“The type or namespace 'name' is missing” when it is included in references and using directives

Question

I have a very weird problem. I am developing a Sharepoint 2013 Project which consists of a WebPart and a couple of Lists. I am using a DateTimeControl in the WebPart. I moved the WebPart into another project yesterday. After I moved it, I got an error that stated "The type or namespace DateTimeControl cannot be found in the namespace 'Microsoft.Sharepoint.WebControls'...". So I checked my using directives and my references. The supposedly missing namespace is definitely there. I even inspected it to check if DateTimeControl was inside it, which it was. The architecture of both projects is also the same.

Am I missing something here?

ADD: I tried adding a new WebPart with an DateTimeControl on it. I still get the same error. When I add another control from the same namespace, I get no errors. Only with DateTimeControl.

Answer 1

Make sure both projects are compiled against the same .Net Framework. If you WebPart project that is referencing your DateTimeControl is compiled at a lower .Net Framework than your DateTimeControl then you will get this error.

In esseense make sure they are compiled at the same .Net Framework, or referenced libraries are compiled at a lower .Net Framework

Answer 2

Ok I found the problem. This is a more Sharepoint-specific error.

It seems that you cannot use the control in the namespace "Microsoft.Sharepoint.WebControls" if you are working on a sandbox solution. I don't really understand Microsoft's reasons behind this, but it has to do with farm solutions being run under the farm administrator account, sandbox solutions don't do this.

So when I set the project's "Sandboxed Solution" property to false, the error was gone. My colleague could confirm this behavior on his machine too.