"An attempt was made to load a program with an incorrect format" even when the platforms are the same

I'm calling functions from a 32-bit unmanaged DLL on a 64-bit system. What I get is:

BadImageFormatException: An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B)

At first, I had my projects set to the Any CPU platform, so I changed them both to x86, but this error is still occurring. That's really the only fix I know for this.

The DLLs aren't corrupt or anything, because I can use them with other programs (that I don't have the source to). I thought that perhaps it wasn't finding a dependency, but I checked and they're all there. Plus, wouldn't it throw a DllNotFoundException in that case?

What else can I do? And before you say "Use a 64-bit unmanaged DLL instead," let me point out that there isn't one. ;)

If you try to run 32-bit applications on IIS 7 (and/or 64-bit OS machine), you will get the same error. So, from the IIS 7, right click on the applications' application pool and go to "advanced settings" and change "Enable 32-Bit Applications" to "TRUE".

Restart your website and it should work.

Somehow, the Build checkbox in the Configuration Manager had been unchecked for my executable, so it was still running with the old Any CPU build. After I fixed that, Visual Studio complained that it couldn't debug the assembly, but that was fixed with a restart.

In Visual Studio, Right Click your project -> On the left pane click the Build tab,

under Platform Target select x86 (or more generally the architecture to match with the library you are linking to)

I hope this helps someone! :)

If you encounter this error when you click green arrow button to run the application, but still want to run the app in 64 bit. You can do this in VS 2013, 2015, 2017, and 2019

Go to: Tools > Options > Projects and Solutions > Web Projects > Use the 64 bit version of IIS Express

Or you can do it per project at Project Properties > Web > Bitness

I just had this problem also. Tried all the suggestions here, but they didn't help.

I found another thing to check that fixed it for me. In Visual Studio, right-click on the project and open "Properties". Click on the "Compile" (or "Build") tab and then click on "Advanced Compile Options" at the bottom.

Check the dropdown "Target CPU". It should match the "Platform" you are building. That is, if you are building "Any CPU" then "Target CPU" should say "Any CPU". Go through all of your Platforms by making them active and check this setting.