Debugging VS2013 Websites Using 64-bit IIS Express

6 April 2016 Update: Added the correct values for VS2015.


If you are working on ASP.NET MVC web sites in Visual Studio 2013 (VS2013) or 2015 (VS2015), you need to make one registry change if you want to run IIS Express as a 64-bit process by default.  Use one of the methods, below.

Within VS2013 or VS2015:

Tools | Options | Projects and Solutions | Web Projects | Use the 64 bit version of IIS Express…

* Thanks to Christian Lundestad, David, Javier Collazo and Eliza for pointing this method out in the comments.


For VS2013, use

reg add HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\12.0\WebProjects /v Use64BitIISExpress /t REG_DWORD /d 1

For VS2015, use

reg add HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\14.0\WebProjects /v Use64BitIISExpress /t REG_DWORD /d 1


  1. For VS2013, navigate to:
  2. For VS2015, navigate to:
  3. Make sure there is a REG_DWORD value named “Use64BitIISExpress”.  If not, create it.
  4. Set its value from 1 (default = 0).

That’s it.  Now you can debug your 64-bit web sites.

If you don't do this, you will likely get a message similar to:

"Could not load file or assembly or one of its dependencies. An attempt was made to load a program with an incorrect format."

I found this solution for Visual Studio 2012 (VS2012) on Stack Overflow here.


Comments (16)
  1. William Gross says:

    Thank you very much Rob! I have a web app that uses the Oracle database and the client library I'm using is platform-specific, so your tip was essential in getting my app running with IIS Express.

    I also noticed that Mads Kristensen said the problem was fixed in Visual Studio 2013, but I don't know what he meant. See…/3254745-allow-for-iis-express-64-bit-to-run-from-visual-st .

  2. Christian Lundestad says:

    There is an now option for this in Visual Studio 2013.

    Tools | Options | Projects and Solutions | Web Projects | Use the 64 bit version of IIS Express…

  3. Mark Hufford says:

    Christian – nice find.  Thanks!

  4. Lester says:

    Thanks guys, I was struggling with an OutOfMemoryException error when testing session state serialization.  Changing my VS 2013 setting to use 64 bit IIS Express solved the problem.

  5. Murari says:

    Thanks Rob!!!.  Do you have any thoughts on configurring the Local IIS to support MVC

  6. Hernaldo says:

    Thank a lot! I'm stuck por 1 day!!

  7. David says:

    In VS2013 you should all be going to –> Tools | Options | Projects and Solutions | Web Projects | Use the 64 bit version of IIS Express for web sites and projects…

  8. Daniel says:

    Christian, thanks a lot!…

  9. Yo says:

    Christian, you made my day ! Thanks a LOT

  10. Trung Pham says:

    Thank you. I did not know that IIS Express default using x86 platform

  11. Scott Weeden says:

    Thanks!   Visual Studio 2015 same problem.   I forgot you had to do this to work with the Office 64-bit dll's too!

  12. Javier Collazo says:

    Thanks Christian, i have the same issue in VS2013 N¿and i fix it with option:

    Tools | Options | Projects and Solutions | Web Projects | Use the 64 bit version of IIS Express…

  13. Ramesha Rangaswamy says:

    they could have made the VS more intelligent by sensing the 32/ 64 bit machine and run it by default…

  14. Duc says:

    Thanks. You've saved my whole day

  15. Eliza says:

    Allow for IIS Express 64 bit to run from Visual Studio 2013

    TOOLS>OPTIONS>Projects and Solutions>WEB PROJECT and allow

  16. Pradnyakar badge says:


    very nice sir

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