Adding and Deleting Source Files

As your project becomes bigger, you should split it into individual source files to keep a clear structure.

Having separate source files also reduces compilation time, at least if you decide to use .CPP files instead of .INO files, see later.
The resulting program will be identical, independent of having one or multiple source files. It's just a matter of organization of your sources.

Adding a new source file

In the menu, click Project > Add New Sketch Item:

You can choose between Arduino items, C Items and Cpp items.

Note

If the Add New Sketch Item menu item is grayed out, then you most likely have selected the whole solution in the Solution Explorer, instead of the project:

Solution Selected - wrong Project selected - correct
(Solutions are a collection of individual projects in Visual Studio/Atmel Studio, but at this point, we don't need to know more about them.)
If the Solution Explorer is not shown, make it visible by pressing [Ctrl+Alt+L]

After clicking the menu item, a new and empty .INO, .C or a .CPP file are added to your project. You will be prompted for a filename. Enter the filename without the extension.

All three file types are used to write source code.

What is the difference between these file types?

Arduino items = .INO files are the Arduino standard. When compiling, Visual Micro treats all .INO files as they were one file. That's why you don't have to deal with include files (.H extension) as long as you are using .INO files only.
.C items are used to write code that is written in the C language. Unless you have special reasons, you should not use .C files, because the C language has no obvious advantages over the language Arduino uses normally and can cause compatibility issues..
.CPP items are used to write code that is written in the C++ language. This is the same language the .INO files use. Along with the .CPP file, Visual studio also creates a matching .H file.
The newly created .CPP file already contains a skeleton for a new class, ready to be used. If that generated code does not fit your needs, simply delete it.

Note:

File names...

must not start with a number
must not be longer than 30 characters
must not contain any blanks

If any of these rules is broken, your sketch will not compile correctly.

Read the Renaming Source Files and Projects section to learn how to change file names

Which file type shall I use?

This article tells you more about the difference between .INO files and .CPP files and helps you in choosing the right file format.

Adding an existing source file

In accordance to the Arduino specification, a project consists of all source files in the project folder on your hard drive, regardless whether they appear in your Visual Studio/Atmel Studio Solution Explorer or not. If you have a file in your hard drive's project folder that is missing in the Solution Explorer, then you can add it with "Project > Add Existing Item" in the menu, so that you can double click and edit it.

With this method, you can not add files to your project which are stored in a different folder on disk. However, you can add them to Solution Explorer, but, according to the rule above, they will not be part of the compilation because they are not in the project folder on disk.

Deleting files

If you want to remove a file from your project, click on it in the solution explorer and press [Del]. If you do so, a box appears that lets you choose between removal and deletion.

Removing a file only means that it won't appear in Solution Explorer anymore, it is still part of your build. After reopening the solution, such files will appear again. That's why it is not recommended to use removal.

Deleting a file will remove it from the project and delete the file on your hard drive as well.
If you want to be sure that the file will be excluded from compilation, but you want to keep the file, then rename it's file extension in Windows Explorer, e.g. from "myfile.cpp" to "myfile.__cpp" or move it to a different folder.