What’s the problem?

One of our colleagues, Petri, had a problem with using extension methods in his Visual Web Parts user control (.ascx file). He had developed some extension methods and tried to use them in user control:

And all he got was this error:

So it seems like this was compiled with a wrong compiler, and this was further revealed by looking at detailed compiler output:

Clearly not a .NET 3.5 C# compiler. All I can say is, that SharePoint is not fully compatible with .NET Framework 3.5 out-of-the-box. You cannot use 3.5 language constructs (inline code) in dynamically compiled files by default, unless…

What’s the solution?

Christian had dealed with this same problem about a year ago, so all the respect for him in documenting the problem.

To fix this issue, you need to add the following in you web application’s web.config inside configuration-element (here is C# example):

You can also use WebConfigModification classes or server-wide web.config merging to make this change in web.config.

Here is your SharePoint 2010 web.config.net35.xml merge file:

Place the web.config.net35.xml in 14\CONFIG folder and run (or do your PowerShell magic):

Popularity: 5% [?]