Cancel acqusition of images from TWAIN device

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You should do the following steps if you want to cancel acquisition of image from TWAIN device:

Call the Device.CancelTransfer method.
Wait for the Device.ScanCanceled event if you are acquiring images asynchronously.
Wait while the Device.AcquireModal method returns AcquireModalState.ScanCanceled if you are acquiring images in modal loop.

Some high speed TWAIN scanners have internal buffer and TWAIN scanners cache the acquired images in the buffer if TWAIN scanner is faster than application which processes acquired images. Images, which are cached in internal buffer, will be lost if you cancel the image acquisition process. You can pause TWAIN image acquisition process (instead of cancelling TWAIN image acquisition) if you need to keep all acquired images.

Here is an example that demonstrates how to cancel current asynchronous acquisition of images from TWAIN device:

static bool _isScanningFinished;
static bool _isScanCanceled;


/// <summary>
/// Asynchronously acquires one image from TWAIN device and cancels image transfer.
/// </summary>
public static void AsynchronouslyAcquireImageFromTwainDeviceAndCancelImageTransfer()
{
    _isScanningFinished = false;
    _isScanCanceled = false;

    // create the device manager
    using (Vintasoft.Twain.DeviceManager deviceManager = new Vintasoft.Twain.DeviceManager())
    {
        deviceManager.IsTwain2Compatible = true;
        // open the device manager
        deviceManager.Open();

        // get the device
        string deviceName = "KODAK Scanner: i5000";
        Vintasoft.Twain.Device device = deviceManager.Devices.Find(deviceName);
        if (device == null)
            throw new System.ApplicationException(string.Format("Device '{0}' is not found.", deviceName));

        // subscribe to device events
        device.ImageAcquired += new System.EventHandler<Vintasoft.Twain.ImageAcquiredEventArgs>(device_ImageAcquired);
        device.ScanCompleted += new System.EventHandler(device_ScanCompleted);
        device.ScanCanceled += new System.EventHandler(device_ScanCanceled);
        device.ScanFailed += new System.EventHandler<Vintasoft.Twain.ScanFailedEventArgs>(device_ScanFailed);
        device.ScanFinished += new System.EventHandler(device_ScanFinished);

        // disable device UI
        device.ShowUI = false;
        // specify that device must be closed after scan
        device.DisableAfterAcquire = true;

        // open the device
        device.Open();
        // specify that 2 images must be acquired from scanner
        device.XferCount = 2;

        // run asynchronous image acqusition
        device.Acquire();
        // wait while scan will be finished
        while (!_isScanningFinished)
        {
            System.Windows.Forms.Application.DoEvents();
        }

        if (!_isScanCanceled)
            throw new System.ApplicationException("Scan is NOT canceled.");

        // unsubscribe from device events
        device.ImageAcquired -= new System.EventHandler<Vintasoft.Twain.ImageAcquiredEventArgs>(device_ImageAcquired);
        device.ScanCompleted -= new System.EventHandler(device_ScanCompleted);
        device.ScanCanceled -= new System.EventHandler(device_ScanCanceled);
        device.ScanFailed -= new System.EventHandler<Vintasoft.Twain.ScanFailedEventArgs>(device_ScanFailed);
        device.ScanFinished -= new System.EventHandler(device_ScanFinished);

        // close the device
        device.Close();
    }
}

static void device_ImageAcquired(object sender, Vintasoft.Twain.ImageAcquiredEventArgs e)
{
    // get reference to the device
    Vintasoft.Twain.Device device = (Vintasoft.Twain.Device)sender;
    // cancel image scan
    device.CancelTransfer();
    System.Console.WriteLine("Feeder is stopped.");
}

static void device_ScanCompleted(object sender, System.EventArgs e)
{
    System.Console.WriteLine("Scan is completed.");
}

static void device_ScanCanceled(object sender, System.EventArgs e)
{
    _isScanCanceled = true;
    System.Console.WriteLine("Scan is canceled.");
}

static void device_ScanFailed(object sender, Vintasoft.Twain.ScanFailedEventArgs e)
{
    System.Console.WriteLine(string.Format("Scan is failed: {0}.", e.ErrorString));
}

static void device_ScanFinished(object sender, System.EventArgs e)
{
    _isScanningFinished = true;
}

