio Unit Testing Framework, with Visual Studio 2010). The class library does have big classes in it, but whenever I run even the simplest test against the simplest class, I get the following exception:

Test method MyClassLibraryTest.MyClassLibraryTests.MySimpleClassTest threw exception: System.TypeLoadException: Could not load type 'MyClassLibrary.MySimpleClass' from assembly 'MyClassLibrary, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.

All of the projects I'm dealing with are in the same solution, and all are compiled for .NET 4.0. All of this is on a Windows 7 64-bit machine.

Here's the weird part: when I "Run" the test, I get the above error. But when I "Debug" the test, it runs fine. Why?

MySimpleClass is not defined in the assembly or it has problems loading. Can you show the source for this class? — Richard Schneider, Apr 20 '11 at 19:48
Like I said, it ran fine in Debug. It shouldn't matter what the definition for MySimpleClass is; ANY class from the project fails to load in Run mode. I can't really fit any of the code on here, but I'm assuming this a build configuration issue (or visual studio setting, or something) rather than a code issue. — Jay Sullivan, Apr 20 '11 at 19:57
Try enabling Fusion logging to see where this assembly is being loaded from. — Anton Gogolev, Apr 20 '11 at 20:50

score 39 · Answer 1 · answered Feb 21 '13 at 18:01

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I just banged my head against this one for an hour. The problem was that I had a command line project named Something.exe, which was using a class library project named Something.dll.

answered Feb 21 '13 at 18:01

Amir Abiri

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Same here. Thanks Amir. I still don't get why having the .dll and .exe the same causing an error – Monacraft Jun 16 '14 at 09:10
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Because for performance reasons the .NET runtime assumes that `MyAssembly` resides in either `MyAssembly.dll` or `MyAssembly.exe` (rather than scanning too many files). It's a feature of the language where you can load an .exe into your app as any other assembly (I've used this on a few occassions). Therefore when the .NET runtime attempts to load `MyAssembly` it tries `MyAssembly.exe`, sees that it's a valid .NET assembly, loads it, and fails to find the relevant class. – Amir Abiri Jun 18 '14 at 11:17
Small error in the last comment - `MyAssembly` is already loaded, since we've executed `MyAssembly.exe`. – Amir Abiri Jun 26 '14 at 12:47

Pavel Melnikov · Answer 2 · 2013-03-30T13:35:02.017

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Happened to me too. In my case the problem arised because the tested project and the unit tests project had the same name. If this is your case too then rename one of the projects and rename output file name to fix it.

edited Mar 30 '13 at 13:35

answered Mar 09 '13 at 12:27

Pavel Melnikov

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fyi: My project name was different. But in the settings, the assembly name / default namespace was the same. – dknaack Sep 05 '13 at 16:27
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I had the same issue - had renamed a project to a different name, then created another project using the old name. Thanks for pointing this out! – joelc Jul 28 '16 at 00:29
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@dknaack 4 years later, still saving lives! Thanks! – digitlworld Feb 17 '18 at 14:14

score 11 · Accepted Answer · answered Apr 20 '11 at 21:10

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The MyClassLibrary assembly was set to x86 mode in the configuration manager. Changing this to x64 fixed it. I really wish Visual Studio would detect this and report it as a less obscure error.

answered Apr 20 '11 at 21:10

Jay Sullivan

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score 4 · Answer 4 · answered Oct 11 '11 at 06:36

Happened to me too. It is related to building for x64, Release and x86 mode. In my case, I deleted folders in my bin (debug/release/x86 in reference assemblies and unit test) and re-run my unit test. VS2010 somewhat reported the error in Output Window. That solved it for me.

score 3 · Answer 5 · answered Aug 11 '17 at 13:35

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It happened to me when i was trying to add test project with the same main project name.

My main project name was : Calculator.OperationsManager my test project name was : Calculator.OperationsManager

i have changed the test project name as Calculator.OperationsManager.Test and everything went well.

answered Aug 11 '17 at 13:35

Wahap

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Same here. Two projects had the same default namespace and this problem arised. – picheto Dec 09 '19 at 22:00

score 2 · Answer 6 · answered Jul 14 '16 at 10:49

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Came through this today and though I would leave my fix.

Specs: VS 2013 / .Net 4.0

Solution: Go into Menu > Test > Test Settings > Default Processor Architechture > X64

answered Jul 14 '16 at 10:49

Tiago Duarte

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score 2 · Answer 7 · answered Nov 18 '16 at 17:26

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Same message here, but I couldn't Debug the test either.

In my case, the DLL I was testing was deployed to the GAC (a BizTalk requirement). I had created a new class and was testing it, but hadn't GAC'd the DLL again since adding the class under test.

answered Nov 18 '16 at 17:26

Trey Mack

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score 1 · Answer 8 · answered Mar 09 '17 at 14:16

In case this helps others with the same error (I realise it doesn't directly answer the question re Release vs Debug); I merged in a legacy project with a namespace that conflicted with an existing one, so renamed it. I got this error when trying to create a form from that project.

I checked the platform target was the same, and deleted the .\bin\ directories to ensure a clean rebuild, removed the reference to the merged project and re-added it, but still the same error.

Eventually (!) I checked the Assembly Name in the project's properties (right click on project, select 'properties', select the 'Application' tab) and changed that to match the Default namespace, and all is now well.

score 1 · Answer 9 · answered Aug 23 '18 at 08:40

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I had similar problem as Trey, but instead of BizTalk, I have SharePoint solution which also uses GAC deployment. GAC had an older assembly. When I removed the GAC assembly by retracting the solution, the test passed.

answered Aug 23 '18 at 08:40

Mika Rinkinen

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For biztalk redeploying the application (Including assemblies to the GAC) worked – Aug 29 '18 at 07:00

score 0 · Answer 10 · answered Sep 01 '16 at 13:52

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I had this error using NUnit 3 in VS 2013. I solved it by deleting the assembly reference in my test project for the assembly that contained the type that was not being found, and then re-added the reference.

answered Sep 01 '16 at 13:52

JamesFaix

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score 0 · Answer 11 · answered Dec 04 '18 at 10:34

Just in case anyone needs this: I had created a test project in Visual Studio and had wondered why some Classes couldn't be found even if Visual Studio repeatedly asked me to add "System.Web" as reference.

I had done that and the error kept occurring. The problem for me was simple (and I should have checked that before, I know): I had created a test project from a template that created a .NET Core project. After changing it to .NET Framework 4.6.1 and adding "System.Web" as reference, everything worked fine.

score 0 · Answer 12 · answered Apr 03 '19 at 06:58

Encountered the same issue. Just in case if it helps anyone - I managed to get it to work by downgrading the nuget package NUnit3TestAdapter from version 3.13.0 to 3.11.2.

You can find more information on this - https://github.com/nunit/nunit-console/issues/424

score 0 · Answer 13 · answered Feb 19 '20 at 22:02

This happened to me as well. I had a nuget package installed on my domain project with version 1.5, but only had version 1.1 on my unit test project. I resolved the issue by updating the version on the unit test project to match the version on the domain project (the project being tested against).

score 0 · Answer 14 · answered Nov 24 '22 at 13:38

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In my case obfuscated code caused the issue. I have disabled the code obfuscation and resolved the problem.

answered Nov 24 '22 at 13:38

Rahul

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What could be causing a System.TypeLoadException in a Visual Studio Unit Test?

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