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Home/ Questions/Q 9227437
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Editorial Team
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Editorial Team
Asked: June 18, 20262026-06-18T05:05:02+00:00 2026-06-18T05:05:02+00:00

I have asp.net web application project in visual studio 2012. When I want to

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I have asp.net web application project in visual studio 2012. When I want to start it, I have the following error:

Unable to launch the IIS Express Web server:
Port”4012″ is in use

This problem came from nothing.How can I resolve it ?

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  1. Editorial Team
    Editorial Team
    2026-06-18T05:05:04+00:00Added an answer on June 18, 2026 at 5:05 am

    From the MSDN library

    Visual Studio cannot guarantee that the port you specify will be
    available when you run your file-system Web site. If the port is in
    use when you run a page, Visual Studio displays an error message.

    To change the port used by IIS Express to run your program you should follow the procedure outlined by this article on MSDN

    How to: Specify a Port for the Development Server

    In short, we need to edit the file %systemdrive%:\Users\<username>\Documents\IISExpress\config and change the binding information found in this file and change other configurations for the IIS Express.

    As a consequence of this not so simple way to fix the problem, I recommend to close the application that tries to use that port access on you dev computer. Look for specific tools like TCPView from Microsoft that could help to spot the application that grabbed your port. Often it is only the browser

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