Error 11: An error occurred when merging assemblies: ILMerge.Merge: ERROR!!: Duplicate type 'xyz' found in assembly 'App_Web_rh2sxhkc'. aspnet_merge

You may have come across this error once or twice while deploying your project if you develop using Web Deployment Projects. It's usually caused when you copy and paste a page and forget to update both the page declaration and code behind file.

But the website builds!?!

You don't usually get the ILMerge error until you build the web deployment project because when you build a website directly, it doesn't compile all the code into a single assembly so the class names are seen as different. Part of the web deployment process is to compile all the websites code into a single assembly hence the duplicate references.

What's the solution?

It's surprisingly simple, all you need to do is open up the offending aspx and aspx.cs files and update two lines:

1. In the code behind file, rename the partial class. By default Visual Studio will name the class FolderName_Pagename which should result in a unique name

2. The page declaration (first line of the page) in the aspx file. You have to make sure that both the Inherits attribute and CodeBehind reference are correct.

Tip: To avoid confusing yourself, open the files independently using the solution browser because if you open the aspx and press F7 to switch to the code behind file before updating the page declaration, you'll end up editing the page you copied rather than the copy.