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I am using a really old website running on .net 2 but I am adding an addition to it that is MVC 3 running on .net 4, created as a virtual directory under the original website.

The problem is that if I get the "Server Application Unavailable" with one of the frameworks I can run aspnet_regiis for that framework and it fixes it but then it make the other framework give this error

I fix one it breaks the other

is there any way around this

I need this asap, client is breathing down my neck

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Are you running both applications under the same application pool? You can't mix and match framework versions in a single app pool. –  Tim Jan 31 '12 at 1:44

3 Answers 3

up vote 7 down vote accepted

Just create two application pools targeting each respective framework.

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thank you so much I can't believe I never thought of this. I guess because of my panic –  user758105 Jan 31 '12 at 1:59
@Gabe On IIS6, I cannot find the place to change aplication pool to target respective framework. How would you do that? –  Jonas T Apr 3 '13 at 0:27
You change it on the website properties, not in the app pool properties, which was confusing for me too –  Jonesy Jul 5 '13 at 18:39
I'll add that I had to force the app pool to recyle through IIS to get my page to come back. –  oscilatingcretin Dec 13 '13 at 19:54
@JonasT - You change the target framework by right clicking on the applciation then going to the ASP.net tab in IIS6 –  Gabe Dec 15 '13 at 16:03

IIS does not support running multiple .Net frameworks within a single application pool. If an application requires a different framework version than other applications in the same application pool, it must be placed in a new application pool.

You need to use different Application pools for each of the framework versions.

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You have three choices:

  1. Upgrade the existing site to .net 4.
    This might be a lot of work.. or it might not.

  2. Start over with your "additions".
    This time using the same tech as the site is built in. Without knowing how involved your additions were, this might be easier than #1.

  3. Create a new app pool just for your 4.0 site.
    This item has the caveat that you aren't going to be sharing session data between the two. But may be the most expedient. Given that you are just now finding the problem I'm going to guess that the addition really isn't that tightly integrated and so you'll probably be just fine.

Either way I hope the lesson you take from this is that using new trinkets on an existing site is probably not the best way to go. ;)

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