Assembly Load problem
Yes, code Assembly.LoadFrom/LoadFile() is the same as NETLOAD command in AutoCAD, if the code code is called inside AutoCAD process.
However, since you are doing EXE, which is a separate process from AutoCAD, even you are automating AutoCAD (via COM API), if you call Assemly.Loadxxx() in your EXE project, the DLL is loaded into your EXE process, not into AutoCAD process. Since the code in the DLL is for AutoCAD and likely has references to acdbmgd/acmgd/accore.dll, which cannot be used outside AutoCAD process, that is why you get error.
If you automate AutoCAD froom EXE via COM, you'd better to make sure the .NET DLL is somehow automatically loaded into AutoCAD on its startup. Or, you want to make things more complicated (and often unnecessarily) by expose your functionality in the .NET DLL to COM, so that you can access it from outside AutoCAD via COM API.
Yes, you can set AutoCAD's system variable via AcadDocument:
acadApp.AcitveDocument.SetVariable "FILEDIA", 0
But, if you use SemdCommand to execute "NETLOAD", and you obviously want to execute some commands defined in your .NET DLL after the DLL is loaded.
However, your EXE app's execution of SendCommand may not wait the NETLOAD being completed (especially, if your DLL implementing IExtensionApplication, which do qite somework) and just jump to next line of code (after SemdCommand("NETLOAD"....). That is, you'd better only call SendCommand() from your external app only once as LAST action it ask AutoCAD to do.
So, if you have to automate AutoCAD this way, you'd better, (as I suggested previously) consider some autoloading .NET DLL approach, or loading them on demaond...
I appologise. I did not read your response thoroughly enough.
I though you meant
Application.SetSystemVariable
My bad 
Although even with acadApp.AcitveDocument.SetVariable , the following exception is thrown:
Public memeber 'ActiveDocument' on type 'IAcadApplication' not found 
Depending on how AutoCAD is started, there may or may not be an ActiveDocument. It looks like you launch AutoCAD from your EXE app. Iin this case, unless you expliciltly open a new or existing drawing, the AutoCAd session will not have an ActiveDocument.
So, before you can use AcadDocument.SetVariable() method, or any methods of the AcadDocument class for that matter, you need to obtain an instance of AcadDocument, be it ActiveDocument or not. Without seeing more of your code as what it exactly does, it is difficult to say more.
Thank you again Norman but I am afraid that's not the case. I put my code here to have a look:
Public Function InAutoCAD(FilePath As String) As Boolean
Dim GivenTaskStatus As Boolean = False
Const progID As String = "AutoCAD.Application.19"
'Const DllPath As String = "C:\DLL file.dll"
Dim acApp As AcadApplication = Nothing
Try
acApp = DirectCast(Marshal.GetActiveObject(progID), AcadApplication)
Catch
Try
Dim acType As Type = Type.GetTypeFromProgID(progID)
acApp = DirectCast(Activator.CreateInstance(acType, True), AcadApplication)
Catch
GivenTaskStatus = False
MessageBox.Show("Cannot create object of type """ + progID + """")
End Try
End Try
If acApp IsNot Nothing Then
Try
acApp.Visible = True
acApp.WindowState = AcWindowState.acMax
Dim doc As AcadDocument = acApp.Documents.Open(FilePath)
doc.Activate()
''╔════════ Loading DLL file ═══════╗
'acApp.AcitveDocument.SetVariable("FILEDIA", 0)
'Try
' acApp.ActiveDocument.SendCommand("NETLOAD" & vbCr & DllPath & vbCr)
'Catch ex As System.Exception
' MsgBox(vbLf & "Cannot load {0}: {1}", DllPath, ex.Message)
'End Try
''╚════════ Loading DLL file ═══════╝
'acApp.ActiveDocument.SendCommand("MYCOMMAND" + vbCr)
'saving document
doc.Close(True)
GivenTaskStatus = True
Catch ex As Exception
GivenTaskStatus = False
MessageBox.Show(Me, "Problem executing component: " + ex.Message)
End Try
End If
Return GivenTaskStatus
End Function
Having said that, I am not happy with AutoCAD launching part of the code as well. It looks bogus to me. It gives me following error at my very first run of the code (cold start).
But after re-running the code couple of times, that error will not pup up again and AutoCAD is simply launched. Any idea what that might be for?
I found this by Google but unfortunately that article is too technical for me to comprehend 😞