uvery9_DailyWallpaper/DailyWallpaper/Helpers/FileLockUtil.cs
using System.Runtime.InteropServices;
using System.Diagnostics;
using System;
using System.Collections.Generic;

// https://stackoverflow.com/users/141172/eric-j
// https://stackoverflow.com/questions/1304/how-to-check-for-file-lock/20623302#20623302
// https://gist.github.com/mlaily/9423f1855bb176d52a327f5874915a97
// https://docs.microsoft.com/en-us/archive/msdn-magazine/2007/april/net-matters-restart-manager-and-generic-method-compilation
// https://github.com/dotnet/msbuild/blob/main/src/Tasks/LockCheck.cs

namespace DailyWallpaper.Helpers { 
    static public class FileLockUtil
    {
        [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);

        /// <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>
        static public List<Process> WhoIsLocking(string path)
        {
            uint handle;
            string key = Guid.NewGuid().ToString();
            List<Process> processes = new List<Process>();

            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<Process>((int)pnProcInfo);

                        // Enumerate all of the results and add them to the 
                        // list to be returned
                        for (int i = 0; i < 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 == 5)
                {
                    throw new Exception($"[{res}] Could not list processes locking resource for folder.");
                    // https://blog.yaakov.online/failed-experiment-what-processes-have-a-lock-on-this-folder/
                    // Particularly, Restart Manager appears to deal with files,
                    // and only files.If you give it a file, it can tell you what's holding that file open.
                    // If you give it a folder without a trailing backslash, it returns Windows error code 5,
                    // or ERROR_ACCESS_DENIED. (Interestingly, this error code is not in the[documented list of error codes for RmGetList][4].)
                    // If you give it a folder with a trailing backslash, it returns an empty list. No matter what I can do,
                    // I cannot get Restart Manager to give me a list of applications holding a reference to a directory.
                }

                else if (res != 0) 
                    throw new Exception($"[{res}] Could not list processes locking resource. Failed to get size of result.");                    
            }
            finally
            {
                RmEndSession(handle);
            }

            return processes;
        }
    }
}