AutoCAD .NET :: Make A New Drawing Or Open One?

In one of my application I want to open a drawing and make a new one.That doesnt work. So I copy following code from the

[CommandMethod("NewDrawing", CommandFlags.Session)]
public static void NewDrawing()
{
string strTemplatePath = "acadiso.dwt";
DocumentCollection acDocMgr  = Application.DocumentManager;
acDocMgr.DocumentActivationEnabled = true;
Autodesk.AutoCAD.ApplicationServices.Document acDoc = DocumentCollectionExtension.Add(acDocMgr, strTemplatePath);
// Alternatively: Autodesk.AutoCAD.ApplicationServices.Document acDoc = Application.DocumentManager.Add(strTemplatePath);
acDocMgr.MdiActiveDocument = acDoc;
}

At the end the MdiActiveDocument is still the drawing which where open before a new document is created.

BTW: I'm using 2013 and 2014. This method works fine in 2010.

Whats the different between 2013/ 2014 and 2010.