ASP.Net error: "The type 'foo' exists in both "temp1.dll" and "temp2.dll"

Question

When running a web application project, at seemingly random times a page may fail with a CS0433 error: type exists in multiple DLL's. The DLL's are all generated DLL's residing in the "Temporary ASP.NET Files" directory.

Answer 1

Add the batch="false" attribute to the "compilation" element of the web.config file.

This problem occurs because of the way in which ASP.NET 2.0 uses the application references and the folder structure of the application to compile the application. If the batch property of the element in the web.config file for the application is set to true, ASP.NET 2.0 compiles each folder in the application into a separate assembly.

http://www.sellsbrothers.com/news/showTopic.aspx?ixTopic=1995

http://support.microsoft.com/kb/919284

Answer 2

Is it the same issuse as described here?

Answer 3

As another data point, I just had this problem without any evidence of circular references as described in the links in Ben's answer. Building my web site project would fail with a few of these errors, and setting compilation batch="false" fixed it, but I didn't want to go that route as this is a large-ish production website.

This solution was in a subfolder of my D:\svn folder, which I had mapped to S:. When I opened the solution from S:, these errors occurred, but if I went straight to D:\svn and opened the solution, no errors.

I also noticed that, despite having compilation batch="true" in my web.config, when opening the solution from the mapped S: drive all my .ascx files get compiled into their own assemblies. If I open it from the physical location, the .ascx files get compiled into their respective folders' assemblies (which is how batch="true" is supposed to work).

Strange.