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Posted 22 Mar 2006


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Eject USB disks using C#

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22 Mar 2006CPOL7 min read
This article shows you how to programmatically eject USB removable disk drives using .NET, with a sample GUI application.


Although removing a USB stick using the Windows shell is quite easy, it tends to be quite difficult programmatically. There are a lot of underlying notions, at the frontier of kernel driver development, one has to understand to achieve what may seem a simple task. When I started this work, I really did not think where it would lead me to. I sure did not think I would have to switch between kernel driver control codes, the Windows setup and Configuration Manager APIs, WMI, etc...

Well, this is the main reason for this article, I think the subject really deserves one. The result is a demo .NET Winforms project that includes reusable source code.

And by the way, as a side benefit, I will also explain how to read the hardware serial number of those disks, a recurring question on newsgroups and forums.


First of all, let's talk about the various Windows API involved here, apart from Win32, and there are many:

  1. The Windows DDK (Driver Development Kit): A driver development kit that provides a build environment, tools, driver samples, and documentation to support driver development for Windows.
  2. The Setup API: An underused and not very well known Windows API. One of the reasons it is underused is because it is part of the DDK, but in fact, these are general Setup routines, which are used in installation applications, and there are many very interesting gems in there, like the CM_Request_Device_Eject function, which is the heart of the UsbEject project.
  3. The DeviceIoControl function: The "Open, Sesame" user mode function to the depths of kernel mode cave. Tons of things (a large part of it is undocumented, or not well document) can be done using this.
  4. WMI (Windows Management Instrumentation): The UsbEject project does not actually uses WMI because WMI does not handle device ejection, as far as I know. I could have used a hybrid approach using WMI for disk management and the rest for device Ejection. I will leave this as an exercise to the reader.
  5. Windows Messages: The WM_DEVICECHANGE Window message is used in this sample GUI application to visually refresh the tree when something happens to the device manager tree.

Now, let's introduce a few terms. The definitions here are my own, not official ones (it is actually quite hard to find official definitions for all these).

  1. Physical Disks: Well, as the name implies, it is the real piece of hardware an end user manipulates. In the case of a USB disk, it is the stick itself.
  2. Volumes: There are actually two types of volumes: volumes as they are understood by Windows, and volumes as we know them, that is, drive letters, from A: to Z:, also called logical disks. A volume can span multiple physical disks.
  3. Devices: The Windows DDK defines both volumes and physical disks as being devices. The code reflects this. Device is a base type, and Volume derives from it.


Using the code

The project is a Visual Studio 2005 (.NET 2.0) Windows form project. There is a folder called Library that contains the heart of it, an object oriented API to handle USB disks. The classes there are described in the following class diagram:

Sample Image - usbeject2.gif

As you can see, there are 5 main classes (each class is defined in its own respective .cs file):

  1. DeviceClass: An abstract class that represents a physical device class. It has a list of devices in this class
  2. DiskDeviceClass: Represents all disk devices in the system
  3. VolumeDeviceClass: Represents all volume devices in the system
  4. Device: Represents a generic device of any type (disk, volume, etc...). Note there is no Disk class, because in this project, a Disk has no specific property, compared to a Device. Note also the code has been designed so it could be extended to other devices, not only volumes and disks
  5. Volume: Represents a Windows volume. A volume has a LogicalDrive property which is filled if it has a drive letter assigned

This is how you can eject all USB volumes for example:

VolumeDeviceClass volumeDeviceClass = new VolumeDeviceClass();
foreach (Volume device in volumeDeviceClass.Devices)
    // is this volume on USB disks?
    if (!device.IsUsb)

    // is this volume a logical disk?
    if ((device.LogicalDrive == null) || (device.LogicalDrive.Length == 0))

    device.Eject(true); // allow Windows to display any relevant UI


Points of Interest

CM_Request_Device_Eject function

This is the SetupApi function that ejects a device (any device that can be ejected). It takes a device instance handle (or devInst) as input. The function behaves differently if the second argument pVetoType is provided or not. If it is provided, the Windows shell will not present the end user with any dialog, and the eject operation will either succeed or fail silently (with an error code, called the Veto). If not, the Windows shell may display the traditional messages or dialog boxes, or balloon windows to inform the end user (Note: Windows 2000 may always display a message under certain circumstances, even when the second argument is not provided) that something interesting has happened.

The main problem is therefore "For a given disk letter, what is the device to eject?"

The device to eject must be a Disk device, not a Volume device (at least for USB disks). So the general algorithm is the following:

  1. Determine all available logical disks, using .NET's Environment.GetLogicalDrives.
  2. For each logical disk (drive letter), determine the real volume name, using Win32's GetVolumeNameForVolumeMountPoint.
  3. For each volume device, determine if there is a matching logical disk.
  4. For each volume device, determine physical disk devices composing (using their number) the volume device (as there may be more than one physical disk per volume), using the DDK's IOCTL_VOLUME_GET_VOLUME_DISK_EXTENTS IO control code.
  5. Because the Plug-And-Play (PNP) Configuration Manager builds a device hierarchy, a physical disk device is not in general the device to eject, so for each physical disk device, determine what is the device to eject among its ascendant devices. The device to eject is the first one in the hierarchy with the CM_DEVCAP_REMOVABLE capability.


