Extender Provider failed to return an Extender

We upgraded from .NET 2.0 to .NET 3.5. A co-worker of mine is getting the following dialog when attempting to run the ASP .NET web project under the debugger in Visual Studio 2008. He can build fine, but he can’t debug (hence, it’s a run-time issue):

Microsoft Visual Studio

“The Extender Provider failed to return an Extender for this object. Please correct before proceeding. (You might rename the current web.config and add a new one.)”


Searching on this error provided one hit that is in French, but Google translated, and the dude deleted his web.config. Well, my co-worker has a web.config.



Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

I encountered the

The Extender Provider failed to return an Extender for this object.

message in a winforms designer window.

I’m aware that your question is about ASP.Net 3.5, but in my case I resolved it by:

  • Closing the solution in Visual Studio
  • Deleting the obj and bin directories from the problematic project.

When I reopened the solution the error no longer occurred in the designer.


VS 2019, resolution still relevant


VS 2022, resolution still relevant

Method 2

I solved it by just closing Visual Studio and opening it again. I was using Visual Studio 2015 Community Edition and didn’t have to delete those folders.

Method 3

In my case run Visual Studio as administrator helped. My project uses local IIS and Studio needs Admin permissions to manage IIS.

Method 4

None of the solutions above worked in VS2019 for me. What did work was deleting the ProjectName.csproj.user file and then reloading the project.

Method 5

In Visual Studio 2019 I had several webservices in a solution failing to load with this error. In my case the error was caused because I had changed the port numbers in IIS for those web services meaning the port numbers in .csproj and/or csproj.user did not match IIS. As soon as I updated the .csproj port numbers to match IIS, the error went away.

Method 6

I had this problem because a project used IIS and HTTPS, there was no https binding on the IIS, adding a HTTPS binding on the default website fixed the problem.

Method 7

VS2019 – restarting Visual Studio did the trick.

Method 8

In VSTO, you can’t use the designer while debugging another VSTO app.

For example, if another VSTO debugging session is running and you try to invoke the Excel Designer on a different project, you’ll get “The Extender Provider failed to return an Extender for this object”. Stop debugging to free up the Designer.

Method 9

I had this issue in Visual Studio 2015, the error kept persisting even after cleaning the project, restarting visual studio and restarting my pc.

What resolved it for me, was right-clicking the xaml file and choosing Open with… > XML Editor. It opened the file and the error didn’t occur after that.

Method 10

For me none of the solutions above worked. I am using VS 2019. In my case, the repository had special characters in the name, so when cloned, the root folder had escaped characters in the name. The compiler could no longer match the file names. Updating the parent folder name and reopening VS fixed the issue.

The detailed error behind ‘Extender Provider failed to return an Extender’ was related to the inability to find the compiler targets in the path.

Method 11

In my case I had deleted ‘SSL’ from the VS2017 C# project properties debug configuration build tab Conditional compilation symbols. Putting it back resolved the error. Apparently SSL require in Debug configuration (but not release).

Method 12

I was also facing the same issue on the new branch checkout in VS 2019. I just did some changes in web.config and then did undo. it’s weird but worked.

Method 13

Well in my case, I did following

  1. cleaned solution
  2. rebuilt solution
  3. Just opened Web.config in the editor

Launch the application and it worked for me.

Project type: ASP.Net MVC
Framework: .Net Framework 4.5.2

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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