ASP.NET App – is not a valid Win32 application

Never seen this one before until this morning. Well, I’ve seen the “not a valid Win32 application” error a ton of times, but this is the first time on with an ASP.NET app (running MVC).

How to Fix

If you ever see this and your app is running IIS7, here’s the first thing to check:

1. Inside IIS (7) click on Application Pools.
2. Find the Application Pool that is configured for your web application in the list.
3. Select it, then click Advanced Settings on the right.
4. Second setting in the list: Enable 32-Bit Applications – must be set to True.

That should get you up and running again. I have no idea how my app pool changed from 32 to 64 unless another dev accidentally changed it.

Why

The reason the 64bit setting wont work? Here’s what I’ve determined from my reading – On a 64-bit Windows machine the World Wide Web Publishing service does not support running 32-bit and 64-bit worker processes concurrently on the same server, which is what happened above.