System.BadImageFormatException: How to fix .NET version mismatch?

Question

I wrote a dll in VS 2005 that will be loaded by another program that was developed in VS 2003. When that program tries to load my dll, it throws a System.BadImageFormatException: The format of the file 'Foo.dll' is invalid.

I searched SO and Google, and it seems that the different version of .NET is the culprit. My question is: How do I fix this?

(I cannot modify the program developed in VS 2003.)

Mesocratic

This link was also helpful: devlicio.us/blogs/ziemowit_skowronski/archive/2008/08/22/… – Mesocratic

A native (unmanaged) library with a different word size can also throw this exception. You will need to get a version for the correct word size or build the hosting application to match. – Deniable

Answer 1

Compile the VS2005 Project with the same .Net Version as your VS2003 Project uses. Or, depending of the size of your VS2003 Project, reflector and rewrite it. That's common practice for projects over here, where for some obscure reason there is no more VSS version available.

Euripus

"Compile the VS2005 Project with the same .Net Version as your VS2003 Project uses." Can you please explain how to do that? I don't see how to choose the .NET version of my project. – Mesocratic

Rightclick your Project in VS, on the very first tab you can select the targetframework. – Euripus

Thank you, Markus. Although that didn't work in VS 2005, it did work in the 2008 version. I appreciate your help. – Mesocratic

As an addition, in my case, I also had to change the project build Platform target to x86 (Project Properties, Build tab) when using x86 libraries. – Janeyjangle

Answer 2

I got this error when I was running the 64bit version of the CLR and trying to load an assembly that was marked 32bit only. The specific assembly in my case was the Oracle.DataAccess.dll that comes as part of ODP.NET.

Fionafionna

I am getting the same error too (with a different assembly), do you have any solution for that? – Paquette

You need to just make sure you run the 32bit CLR when loading 32bit only assemblies – Fionafionna

This was the answer to my problem. I had not faced this in quite a while, but when one is using COM interop it's fairly common to run into this scenario when using the 'Any CPU' build config and then deploying to a 64-bit server. The x86 configs are handy in those cases. – Autocratic

Answer 3

SOLUTION: If you are on IIS7, Right Click/ Advanced Settings on the Specific Application Pool of the website and select True on Enable 32-Bit Applications. It should work.

Nietzsche