Unlockfile

[DllImport("Kernel32.dll")][return: MarshalAs(UnmanagedType.Bool)]
public static extern bool UnlockFile(IntPtr hFile,
   uint dwFileOffsetLow,
   uint dwFileOffsetHigh,
   uint nNumberOfBytesToUnlockLow,
   uint nNumberOfBytesToUnlockHigh
);

Microsoft documentation: Link