external event handler

Hi

I have a C# (VS2010) class written to be a COM Dll used by Outlook VBA

It supports serial port having ReadEvent which handler is external (will be implemented  in VBA)
coded :

[DllImport("Cmdll.dll")]
public static extern void DataReceivedHandler(object sender, SerialDataReceivedEventArgs e);
Port.DataReceived += new SerialDataReceivedEventHandler(DataReceivedHandler);

How do i eliminate the DllImpot ?  or is there other alternative ?

Would appreciate your advice

Thanks