VB.NET

Shared _isScanningFinished As Boolean
Shared _isScanCanceled As Boolean


''' <summary>
''' Asynchronously acquires one image from TWAIN device and cancels image transfer.
''' </summary>
Public Shared Sub AsynchronouslyAcquireImageFromTwainDeviceAndCancelImageTransfer()
        _isScanningFinished = False
        _isScanCanceled = False

        ' create the device manager
        Using deviceManager As New Vintasoft.Twain.DeviceManager()
                deviceManager.IsTwain2Compatible = True
                ' open the device manager
                deviceManager.Open()

                ' get the device
                Dim deviceName As String = "KODAK Scanner: i5000"
                Dim device As Vintasoft.Twain.Device = deviceManager.Devices.Find(deviceName)
                If device Is Nothing Then
                        Throw New System.ApplicationException(String.Format("Device '{0}' is not found.", deviceName))
                End If

                ' subscribe to device events
                AddHandler device.ImageAcquired, New System.EventHandler(Of Vintasoft.Twain.ImageAcquiredEventArgs)(AddressOf device_ImageAcquired)
                AddHandler device.ScanCompleted, New System.EventHandler(AddressOf device_ScanCompleted)
                AddHandler device.ScanCanceled, New System.EventHandler(AddressOf device_ScanCanceled)
                AddHandler device.ScanFailed, New System.EventHandler(Of Vintasoft.Twain.ScanFailedEventArgs)(AddressOf device_ScanFailed)
                AddHandler device.ScanFinished, New System.EventHandler(AddressOf device_ScanFinished)

                ' disable device UI
                device.ShowUI = False
                ' specify that device must be closed after scan
                device.DisableAfterAcquire = True

                ' open the device
                device.Open()
                ' specify that 2 images must be acquired from scanner
                device.XferCount = 2

                ' run asynchronous image acqusition
                device.Acquire()
                ' wait while scan will be finished
                While Not _isScanningFinished
                        System.Windows.Forms.Application.DoEvents()
                End While

                If Not _isScanCanceled Then
                        Throw New System.ApplicationException("Scan is NOT canceled.")
                End If

                ' unsubscribe from device events
                RemoveHandler device.ImageAcquired, New System.EventHandler(Of Vintasoft.Twain.ImageAcquiredEventArgs)(AddressOf device_ImageAcquired)
                RemoveHandler device.ScanCompleted, New System.EventHandler(AddressOf device_ScanCompleted)
                RemoveHandler device.ScanCanceled, New System.EventHandler(AddressOf device_ScanCanceled)
                RemoveHandler device.ScanFailed, New System.EventHandler(Of Vintasoft.Twain.ScanFailedEventArgs)(AddressOf device_ScanFailed)
                RemoveHandler device.ScanFinished, New System.EventHandler(AddressOf device_ScanFinished)

                ' close the device
                device.Close()
        End Using
End Sub

Private Shared Sub device_ImageAcquired(sender As Object, e As Vintasoft.Twain.ImageAcquiredEventArgs)
        ' get reference to the device
        Dim device As Vintasoft.Twain.Device = DirectCast(sender, Vintasoft.Twain.Device)
        ' cancel image scan
        device.CancelTransfer()
        System.Console.WriteLine("Feeder is stopped.")
End Sub

Private Shared Sub device_ScanCompleted(sender As Object, e As System.EventArgs)
        System.Console.WriteLine("Scan is completed.")
End Sub

Private Shared Sub device_ScanCanceled(sender As Object, e As System.EventArgs)
        _isScanCanceled = True
        System.Console.WriteLine("Scan is canceled.")
End Sub

Private Shared Sub device_ScanFailed(sender As Object, e As Vintasoft.Twain.ScanFailedEventArgs)
        System.Console.WriteLine(String.Format("Scan is failed: {0}.", e.ErrorString))
End Sub

Private Shared Sub device_ScanFinished(sender As Object, e As System.EventArgs)
        _isScanningFinished = True
End Sub

Here is an example that demonstrates how to cancel current synchronous acquisition of images from TWAIN device:

/// <summary>
/// Synchronously acquires one image from TWAIN device and cancels image transfer.
/// </summary>
public static void SynchronouslyAcquireImageFromTwainDeviceAndCancelImageTransfer()
{
    bool isScanCanceled = false;

    // create the device manager
    using (Vintasoft.Twain.DeviceManager deviceManager = new Vintasoft.Twain.DeviceManager())
    {
        deviceManager.IsTwain2Compatible = true;
        // open the device manager
        deviceManager.Open();

