OpenXml V2.5 has deprecated SmartTags and also can't be used with with .Net 3.5 builds due some internal dependencies on .Net.4.0.
You can recompile the source to ClosedXml with a reference to the OpenXml SDK V2.5 if you don't need to target .Net 3.5 and if you comment out the line that refers to the SmartTag class
For more info on the SmartTag issue, Google to:
"What's new in the Open XML SDK 2.5 for Office - MSDN - Microsoft"
I'm at the point of testing now so there may be a few other things but even Resharper was happy so it can't be too bad.
You can recompile the source to ClosedXml with a reference to the OpenXml SDK V2.5 if you don't need to target .Net 3.5 and if you comment out the line that refers to the SmartTag class
For more info on the SmartTag issue, Google to:
"What's new in the Open XML SDK 2.5 for Office - MSDN - Microsoft"
I'm at the point of testing now so there may be a few other things but even Resharper was happy so it can't be too bad.