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I installed .NET 4 on the server I would be installing SP 2010, and everything worked out just fine.

Later, I learned that just putting 4 on a machine doesn't change the IIS setting from .NET 2 to (the new) 4. I tried that, and get HTTP 500 errors.

PUtting IIS back to .NET 2 fixes the problem, but I suspect there must be a way to use .NET 4?

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SharePoint 2010 requires .NET Framework 3.5 SP1

Agreed with holding back from using .NET 4 until WWF supports it.

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The workflows and such are based on the .NET 3.5 framework SP1, so I would be hesitant to try to use .NET 4 until it is supported.

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As per the SharePoint Developer Team blog's posting: .NET Framework support in SharePoint 2010 - there are limitations in using the .NET 4.0 framework. The SP 2010 Object model cannot be accessible and any calls to methods or objects other than using .NET 3.5/SP1 will throw an exception. But, a .NET 4 application may call the SP 2010 web services (see the above post regarding SilverLight apps).

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From my understanding you need .NET 3.5 to run SharePoint. SharePoint 2010 will not run on 4.0, and deployed packages must be in .NET 3.5. Sure, you can have 1.1, 2.x, or 4.x versions of .NET on the server/machine for other purposes, but the application pools assigned to the applications must be running under 3.5 (which are listed as v2.0.5x).

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