Specifies whether the contents of an existing file are preserved or overwritten and whether requests to create an existing file cause an exception.

Append

It opens the file if it exists and seeks to the end of the file, or creates a new file.  FileMode.Append can only be used in conjunction with FileAccess.Write.

Create

It specifies that the operating system should create a new file.  The file will be overwritten if it already exists.

CreateNew

It specifies that the operating system should create a new file.

Open

It specifies that the operating system should open an existing file.  The ability to open the file is dependent on the value of enumeration specified by FileAccess.

OpenCreate

It specifies that the operating system should open a file if it exists.  Otherwise, a new file will be created.

Truncate

It specifies that the operating system should open an existing file.  Once opened, the file should be truncated so that its size is zero bytes.