You can specify file associations that determine the editor you want to use for each file type. For example, you can instruct the IDE to use an external program such as WordPad to edit all .h files. After you set that preference, you can double-click a file in the Project Explorer view, and the IDE automatically opens the file in your selected program.
If you have multiple associations for the same file extension, the IDE always opens the same editor, which is marked as the default in the list of editors for this extension, but other associations continue to be available. To open a file in an alternative editor, right-click on the file you want to open, select, and then choose the editor.
If the IDE doesn't have an association set for a certain file type, it uses the host OS defaults. For example, on a Windows host, if you double-click a .doc file, Word or WordPad is automatically launched and opens the file.