Azure app service error 'An attempt was made to load a program with an incorrect format'

I was deploying a new website today and came across an issue running the site on azure. The site ran fine on my dev machine both running under vs2015 and when setup as a website under my local IIS.

But when running on a freshly created Azure app service i got the error:

Could not load file or assembly 'System.Web' or one of its dependencies. An attempt was made to load a program with an incorrect format.

Exception Details: System.BadImageFormatException: Could not load file or assembly 'System.Web' or one of its dependencies. An attempt was made to load a program with an incorrect format.

[BadImageFormatException: Could not load file or assembly 'System.Web' or one of its dependencies. An attempt was made to load a program with an incorrect format.] System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly ....


After digging around on the interwebs for a while it became apparent it was an issue with either the target architecture or the targetting of the dlls. My solution was built to target 'any cpu' but somehow the version of System.Web that was brought down by nuget was the 64bit version.

So i figured that changing the target architecture would have the desired effect and changed the azure platform from 32 bit (the default for new app services) to 64 bit in the application settings of the azure portal (portal.azure.com).


Problem solved. I hope this will point someone else to a quick resolution in the future.