        // get the device
        string deviceName = "KODAK Scanner: i5000";
        Vintasoft.Twain.Device device = deviceManager.Devices.Find(deviceName);
        if (device == null)
            throw new System.ApplicationException(string.Format("Device '{0}' is not found.", deviceName));

        // disable device UI
        device.ShowUI = false;
        // specify that device must be closed after scan
        device.DisableAfterAcquire = true;

        // open the device
        device.Open();
        // specify that 2 images must be acquired from scanner
        device.XferCount = 2;

        // run asynchronous image acquisition
        Vintasoft.Twain.AcquireModalState acquireModalState = Vintasoft.Twain.AcquireModalState.None;
        do
        {
            acquireModalState = device.AcquireModal();
            switch (acquireModalState)
            {
                case Vintasoft.Twain.AcquireModalState.ImageAcquired:
                    System.Console.WriteLine("Image is acquired.");
                    if (device.AcquiredImage != null)
                        device.AcquiredImage.Dispose();

                    // cancel image scan
                    device.CancelTransfer();
                    break;

                case Vintasoft.Twain.AcquireModalState.ScanCompleted:
                    System.Console.WriteLine("Scan is completed.");
                    break;

                case Vintasoft.Twain.AcquireModalState.ScanCanceled:
                    isScanCanceled = true;
                    System.Console.WriteLine("Scan is canceled.");
                    break;

                case Vintasoft.Twain.AcquireModalState.ScanFailed:
                    System.Console.WriteLine(string.Format("Scan is failed: {0}.", device.ErrorString));
                    break;
            }
        }
        while (acquireModalState != Vintasoft.Twain.AcquireModalState.None);

        if (!isScanCanceled)
            throw new System.ApplicationException("Scan is NOT canceled.");

        // close the device
        device.Close();
    }
}

VB.NET

''' <summary>
''' Synchronously acquires one image from TWAIN device and cancels image transfer.
''' </summary>
Public Shared Sub SynchronouslyAcquireImageFromTwainDeviceAndCancelImageTransfer()
        Dim isScanCanceled As Boolean = False

        ' create the device manager
        Using deviceManager As New Vintasoft.Twain.DeviceManager()
                deviceManager.IsTwain2Compatible = True
                ' open the device manager
                deviceManager.Open()

                ' get the device
                Dim deviceName As String = "KODAK Scanner: i5000"
                Dim device As Vintasoft.Twain.Device = deviceManager.Devices.Find(deviceName)
                If device Is Nothing Then
                        Throw New System.ApplicationException(String.Format("Device '{0}' is not found.", deviceName))
                End If

                ' disable device UI
                device.ShowUI = False
                ' specify that device must be closed after scan
                device.DisableAfterAcquire = True

                ' open the device
                device.Open()
                ' specify that 2 images must be acquired from scanner
                device.XferCount = 2

                ' run asynchronous image acquisition
                Dim acquireModalState As Vintasoft.Twain.AcquireModalState = Vintasoft.Twain.AcquireModalState.None
                Do
                        acquireModalState = device.AcquireModal()
                        Select Case acquireModalState
                                Case Vintasoft.Twain.AcquireModalState.ImageAcquired
                                        System.Console.WriteLine("Image is acquired.")
                                        If device.AcquiredImage IsNot Nothing Then
                                                device.AcquiredImage.Dispose()
                                        End If

                                        ' cancel image scan
                                        device.CancelTransfer()
                                        Exit Select

                                Case Vintasoft.Twain.AcquireModalState.ScanCompleted
                                        System.Console.WriteLine("Scan is completed.")
                                        Exit Select

                                Case Vintasoft.Twain.AcquireModalState.ScanCanceled
                                        isScanCanceled = True
                                        System.Console.WriteLine("Scan is canceled.")
                                        Exit Select

                                Case Vintasoft.Twain.AcquireModalState.ScanFailed
                                        System.Console.WriteLine(String.Format("Scan is failed: {0}.", device.ErrorString))
                                        Exit Select
                        End Select
                Loop While acquireModalState <> Vintasoft.Twain.AcquireModalState.None

                If Not isScanCanceled Then
                        Throw New System.ApplicationException("Scan is NOT canceled.")
                End If

                ' close the device
                device.Close()
        End Using
End Sub

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