GetVolumeNameForVolumeMountPoint function

To match Windows volumes with logical disks (algorithm step 3), there is a trick to know. First of all, let me say that all Windows volume can be uniquely addressed with a specific syntax of the form "\\?\Volume{GUID}\" where GUID is the GUID that identifies the volume.

When enumerating volumes devices (using the Volume Device Class Guid GUID_DEVINTERFACE_VOLUME, do not worry, the VolumeDeviceClass wraps it), the volume device path can be used directly in a call to GetVolumeNameForVolumeMountPoint, if "\" is appended to it, although it is not strictly speaking a volume name.

Enumerating all devices for a given class

I have reproduced here a code snippet that shows how .NET P/Invoke interop is used to enumerate all physical devices for a given type.

int index = 0;
while (true)
    // enumerate device interface for a given index
    if (!SetupDiEnumDeviceInterfaces(
        _deviceInfoSet, null, ref _classGuid, index, interfaceData))
        int error = Marshal.GetLastWin32Error();
        // this is not really an error...
        if (error != Native.ERROR_NO_MORE_ITEMS)
            throw new Win32Exception(error);

    int size = 0;
    // get detail for all the device interface
    if (!SetupDiGetDeviceInterfaceDetail(
        _deviceInfoSet, interfaceData, IntPtr.Zero, 0, ref size, devData))
        int error = Marshal.GetLastWin32Error();
        if (error != Native.ERROR_INSUFFICIENT_BUFFER)
        throw new Win32Exception(error);

    // allocate unmanaged Win32 buffer
    IntPtr buffer = Marshal.AllocHGlobal(size);
                    new SP_DEVICE_INTERFACE_DETAIL_DATA();
    detailData.cbSize = Marshal.SizeOf(

    // copy managed struct buffer into unmanager win32 buffer
    Marshal.StructureToPtr(detailData, buffer, false);

    if (!SetupDiGetDeviceInterfaceDetail(
        _deviceInfoSet, interfaceData, buffer, size, ref size, devData))
        Marshal.FreeHGlobal(buffer); // don't forget to free memory
        throw new Win32Exception(Marshal.GetLastWin32Error());

    // a bit of voodoo magic. This code is not 64 bits portable :-)
    IntPtr pDevicePath = (IntPtr)((int)buffer + Marshal.SizeOf(typeof(int)));
    string devicePath = Marshal.PtrToStringAuto(pDevicePath);


Refreshing the UI when a disk is inserted or removed

There are multiple ways of doing this. I have chosen the most simple one: capture the WM_DEVICECHANGE Windows message. You just have to override the default Window procedure of any Winform in your application, like this:

protected override void WndProc(ref Message m)
    if (m.Msg == Native.WM_DEVICECHANGE)
        if (!_loading)
            LoadItems(); // do the refresh work here
    base.WndProc(ref m);


A word on Serial Numbers

This is not strictly speaking related to the Eject subject, but I have seen many questions about this too, so I will talk a bit about hard disk's "serial numbers". When speaking about Windows volumes and disks, there are actually (at least) two serial numbers:

  1. A volume software serial number, assigned during the formatting process. This 32bits value can be read easily using the GetVolumeInformation regular Win32 function.
  2. A disk vendor's hardware serial number: This serial number is setup by the vendor during the manufacturing process. It's a string. Of course, it cannot be changed. Unfortunately, you have to know that serial numbers are optional for USB devices, so USB storage sticks may not have one, and indeed, many do not.

Ok, so the question is "how do I read the hardware serial number?". There are at least two ways:

  1. WMI: By far, the easiest way, although it looks like black magic first. Here is an example of such a code (not provided in the project .zip package).


    // browse all USB WMI physical disks
    foreach(ManagementObject drive in new ManagementObjectSearcher(
        "select * from Win32_DiskDrive where InterfaceType='USB'").Get())
        // associate physical disks with partitions
        foreach(ManagementObject partition in new ManagementObjectSearcher(
            "ASSOCIATORS OF {Win32_DiskDrive.DeviceID='" + drive["DeviceID"]
              + "'} WHERE AssocClass = 
            Console.WriteLine("Partition=" + partition["Name"]);
            // associate partitions with logical disks (drive letter volumes)
            foreach(ManagementObject disk in new ManagementObjectSearcher(
                "ASSOCIATORS OF {Win32_DiskPartition.DeviceID='"
                  + partition["DeviceID"]
                  + "'} WHERE AssocClass =
                Console.WriteLine("Disk=" + disk["Name"]);
        // this may display nothing if the physical disk
        // does not have a hardware serial number
         + new ManagementObject("Win32_PhysicalMedia.Tag='"
         + drive["DeviceID"] + "'")["SerialNumber"]);
  2. Using raw techniques described here. I have not ported this to C# although it is perfectly feasible.



