Specifies that the operating system should open a file if it exists; otherwise, a new file should be created. If the file is opened with FileAccess.Read, System.Security.Permissions.FileIOPermissionAccess.Read permission is required. If the file access is FileAccess.Write, System.Security.Permissions.FileIOPermissionAccess.Write permission is required. If the file is opened with FileAccess.ReadWrite, both System.Security.Permissions.FileIOPermissionAccess.Read and System.Security.Permissions.FileIOPermissionAccess.Write permissions are required.