You should consider several factors when using file locks in .NET

Creating Quick Response Code in .NET You should consider several factors when using file locks
You should consider several factors when using file locks
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The unlock must use exactly the same range as a preceding lock It is not possible, for example, to combine two previous lock ranges or unlock a portion of a locked range An attempt to unlock a region that does not correspond exactly with an existing lock will fail; the function returns FALSE and the system error message indicates that the lock does not exist Locks cannot overlap existing locked regions in a file if a conflict would result It is possible to lock beyond the range of a file's length This approach could be useful when a process or thread extends a file Locks are not inherited by a newly created process
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Table 3-1 shows the lock logic when all or part of a range already has a lock This logic applies even if the lock is owned by the same process that is making the new request
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Table 3-1 Lock Request Logic
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Requested Lock Type
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Existing Lock
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Shared Lock Exclusive Lock
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None
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Granted
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Granted Refused Refused
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Shared lock (one or more) Granted Exclusive lock Refused
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Table 3-2 shows the logic when a process attempts a read or write operation on a file region with one or more locks, owned by a separate process, on all or part of the read-write region A failed read or write may take the form of a partially completed operation if only a portion of the read or write record is locked
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Table 3-2 Locks and I/O Operation
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I/O Operation
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Existing Lock
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Read
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Write
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Succeeds
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Succeeds Fails
Shared lock (one or more) Succeeds It is not necessary for the calling process to own a lock on the file region Exclusive lock Succeeds if the calling process owns the lock Fails otherwise
Succeeds if the calling process owns the lock Fails otherwise
Read and write operations are normally in the form of ReadFile and WriteFile calls or their extended versions, ReadFileEx and WriteFileEx Diagnosing a read or write failure requires calling GetLastError Accessing memory that is mapped to a file is another form of file I/O, as will be discussed in 5 Lock conflicts are not detected at the time of memory reference; rather, they are detected at the time that the MapViewOfFile function is called This function makes a part of the file available to the process, so the lock must be checked at that time The LockFile function is a limited, special case and is a form of advisory locking It can be used on Windows 9x, which does not support LockFileEx Only exclusive access is available, and LockFile returns immediately That is, LockFile does not block Test the return value to determine whether you obtained the lock
Releasing File Locks
Every successful LockFileEx call must be followed by a single matching call to UnlockFileEx (the same is true for LockFile and UnlockFile) If a program fails to release a lock or holds the lock longer than necessary, other programs may not be able to proceed, or, at the very least, their performance will be negatively impacted Therefore, programs should be carefully designed and implemented so that locks are released as soon as possible, and logic that might cause the program to skip the unlock should be avoided Termination handlers ( 4) are a useful way to ensure that the unlock is performed
Lock Logic Consequences
Although the file lock logic shown in Tables 3-1 and 3-2 is natural, it has consequences that may be unexpected and cause unintended program defects Here are some examples
Suppose that process A and process B periodically obtain shared locks on a file, and process C blocks when attempting to gain an exclusive lock on the same file after process A gets its shared lock Process B may now gain its shared lock even though C is still blocked, and C will remain
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blocked even after A releases the lock C will remain blocked until all processes release their shared locks even if they obtained them after C blocked In this scenario, it is possible that C will be blocked forever even though all the other processes manage their shared locks properly Assume that process A has a shared lock on the file and that process B attempts to read the file without obtaining a shared lock first The read will still succeed even though the reading process does not own any lock on the file because the read operation does not conflict with the existing shared lock These statements apply both to entire files and to regions A read or write may be able to complete a portion of its request before encountering a conflicting lock The read or write will return FALSE, and the byte transfer count will be less than the number requested