Check last modified date of file in C

Question

I m looking to find out a way of seeing when a file was last modified in C   I have full access to the file

User · Answer

You simply want the File GetLastWriteTime static method  Example  var lastModified   System IO File GetLastWriteTime  quot C  foo bar quot     Console WriteLine lastModified ToString  quot dd MM yy HH mm ss quot      Note however that in the rare case the last-modified time is not updated by the system when writing to the file  this can happen intentionally as an optimisation for high-frequency writing  e g  logging  or as a bug   then this approach will fail  and you will instead need to subscribe to file write notifications from the system  constantly listening

User · Answer

System IO File GetLastWriteTime is what you need

User · Answer

Be aware that the function File GetLastWriteTime does not always work as expected  the values are sometimes not instantaneously updated by the OS  You may get an old Timestamp  even if the file has been modified right before    The behaviour may vary between OS versions  For example  this unit test worked well every time on my developer machine  but it always fails on our build server      TestMethod    public void TestLastModifiedTimeStamps            var tempFile   Path GetTempFileName         var lastModified   File GetLastWriteTime tempFile        using  new FileStream tempFile  FileMode Create  FileAccess Write  FileShare None                       Assert AreNotEqual lastModified  File GetLastWriteTime tempFile          See File GetLastWriteTime seems to be returning   39 out of date  39  value  Your options   a  live with the occasional omissions   b  Build up an active component realising the observer pattern  eg  a tcp server client structure   communicating the changes directly instead of writing   reading files  Fast and flexible  but another dependency and a possible point of failure  and some work  of course    c  Ensure the signalling process by replacing the content of a dedicated signal file that other processes regularly read  It  s not that smart as it  s a polling procedure and has a greater overhead than calling File GetLastWriteTime  but if not checking the content from too many places too often  it will do the work        lt summary gt      type to set signals or check for them using a central file       lt  summary gt  public class FileSignal            lt summary gt          path to the central file for signal control          lt  summary gt      public string FilePath   get  private set              lt summary gt          numbers of retries when not able to retrieve  exclusive  file access          lt  summary gt      public int MaxCollisions   get  private set              lt summary gt          timespan to wait until next try          lt  summary gt      public TimeSpan SleepOnCollisionInterval   get  private set              lt summary gt          Timestamp of the last signal          lt  summary gt      public DateTime LastSignal   get  private set              lt summary gt          constructor          lt  summary gt           lt param name  filePath  gt path to the central file for signal control lt  param gt           lt param name  maxCollisions  gt numbers of retries when not able to retrieve  exclusive  file access lt  param gt           lt param name  sleepOnCollisionInterval  gt timespan to wait until next try  lt  param gt      public FileSignal string filePath  int maxCollisions  TimeSpan sleepOnCollisionInterval                FilePath   filePath          MaxCollisions   maxCollisions          SleepOnCollisionInterval   sleepOnCollisionInterval          LastSignal   GetSignalTimeStamp                    lt summary gt          constructor using a default value of 50 ms for sleepOnCollisionInterval          lt  summary gt           lt param name  filePath  gt path to the central file for signal control lt  param gt           lt param name  maxCollisions  gt numbers of retries when not able to retrieve  exclusive  file access lt  param gt              public FileSignal string filePath  int maxCollisions   this  filePath  maxCollisions  TimeSpan FromMilliseconds 50                         lt summary gt          constructor using a default value of 50 ms for sleepOnCollisionInterval and a default value of 10 for maxCollisions          lt  summary gt           lt param name  filePath  gt path to the central file for signal control lt  param gt              public FileSignal string filePath    this filePath  10                   private Stream GetFileStream FileAccess fileAccess                var i   0          while  true                        try                               return new FileStream FilePath  FileMode Create  fileAccess  FileShare None                             catch  Exception e                                i                    if  i  gt   MaxCollisions                                        throw e                                    Thread Sleep SleepOnCollisionInterval                                        private DateTime GetSignalTimeStamp                 if   File Exists FilePath                         return DateTime MinValue                    using  var stream   new FileStream FilePath  FileMode Open  FileAccess Read  FileShare None                         if stream Length    0                                return DateTime MinValue                            using  var reader   new BinaryReader stream                                 return DateTime FromBinary reader ReadInt64                                                              lt summary gt          overwrites the existing central file and writes the current time into it           lt  summary gt      public void Signal                 LastSignal   DateTime Now          using  var stream   new FileStream FilePath  FileMode Create  FileAccess Write  FileShare None                         using  var writer   new BinaryWriter stream                                 writer Write LastSignal ToBinary                                             lt summary gt          returns true if the file signal has changed  otherwise false           lt  summary gt              public bool CheckIfSignalled                 var signal   GetSignalTimeStamp            var signalTimestampChanged   LastSignal    signal          LastSignal   signal          return signalTimestampChanged            Some tests for it        TestMethod      public void TestSignal                 var fileSignal   new FileSignal Path GetTempFileName             var fileSignal2   new FileSignal fileSignal FilePath           Assert IsFalse fileSignal CheckIfSignalled             Assert IsFalse fileSignal2 CheckIfSignalled             Assert AreEqual fileSignal LastSignal  fileSignal2 LastSignal           fileSignal Signal            Assert IsFalse fileSignal CheckIfSignalled             Assert AreNotEqual fileSignal LastSignal  fileSignal2 LastSignal           Assert IsTrue fileSignal2 CheckIfSignalled             Assert AreEqual fileSignal LastSignal  fileSignal2 LastSignal           Assert IsFalse fileSignal2 CheckIfSignalled

User · Answer

Just use File GetLastWriteTime  There s a sample on that page showing how to use it

[c#] Check last modified date of file in C#

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