Using C#, how does one figure out what process locked a file?

Question

In Windows, how do I determine (using C#) what process locked a file?

Third-party tools are helpful, but not what I'm looking for.

Answer 1

This question had an original answer that is now over 7 years old. That code is preserved at https://gist.github.com/i-e-b/2290426 This old version might work for you if you need to use Windows XP for some reason.

A much better answer is at How to check for file lock?

I've replicated Eric J's answer below (with using statements added, and class & method names to match the old code that was here) Please note that the comments to this answer may be out-of-date.

Research by user 'Walkman' is ongoing to improve the older code, as there are some conditions where the Restart Manager does not list all locks. See Github repo: https://github.com/Walkman100/FileLocks

Use like:

List<Process> locks = Win32Processes.GetProcessesLockingFile(@"C:\Hello.docx"); 

Code:

using System; using System.Collections.Generic; using System.Diagnostics; using System.Runtime.InteropServices;  namespace FileLockInfo {     public static class Win32Processes     {         /// <summary>         /// Find out what process(es) have a lock on the specified file.         /// </summary>         /// <param name="path">Path of the file.</param>         /// <returns>Processes locking the file</returns>         /// <remarks>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)         /// </remarks>         public static List<Process> GetProcessesLockingFile(string path)         {             uint handle;             string key = Guid.NewGuid().ToString();             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 MORE_DATA = 234;                 uint pnProcInfoNeeded, pnProcInfo = 0, lpdwRebootReasons = RmRebootReasonNone;                  string[] resources = {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 == MORE_DATA)                 {                     return EnumerateProcesses(pnProcInfoNeeded, handle, lpdwRebootReasons);                 }                 else if (res != 0) throw new Exception("Could not list processes locking resource. Failed to get size of result.");             }             finally             {                 RmEndSession(handle);             }              return new List<Process>();         }           [StructLayout(LayoutKind.Sequential)]         public 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;          public enum RM_APP_TYPE         {             RmUnknownApp = 0,             RmMainWindow = 1,             RmOtherWindow = 2,             RmService = 3,             RmExplorer = 4,             RmConsole = 5,             RmCritical = 1000         }          [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]         public 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, uint nFiles, string[] rgsFilenames,             uint nApplications, [In] RM_UNIQUE_PROCESS[] rgApplications, uint 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);          private static List<Process> EnumerateProcesses(uint pnProcInfoNeeded, uint handle, uint lpdwRebootReasons)         {             var processes = new List<Process>(10);             // Create an array to store the process results             var processInfo = new RM_PROCESS_INFO[pnProcInfoNeeded];             var pnProcInfo = pnProcInfoNeeded;              // Get the list             var res = RmGetList(handle, out pnProcInfoNeeded, ref pnProcInfo, processInfo, ref lpdwRebootReasons);              if (res != 0) throw new Exception("Could not list processes locking resource.");             for (int i = 0; i < pnProcInfo; i++)             {                 try                 {                     processes.Add(Process.GetProcessById(processInfo[i].Process.dwProcessId));                 }                 catch (ArgumentException) { } // catch the error -- in case the process is no longer running             }             return processes;         }     } }