Right-click on the Project References folder and select Add Reference and then select the assembly MicroFocus Runtime (Interop RuntimeServices) 4.0 which is listed under the Extensions section.
This will add MicroFocus.COBOL.RuntimeServices.dll to your project. This contains the RunUnit class.
Then add the following directive to the top of Default.aspx.cbl
$set ilusing"MicroFocus.COBOL.RuntimeServices"
Your project will then compile successfully.
In a multi-threaded environment like ASP.NET where each user request is handled by a separate thread, you must protect the resources that the COBOL programs use to make them thread safe. This is what the RunUnit class does. By creating an instance of the RunUnit class for each user request and then adding your top level COBOL program to the RunUnit and calling it, you will be protecting all resources of that program and any other program that it may call. This includes resources source as open files, database connections and working-storage data. If you did not use a RunUnit and just executed a CALL statement directly from your .aspx program then only one instance of the program would be created and shared by all threads. These threads would quickly be stomping all over each other.
Documentation for RunUnits and multiuser programming can be found in the docs here:
The actual documentation for the RunUnit class is in a separate help file which is documented here: