I use invoke via vb.net. Here are some samples that call user defined arx function and autocad function.
' override with resultbuffer (user defined arx function)
Dim result as ResultBuffer
Dim args as New ResultBuffer()
args.Add( New TypedValue( Import.aced.valType.RTSTR, "my_arx_function" ) )
args.Add( New TypedValue( Import.aced.valType.RTSTR, "a string" ) )
args.Add( New TypedValue( Import.aced.valType.RTSHORT, 0 ) )
Dim ret as Integer = Import.aced.Invoke( args, result )
' override with string parameter array (autocad command "units")
Dim ret as Integer = Import.aced.Invoke( "command", "_.units", "2", "4", "1", "6", "0", "_N" )
Hope this help.