I tried to debug one of my WebAPI projects in Visual Studio 2019 RC4 today and received an error message:
Cannot add duplicate collection entry of type ‘add’ with unique key attribute ‘name’ set to ‘aspNetCore
First I tried restarting Visual Studio but got the same error. I researched a little and did find a StackOverflow post with some workarounds but they didn’t seem to fix the underlying issue.
By opening the associated error log after the error message I found the problem seems to be a duplicate http handler in the generated applicationHost.config file under the hidden “.vs” folder in my root solution folder. I didn’t see any duplicate handlers in this file for my project so it must be duplicated somewhere else.
After further thought I remember reading a post at some point that talked about the applicationHost.config file becoming corrupt. So I closed VS, opened a PowerShell console as Admin, navigated to the root solution folder and renamed the “.vs” folder.
> Rename-Item .vs old.vs
That did it! Visual Studio regenerated the “.vs” folder and everything worked again. I’m not sure if this was a 2019 RC specific issue or not but this solution will probably fix multiple similar issues in 2017/2019.
Hope this helps others. If so or you have further question/info please leave a comment below.
Happy Debugging!