How to check for file lock

Question

Is there any way to check whether a file is locked without using a try catch block     Right now  the only way I know of is to just open the file and catch any System IO IOException

User · Answer

You could call LockFile via interop on the region of file you are interested in  This will not throw an exception  if it succeeds you will have a lock on that portion of the file  which is held by your process   that lock will be held until you call UnlockFile or your process dies

User · Answer

The other answers rely on old information   This one provides a better solution   Long ago it was impossible to reliably get the list of processes locking a file because Windows simply did not track that information   To support the Restart Manager API  that information is now tracked   The Restart Manager API is available beginning with Windows Vista and Windows Server 2008  Restart Manager  Run-time Requirements    I put together code that takes the path of a file and returns a List lt Process gt  of all processes that are locking that file   static public class FileUtil        StructLayout LayoutKind Sequential       struct RM UNIQUE PROCESS               public int dwProcessId          public System Runtime InteropServices ComTypes FILETIME ProcessStartTime             const int RmRebootReasonNone   0      const int CCH RM MAX APP NAME   255      const int CCH RM MAX SVC NAME   63       enum RM APP TYPE               RmUnknownApp   0          RmMainWindow   1          RmOtherWindow   2          RmService   3          RmExplorer   4          RmConsole   5          RmCritical   1000             StructLayout LayoutKind Sequential  CharSet   CharSet Unicode       struct RM PROCESS INFO               public RM UNIQUE PROCESS Process            MarshalAs UnmanagedType ByValTStr  SizeConst   CCH RM MAX APP NAME   1           public string strAppName            MarshalAs UnmanagedType ByValTStr  SizeConst   CCH RM MAX SVC NAME   1           public string strServiceShortName           public RM APP TYPE ApplicationType          public uint AppStatus          public uint TSSessionId           MarshalAs UnmanagedType Bool           public bool bRestartable              DllImport  rstrtmgr dll   CharSet   CharSet Unicode       static extern int RmRegisterResources uint pSessionHandle                                            UInt32 nFiles                                            string   rgsFilenames                                            UInt32 nApplications                                             In  RM UNIQUE PROCESS   rgApplications                                            UInt32 nServices                                            string   rgsServiceNames         DllImport  rstrtmgr dll   CharSet   CharSet Auto       static extern int RmStartSession out uint pSessionHandle  int dwSessionFlags  string strSessionKey         DllImport  rstrtmgr dll        static extern int RmEndSession uint pSessionHandle         DllImport  rstrtmgr dll        static extern int RmGetList uint dwSessionHandle                                  out uint pnProcInfoNeeded                                  ref uint pnProcInfo                                   In  Out  RM PROCESS INFO   rgAffectedApps                                  ref uint lpdwRebootReasons             lt summary gt          Find out what process es  have a lock on the specified file           lt  summary gt           lt param name  path  gt Path of the file  lt  param gt           lt returns gt Processes locking the file lt  returns gt           lt remarks gt See also          http   msdn microsoft com en-us library windows desktop aa373661 v vs 85  aspx         http   wyupdate googlecode com svn-history r401 trunk frmFilesInUse cs  no copyright in code at time of viewing                    lt  remarks gt      static public List lt Process gt  WhoIsLocking string path                uint handle          string key   Guid NewGuid   ToString            List lt Process gt  processes   new List lt Process gt              int res   RmStartSession out handle  0  key            if  res    0              throw new Exception  Could not begin restart session   Unable to determine file locker              try                       const int ERROR MORE DATA   234              uint pnProcInfoNeeded   0                   pnProcInfo   0                   lpdwRebootReasons   RmRebootReasonNone               string   resources   new string     path       Just checking on one resource               res   RmRegisterResources handle   uint resources Length  resources  0  null  0  null                if  res    0                   throw new Exception  Could not register resource                                                        Note  there s a race condition here -- the first call to RmGetList   returns                     the total number of process  However  when we call RmGetList   again to get                     the actual processes this number may have increased              res   RmGetList handle  out pnProcInfoNeeded  ref pnProcInfo  null  ref lpdwRebootReasons                if  res    ERROR MORE DATA                                   Create an array to store the process results                 RM PROCESS INFO   processInfo   new RM PROCESS INFO pnProcInfoNeeded                   pnProcInfo   pnProcInfoNeeded                      Get the list                 res   RmGetList handle  out pnProcInfoNeeded  ref pnProcInfo  processInfo  ref lpdwRebootReasons                    if  res    0                                        processes   new List lt Process gt   int pnProcInfo                           Enumerate all of the results and add them to the                         list to be returned                     for  int i   0  i  lt  pnProcInfo  i                                                  try                                                       processes Add Process GetProcessById processInfo i  Process dwProcessId                                                         catch the error -- in case the process is no longer running                         catch  ArgumentException                                                              else                     throw new Exception  Could not list processes locking resource                                                   else if  res    0                  throw new Exception  Could not list processes locking resource  Failed to get size of result                                           finally                       RmEndSession handle                      return processes            UPDATE  Here is another discussion with sample code on how to use the Restart Manager API

