SPFile.LockId property: whenever somebody checked out file it will be locked automatically, is that correct? then what will be the value of LockId property at that time? secondly, what is the default value of LockId? When the LockId value will be null?
Yes, the file will be locked automatically whenever anyone checks it out. A GUID will be assigned to the SPFIle.LockId property. The default value is Null, if the file is not locked.
Refer this MSDN link