Open AutoCAD Using C#

For the last few years I've been using this code in VB.NET to open a drawing in AutoCAD. This works great and it looks for an existing instance to use before opening a new one:

       Dim vAcadApp As AcadApplication
       Dim vAcadDoc As AcadDocument
Try
           If Process.GetProcessesByName("acad").Length > 0 Then
               vAcadApp = GetObject(, "AutoCAD.Application.19")
           Else
               vAcadApp = New AcadApplication
           End If

           vAcadApp.Visible = True
           vAcadApp.WindowState = AcWindowState.acMax
           vAcadDoc = vAcadApp.Documents.Open(DwgPath & DwgName, True)

We've now begun working more with C# so I'm searching for how to do this very same thing but in C#.

And there is one other caveat on this I must deal with. This will be run at the users' desktop and we have various versions of AutoCAD running. Some 2009's sprinkled here and there but mostly 2013 and 2014. I'm fearing that this will make loading the appropriate reference a complicated task.