User · Answer

You can also check if any process is using this file and show a list of programs you must close to continue like an installer does   public static string GetFileProcessName string filePath        Process   procs   Process GetProcesses        string fileName   Path GetFileName filePath        foreach  Process proc in procs                if  proc MainWindowHandle    new IntPtr 0   amp  amp   proc HasExited                        ProcessModule   arr   new ProcessModule proc Modules Count                foreach  ProcessModule pm in proc Modules                                if  pm ModuleName    fileName                      return proc ProcessName                                     return null

User · Answer

Same thing but in Powershell  function Test-FileOpen       Param       string  FileToOpen      try                openFile    system io file   Open  FileToOpen  system io filemode   Open            open   true          openFile close             catch                open    false            open

User · Answer

Here s a variation of DixonD s code that adds number of seconds to wait for file to unlock  and try again   public bool IsFileLocked string filePath  int secondsToWait        bool isLocked   true      int i   0       while  isLocked  amp  amp     i  lt  secondsToWait      secondsToWait    0                  try                       using  File Open filePath  FileMode Open                   return false                    catch  IOException e                        var errorCode   Marshal GetHRForException e   amp    1  lt  lt  16  - 1               isLocked   errorCode    32    errorCode    33              i                 if  secondsToWait   0                  new System Threading ManualResetEvent false  WaitOne 1000                        return isLocked      if   IsFileLocked file  10               else       throw new Exception

User · Answer

No  unfortunately  and if you think about it  that information would be worthless anyway since the file could become locked the very next second  read  short timespan    Why specifically do you need to know if the file is locked anyway  Knowing that might give us some other way of giving you good advice   If your code would look like this   if not locked then     open and update file   Then between the two lines  another process could easily lock the file  giving you the same problem you were trying to avoid to begin with  exceptions

User · Answer

What I ended up doing is   internal void LoadExternalData         FileStream file       if  TryOpenRead  filepath filename   5  out file             using  file          using  StreamReader reader   new StreamReader file                 do something                      internal bool TryOpenRead string path  int timeout  out FileStream file        bool isLocked   true      bool condition   true       do           try               file   File OpenRead path               return true                    catch  IOException e                var errorCode   Marshal GetHRForException e   amp    1  lt  lt  16  - 1               isLocked   errorCode    32    errorCode    33              condition    isLocked  amp  amp  timeout  gt  0                if  condition                       we only wait if the file is locked  If the exception is of any other type  there s no point on keep trying  just return false and null                  timeout--                  new System Threading ManualResetEvent false  WaitOne 1000                                     while  condition        file   null      return false

User · Answer

Instead of using interop you can use the  NET FileStream class methods Lock and Unlock   FileStream Lock http   msdn microsoft com en-us library system io filestream lock aspx  FileStream Unlock http   msdn microsoft com en-us library system io filestream unlock aspx

User · Answer

A variation of DixonD s excellent answer  above    public static bool TryOpen string path                             FileMode fileMode                             FileAccess fileAccess                             FileShare fileShare                             TimeSpan timeout                             out Stream stream        var endTime   DateTime Now   timeout       while  DateTime Now  lt  endTime                if  TryOpen path  fileMode  fileAccess  fileShare  out stream               return true             stream   null      return false     public static bool TryOpen string path                             FileMode fileMode                             FileAccess fileAccess                             FileShare fileShare                             out Stream stream        try               stream   File Open path  fileMode  fileAccess  fileShare           return true            catch  IOException e                if   FileIsLocked e               throw           stream   null          return false           private const uint HRFileLocked   0x80070020  private const uint HRPortionOfFileLocked   0x80070021   private static bool FileIsLocked IOException ioException        var errorCode    uint Marshal GetHRForException ioException       return errorCode    HRFileLocked    errorCode    HRPortionOfFileLocked      Usage   private void Sample string filePath        Stream stream   null       try               var timeOut   TimeSpan FromSeconds 1            if   TryOpen filePath                       FileMode Open                       FileAccess ReadWrite                       FileShare ReadWrite                       timeOut                       out stream               return              Use stream              finally               if  stream    null              stream Close

User · Answer

When I faced with a similar problem  I finished with the following code   public bool IsFileLocked string filePath        try               using  File Open filePath  FileMode Open               catch  IOException e                var errorCode   Marshal GetHRForException e   amp    1  lt  lt  16  - 1            return errorCode    32    errorCode    33             return false

User · Answer

Then between the two lines  another process could easily lock the file  giving you the same problem you were trying to avoid to begin with  exceptions    However  this way  you would know that the problem is temporary  and to retry later   E g   you could write a thread that  if encountering a lock while trying to write  keeps retrying until the lock is gone    The IOException  on the other hand  is not by itself specific enough that locking is the cause of the IO failure  There could be reasons that aren t temporary

User · Answer

You can see if the file is locked by trying to read or lock it yourself first    Please see my answer here for more information

[c#] How to check for file lock?

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