System.IO.IOException

The exception that is thrown when an I/O error occurs.

Minimum version: >= 1.1 >= Core 1.0

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How to handle it

try
{

}
catch (System.IO.IOException e)
{

}
try
{

}
catch (System.IO.IOException e) when (e.Message.Contains("something"))
{

}
try
{

}
catch (System.IO.IOException e) when (LogException(e))
{

}

private static bool LogException(Exception e)
{
    logger.LogError(...);
    return false;
}

How to avoid it

We haven't written anything about avoiding this exception yet. Got a good tip on how to avoid throwing System.IO.IOException? Feel free to reach out through the support widget in the lower right corner with your suggestions.

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Possible fixes from StackOverflow

From How to tell if path is file or directory:

// get the file attributes for file or directory
FileAttributes attr = File.GetAttributes(@"c:\Temp");

//detect whether its a directory or file
if ((attr & FileAttributes.Directory) == FileAttributes.Directory)
    MessageBox.Show("Its a directory");
else
    MessageBox.Show("Its a file");

Update for .NET 4.0+

Per the comments below, if you are on .NET 4.0 or later (and maximum performance is not critical) you can write the code in a cleaner way:

// get the file attributes for file or directory
FileAttributes attr = File.GetAttributes(@"c:\Temp");

if (attr.HasFlag(FileAttributes.Directory))
    MessageBox.Show("Its a directory");
else
    MessageBox.Show("Its a file");

You can just specify the generic octet-stream MIME type:

public FileResult Download()
{
    byte[] fileBytes = System.IO.File.ReadAllBytes(@"c:\folder\myfile.ext");
    string fileName = "myfile.ext";
    return File(fileBytes, System.Net.Mime.MediaTypeNames.Application.Octet, fileName);
}

When I faced with a similar problem, I finished with the following code:

public class FileManager
{
    private string _fileName;

    private int _numberOfTries;

    private int _timeIntervalBetweenTries;

    private FileStream GetStream(FileAccess fileAccess)
    {
        var tries = 0;
        while (true)
        {
            try
            {
                return File.Open(_fileName, FileMode.Open, fileAccess, Fileshare.None); 
            }
            catch (IOException e)
            {
                if (!IsFileLocked(e))
                    throw;
                if (++tries > _numberOfTries)
                    throw new MyCustomException("The file is locked too long: " + e.Message, e);
                Thread.Sleep(_timeIntervalBetweenTries);
            }
        }
    }

    private static bool IsFileLocked(IOException exception)
    {
        int errorCode = Marshal.GetHRForException(exception) & ((1 << 16) - 1);
        return errorCode == 32 || errorCode == 33;
    }

    // other code

}

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<Process> 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);

    /// <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 != 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.

If this is happening to you on a production environment or with an app that you can't change, the quick fix is to empty the Temp folder.

Depending on the user that is running the application you should either

  • Empty C:\Windows\Temp (for IIS or services running under LocalSystem account)
  • Or %temp% for locally logged on users (which for me is C:\Users\MyUserName\AppData\Local\Temp).

On the other side, if your own code is throwing this, and you want to prevent this from happening ever again:

  1. Do not use System.IO.Path.GetTempFileName()!

GetTempFileName() is a wrapper of the two decades old Win32 Api. It generate file names that will very easily collide. It circumvents those collitions by heavily looping on the file system, iterating possible file names from "%temp%\tmp0000.tmp" to "tmpFFFF.tmp" and skipping already existing ones. This is a I/O intensive, slow, and frankly terrible algorithm. Also using only 4 hex characters is what makes the artificial limit of 65536 files before failing.

The alternative is to generate file names that will not collide. For example, lets reuse GUID's logic: 32 hex digits will almost never collide.

private string GetTempFileName()
{
    return Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());
}
// Sample: c:\Windows\Temp\2e38fe87-f6bb-4b0d-90b3-2d07016324c1

This expands the limit from 65k to 4k millions files max (theoretically)... Of course, having leaked 65k files is already terrible, so...

  1. Do not leak temp files!

Double check your app for all happy and unhappy paths (like unexpected exceptions). Ensure it's correctly disposing each FileStream and deleting the temp files in Finally blocks .

  1. Clean the temp folder

Clean it now, and educate the system administrator to clean it periodically, because you can't trust every app in the wild. On my own servers I would automate this task using:

  • For global Windows\Temp

schtasks /Create /TR "cmd /c call DEL /F /S /Q %^TEMP%" /TN "Delete Global Temp Files" /sc WEEKLY /ST 12:00 /ru system

  • For current user:

schtasks /Create /TR "cmd /c call DEL /F /S /Q %^TEMP%" /TN "Delete %username% Temp Files" /sc WEEKLY /ST 12:00