  1. The source code has been designed using the .NET Framework 2.0, but should work on the .NET framework 1.1, although this has not been tested. I do not think it can work on 64 bits CLR as is because of interop structs size that may vary. It could be ported quite easily though.
  2. The code has been tested on Windows XP SP2, and Windows Server 2003, not on Windows 2000, but it should work. I don't think the code can work or can be ported on Windows 9x.
  3. I have not researched the security implications of all this. In particular, the Windows user calling the API must obviously have some rights to be able to access physical devices. I have left these experiments as an exercise for the reader.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Simon Mourier
Web Developer
France France
Simon Mourier is Software Architect & CTO for SoftFluent in France. He can be reached at simon "dot" mourier "at"

Comments and Discussions

QuestionTrying to eject one partition from a USb with 2 partitions Pin
Member 1254355323-Dec-16 4:23
MemberMember 1254355323-Dec-16 4:23 
QuestionDownload links don't work.. Pin
Rick Rockhold20-Jul-15 11:29
MemberRick Rockhold20-Jul-15 11:29 
AnswerRe: Download links don't work.. Pin
Israel Marsilli4-Aug-15 7:37
MemberIsrael Marsilli4-Aug-15 7:37 
AnswerRe: Download links don't work.. Pin
Sascha Lefèvre12-Jan-16 0:09
professionalSascha Lefèvre12-Jan-16 0:09 
QuestionI found the problem that is causing mixed up drive letters. Pin
mthiffault26-Feb-15 13:25
Membermthiffault26-Feb-15 13:25 
Questionhi hello ... give an errore that device not ready Pin
mehdi golzary30-Nov-14 20:37
Membermehdi golzary30-Nov-14 20:37 
QuestionIn VS 2010 Pin
hiravn164-Feb-14 6:56
Memberhiravn164-Feb-14 6:56 
AnswerRe: In VS 2010 Pin
arifin9-Oct-14 0:41
Memberarifin9-Oct-14 0:41 
QuestionEject USB Drive and other equipment hosted on the same device Pin
d.barile10-Dec-13 23:53
professionald.barile10-Dec-13 23:53 
GeneralMy vote of 5 Pin
ChrisCheung21-Jul-13 20:20
MemberChrisCheung21-Jul-13 20:20 
NewsGreat article, support Windows 7 x64 Pin
ChrisCheung21-Jul-13 20:19
MemberChrisCheung21-Jul-13 20:19 
GeneralMy vote of 4 Pin
Jayke Huempfner15-Jul-13 17:14
MemberJayke Huempfner15-Jul-13 17:14 
GeneralMy vote of 4 Pin
VictorAche19-Apr-13 13:28
MemberVictorAche19-Apr-13 13:28 
QuestionDifferent Drive Letters (Windows-Explorer / Demo-Project)! Pin
Dave Mustjuk20-Mar-13 22:02
MemberDave Mustjuk20-Mar-13 22:02 
BugMenu->Veiw->USB Only Unchecked my Win7 return win32 error ERROR_NO_TOKEN Pin
Member 793228413-Mar-13 20:52
MemberMember 793228413-Mar-13 20:52 
Member 835814820-Feb-13 2:25
MemberMember 835814820-Feb-13 2:25 
QuestionGet Instance Handler for single pendrive. Pin
Hetal Jariwala28-Dec-12 0:42
MemberHetal Jariwala28-Dec-12 0:42 
Questionthe code doesn't work Pin
Anchik1227-Nov-12 4:49
MemberAnchik1227-Nov-12 4:49 
AnswerRe: the code doesn't work Pin
Anchik1228-Nov-12 7:03
MemberAnchik1228-Nov-12 7:03 
GeneralSample Works Pin
ledtech317-Sep-12 5:42
Memberledtech317-Sep-12 5:42 
QuestionI built the source code & ran the debug version and I get an exception on line 127 using vs 2010 Pin
Member 44867916-Sep-12 12:28
MemberMember 44867916-Sep-12 12:28 
AnswerRe: I built the source code & ran the debug version and I get an exception on line 127 using vs 2010 Pin
Kansai Robot5-Nov-18 14:52
MemberKansai Robot5-Nov-18 14:52 
QuestionThe supplied user buffer is not valid for the requested operation Pin
me_4274-Sep-12 17:01
Memberme_4274-Sep-12 17:01 
Questionpoor code Pin
Member 964871811-Jul-12 12:08
MemberMember 964871811-Jul-12 12:08 
GeneralAnyone have a fix for the wrong drive letter being ejected? Pin
Anarchi27-Jun-12 4:16
MemberAnarchi27-Jun-12 4:16 

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