EOL conversion in notepad ++

That functionality is already built into Notepad++. From the "Edit" menu, select "EOL Conversion" -> "UNIX/OSX Format".

screenshot of the option for even quicker finding (or different language versions)

You can also set the default EOL in notepad++ via "Settings" -> "Preferences" -> "New Document/Default Directory" then select "Unix/OSX" under the Format box.

In Notepad++, use replace all with regular expression. This has advantage over conversion command in menu that you can operate on entire folder w/o having to open each file or drag n drop (on several hundred files it will noticeably become slower) plus you can also set filename wildcard filter.

(\r?\n)|(\r\n?)

to

\n

This will match every possible line ending pattern (single \r, \n or \r\n) back to \n. (Or \r\n if you are converting to windows-style)

To operate on multiple files, either:

• Use "Replace All in all opened document" in "Replace" tab. You will have to drag and drop all files into Notepad++ first. It's good that you will have control over which file to operate on but can be slow if there several hundreds or thousands files.
• "Replace in files" in "Find in files" tab, by file filter of you choice, e.g., *.cpp *.cs under one specified directory.