C# how obtain parameter values

I have a C# 2008/2010 application that is currently accessed as a dll file and  I would like to change the application so that the dll is accessed though as a reference. The code that calls the dll is a process and the code looks like the following:

Process theProcess = new Process();

theProcess.StartInfo = new ProcessStartInfo("filename.ext");

theProcess.StartInfo.Arguments = "args here"

theProcess.WaitForExit();

theProcess.Start();

I want to be able to pass argument values to the dll when it becomes a reference in the 5 programs that call it.
My problem is that I do not know how to pass the parameter values when the code is accessed as a reference. Thus can  you tell me how to pass argument values to the application when it is accessed as a reference. Is it something in the reference call? Can you show me code and/or tell me how to acoomplish my goal?