System.BadImageFormatException: Could not load file or assembly

I found a different solution to this issue. Apparently my IIS 7 did not have 32bit mode enabled in my Application Pool by default.

To enable 32bit mode, open IIS and select your Application Pool. Mine was named “ASP.NET v4.0”.
Right click, go to “Advanced Settings” and change the section named: “Enabled 32-bit Applications” to true.

Restart your web server and try again.

I found the fix from this blog reference:



Running IIS 7 In 32-bit Mode

I recently got a new machine at work with a 64-bit quad-core CPU, dual 15k RPM SCSI drives, 4G RAM, and, *gulp*, Vista. I’ve never used Vista before, and after all the horror stories I’ve heard, I was pretty hesitant. We’re using ASP.NET MVC with our new project, though, and deploying out to Windows Server 2008, which runs IIS7, so I figured developing with the same thing on my local box was a good idea. That means running the apps through IIS and not the built in Visual Studio 2008 Cassini server, which is what I was using back on XP.

So I set everything up, check out our project, fire up Visual Studio, hit debug, and BAM! I get a nice looking yellow screen of death, which, incidentally, isn’t our application’s start-up page. Here’s the exception it throws:

System.BadImageFormatException: Could not load file or assembly ‘OurProject.Core’ or one of its dependencies. An attempt was made to load a program with an incorrect format.

WTF? It was working fine on my old box. I recompiled our app after changing all the projects to explicitly compile in 32-bit mode and still got the error. As it turns out, IIS will, by default, run in 64-bit mode on a 64-bit box. Makes sense. If you open Task Manager, you can see that’s how it works (note there’s no *32 next to the process name like all the other 32-bit apps that are running, like Fire Fox and Visual Studio):

Task Manager showing IIS 7 running in 64-bit mode.

So how do you tell IIS 7 to run in 32-bit mode? I found a terribly helpful post here that details of the situation (both a fix and the reasoning behind it). Basically, go into your IIS 7 manager console, find the application pool your app is running in, right-click on it, go to Advanced Settings, and change the Enable 32-bit Applications setting to true:

Editing the application pool properties in the IIS 7 Manager snap-in.

Now restart IIS (either via the snap-in or the command line w/net stop w3svc & net start w3svc) and you’re good to go. Easy enough, right? Pop open Task Manager again and you should see a lovely *32 next to the World Wide Web Worker Process (w3wp.exe), signifying that it’s basically running in 32-bit compatibility mode:

Task Manager showing IIS 7 running in 32-bit mode.

Hopefully your app should load fine now. Well, at least mine did.


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