cannot find embedded type 'autocad.iacadapplication'

[mtwohig]

Hello all.

I am writing a Windows Service application using Visual Studio 2013 which is to start AutoCAD Mechanical 2016, make the app visible, do some stuff and issue a SendCommand. The command that I am sending works when I run it manually in the AutoCAD Mechanical 2016 UI.

The problem is that the SendCommand never fires.

My only relevant import is System.Runtime.InteropServices.

My code:

        'startup autocad
        Process.Start("C:\Program Files\Autodesk\AutoCAD 2016\acad.exe", "/p <<ACADMPP>> /product ACADM /language ""en-US""")

        'wait till the thing opens. 0 min 30 seconds maybe?
        System.Threading.Thread.Sleep(New TimeSpan(0, 0, 30))

        Dim acAppComObj As AutoCAD.AcadApplication = Nothing
        Const strProgId As String = "AutoCAD.Application.20.1"

        Try
            ' Get a running instance of AutoCAD
            acAppComObj = DirectCast(Marshal.GetActiveObject(strProgId), AutoCAD.AcadApplication) ' .AcadApplication)
        Catch ex As Exception
            ' If an instance of AutoCAD is not created then message and exit
            EventLog1.WriteEntry(String.Format("Instance of 'AutoCAD.Application' could not be attached.{0}{1}", vbCrLf, ex.Message))
            'Return False
            GoTo NOGOOD
        End Try
        'EventLog1.WriteEntry("Acad successfully attached.")

        acAppComObj.Visible = True

        'open up a new document - need to copy local for security reasons
        acAppComObj.Documents.Add(fi2.FullName)
        EventLog1.WriteEntry(String.Format("Opened doc: {0}", fi2.FullName))

        'wait for a few seconds
        System.Threading.Thread.Sleep(5000)

        ' Get the active document
        Dim acDocComObj As AutoCAD.AcadDocument

        Try
            acDocComObj = acAppComObj.ActiveDocument
        Catch ex As Exception
            EventLog1.WriteEntry(String.Format("Could not open the active document.{0}{1}", vbCrLf, ex.Message))
            'Return False
            GoTo NOGOOD
        End Try

[...]
[... OTHER CODE THAT REFERENCES acAppComObj AND acDocComObj WORKS FINE]
[...]

        'plot pdf
        acDocComObj.SendCommand("-AUTOPLOTTOPDF ")

So the SendCommand never fires. Also, the application never becomes visible. Is it possible that the SendCommand will only fire in a visible app?

If I break on the acAppComObj.Visible = True line and step through it, .Visible always stays False. It does not error.
However, if I break on that same line and manually set it to True using an immediate command, This messagebox pops up:

cannot find the interop type that matches the embedded type 'autocad.iacadapplication'. are you missing an assembly reference?

So it is refernecing IAcadApplication rather than AcadApplication.

Is this because I am writing a Service app? How can I fix this?

Thanks!!