You shouldn't run into any problems, you'll just need to open the solution in VS2010 when it's installed, it should handle the rest. I would recommend backing up your files before you open with VS2010 though just in case the conversion process has any issues.

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Chris Hammond
Former DNN Corp Employee, MVP, Core Team Member, Trustee
Christoc.com Software Solutions DotNetNuke Module Development, Upgrades and consulting.
dnnCHAT.com a chat room for DotNetNuke discussions

I'm afraid the problems seem to be countless.

I started out as you suggested (after creating a backup) by opening my solution in Visual Studio 2010. The conversion wizard started and asked me if I wanted to upgrade my "website" to .NET Framework version 4.0. I clicked on No. After the conversion a number of error messages resulted coming from the AjaxControlToolkit (version: 3.0.30930.0) and a DLL-project that is in the same solution as my DNN module project. Both because they target a higher version of the .NET framework. (For 2 error messages, see below.) (Visual Studio 2008 was not so picky.)

I decided first to upgrade my DNN version from 5.1.4 to 5.6.1. (This resulted in itself in quite some error messages and took me a lot of time to accomplish with luckily some help.)

After this upgrade the previous error messages remained and were supplemented with error messages suggesting that DotNetNuke namespaces have changed / can't be referenced.

I've got .NET framework version 4 installed on my PC. Do you know if this means that DotNetNuke automatically uses it? (Even if I did not freshly install DNN, but upgraded it from a version (5.1.4) that did not support .NET 4?)

Error Messages (from AjaxControlToolkit):
"Warning           1          The primary reference "AjaxControlToolkit, Version=3.0.30930.28736, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e, processorArchitecture=MSIL" could not be resolved because it has an indirect dependency on the framework assembly "System.Xml.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" which could not be resolved in the currently targeted framework. ".NETFramework,Version=v2.0". To resolve this problem, either remove the reference "AjaxControlToolkit, Version=3.0.30930.28736, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e, processorArchitecture=MSIL" or retarget your application to a framework version which contains "System.Xml.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"."

Error Messages (from DLL-project):
"Warning           2          The primary reference "D:\DotNetNuke\DesktopModules\DLL\bin\Debug\DLL.dll" could not be resolved because it has an indirect dependency on the framework assembly "System.Xml.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" which could not be resolved in the currently targeted framework. ".NETFramework,Version=v2.0". To resolve this problem, either remove the reference "D:\DotNetNuke\DesktopModules\DLL\bin\Debug\DLL.dll" or retarget your application to a framework version which contains "System.Xml.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"."