1.       Using Microsoft Word 2007 as an example, you can open test1.docx and test2.docx at same time. The two documents are opened within single instance of the word application. World application supports command line argument of passing multiple documents.

2.       Again, Using Microsoft Word 2007 as an example, you can open test1.docx first and then test2.docx. The two documents are opened within single instance of the Word application. Word application supports Multiple Document Interface (MDI).

3.       Using Notepad as an example, you receive error message of “The filename, directory name, or volume label syntax is incorrect” if you want to open two documents at the same time. Notepad does not support command line argument of passing multiple documents

4.       Again, using Notepad as an example, you can open test1.txt first and then test2.txt. The two documents are opened to two different instances of Notepad application. Notepad does not support Multiple Document Interface (MDI).

5.       In conclusion, there is nothing you can do trying to rely on System.Diagnostics.Process class to open multiple documents within a single instance of an application because it is controlled by the application itself. The best approach is to read any developer or user guide of the application and make sure: 1. The application supports Multiple Document Interface (MDI) 2. The application provides command line argument of passing multiple documents. Then, you can use Process class and the command line argument syntax to open multiple documents for the application.