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Editorial Team
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Asked: 2026-05-20T11:01:34+00:00

The message sent to browser is as follows: The type My.API.Class is ambiguous: it

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The message sent to browser is as follows:

The type My.API.Class is ambiguous: it could come from assembly ‘[on Temporary ASP.NET Files]’ or from assembly ‘[on bin folder]’

The problem occurs when debugging a Web App, specifically when making a request to a WebMethod of a WebService.

The project compiles just right. It generates My.Website.dll on bin folder and if I publish the Web Application. It works fine.

The asmx file is on the root of the application. The CodeBehind file is on App_Code and its marked to be compiled to generate My.Website.dll.

I should be missing something really important.

I found someone having the same issue with a possible related cause. Check it out. The way this person exposes its problem is somehow similar but I get starting to be lost when he talks about a proxy class and shared dlls I don’t use.

Any help is appreciated.

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1 Answer

  1. Editorial Team

    According to this, the App_Code folder should be used only on Web Site projects. That’s the reason the CodeBehind of the asmx was compiled at runtime too.

    The initial question was made based on a Web App. But I didn’t specify this Web App. was been manually changed from a Web Site project.

    To solve my problem I did the following:

    1. Convert the project to Web Application. This will make App_Code to be renamed to Old_App_Code.
    2. Moved all the Old_App_Code resources to a Class Library and then referenced this library into the Web Application.

    @Tony: Thank you for guiding me.

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