Download​Operation Download​Operation Download​Operation Class

Definition

Performs an asynchronous download operation. The Background Transfer sample demonstrates this functionality. For an overview of Background Transfer capabilities, see Transferring data in the background. Download the Background Transfer sample for examples in JavaScript, C#, and C++.

public : sealed class DownloadOperation : IBackgroundTransferOperation, IBackgroundTransferOperationPriority, IDownloadOperation, IDownloadOperation2public sealed class DownloadOperation : IBackgroundTransferOperation, IBackgroundTransferOperationPriority, IDownloadOperation, IDownloadOperation2Public NotInheritable Class DownloadOperation Implements IBackgroundTransferOperation, IBackgroundTransferOperationPriority, IDownloadOperation, IDownloadOperation2
Attributes
Windows 10 requirements
Device family
Windows 10 (introduced v10.0.10240.0)
API contract
Windows.Foundation.UniversalApiContract (introduced v1)
Capabilities
internetClientServer privateNetworkClientServer internetClient

Examples

The following example demonstrates how to configure and begin a basic download operation, and is based on the Background Transfer sample offered in the Windows Sample Gallery.

        var download = null;
        var promise = null;

        function DownloadFile (uriString, fileName) {
            try {
                // Asynchronously create the file in the pictures folder.
                Windows.Storage.KnownFolders.picturesLibrary.createFileAsync(fileName, Windows.Storage.CreationCollisionOption.generateUniqueName).done(function (newFile) {
                    var uri = Windows.Foundation.Uri(uriString);
                    var downloader = new Windows.Networking.BackgroundTransfer.BackgroundDownloader();

                    // Create a new download operation.
                    download = downloader.createDownload(uri, newFile);

                    // Start the download and persist the promise to be able to cancel the download.
                    promise = download.startAsync().then(complete, error, progress);
                }, error);
            } catch (err) {
                displayException(err);
            }
        };
       using Windows.Foundation;
       using Windows.Networking.BackgroundTransfer;
       using Windows.Storage;

       private async void StartDownload_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                Uri source = new Uri(serverAddressField.Text.Trim());
                string destination = fileNameField.Text.Trim();

                StorageFile destinationFile = await KnownFolders.PicturesLibrary.CreateFileAsync(
                    destination, CreationCollisionOption.GenerateUniqueName);

                BackgroundDownloader downloader = new BackgroundDownloader();
                DownloadOperation download = downloader.CreateDownload(source, destinationFile);

                // Attach progress and completion handlers.
                HandleDownloadAsync(download, true);
            }
            catch (Exception ex)
            {
                LogException("Download Error", ex);
            }
        }

Remarks

After app termination, an app should enumerate all existing DownloadOperation instances at next start-up using GetCurrentDownloadsAsync. When a Windows Store app using Background Transfer is terminated, incomplete downloads will persist in the background. If the app is restarted after termination and these incomplete operations are not enumerated and re-introduced to the current session, they will go stale and continue to occupy device resources.

Background transfer doesn't support concurrent downloads of the same Uri. So an app can download http://example.com/myfile.wmv once, or download it again after a previous download completed. An app shouldn't start two downloads of the same Uri concurrently, since this may result in truncated files.

Note

Paused or incomplete download operations can only be resumed if the server accepts range-requests.

Timeout considerations

  • 1.) When establishing a new connection for a download over TCP/SSL, the connection attempt is aborted if not established within five minutes.
  • 2.) After the connection has been established, an HTTP request message that has not received a response within two minutes is aborted. Assuming there is Internet connectivity, Background Transfer will retry a download up to three times. In the event Internet connectivity is not detected, additional attempts will not be made until it is.

Debugging Guidance

Stopping a debugging session in Microsoft Visual Studio is comparable to closing your app; downloads are paused and POST uploads are terminated. Even while debugging, your app should enumerate and then pause, resume, restart, or cancel any persisted downloads.

However, if Microsoft Visual Studio project updates, like changes to the app manifest, require that the app is uninstalled and re-deployed for debugging, GetCurrentDownloadsAsync cannot enumerate persisted operations created using the previous app deployment.

Properties

CostPolicy CostPolicy CostPolicy

Gets and sets the cost policy for the download.

public : BackgroundTransferCostPolicy CostPolicy { get; set; }public BackgroundTransferCostPolicy CostPolicy { get; set; }Public ReadWrite Property CostPolicy As BackgroundTransferCostPolicy
Value
BackgroundTransferCostPolicy BackgroundTransferCostPolicy BackgroundTransferCostPolicy

Specifies whether the transfer can happen over costed networks.

Attributes

Group Group Group

Note

Group may be altered or unavailable for releases after Windows 8.1. Instead, use TransferGroup.

Gets a string value indicating the group the transfer belongs to.

public : PlatForm::String Group { get; }public string Group { get; }Public ReadOnly Property Group As string
Value
PlatForm::String string string

The group name.

Attributes

Guid Guid Guid

This is a unique identifier for a specific download operation. A GUID associated to a download operation will not change for the duration of the download.

public : PlatForm::Guid Guid { get; }public Guid Guid { get; }Public ReadOnly Property Guid As Guid
Value
PlatForm::Guid Guid Guid

The unique ID for this download operation.

Attributes

Method Method Method

Gets the method to use for the download.

public : PlatForm::String Method { get; }public string Method { get; }Public ReadOnly Property Method As string
Value
PlatForm::String string string

The method to use for the download. This value can be GET, PUT, RETR, or STOR.

Attributes

Priority Priority Priority

Gets or sets the transfer priority of this download operation when within a BackgroundTransferGroup. Possible values are defined by BackgroundTransferPriority.

public : BackgroundTransferPriority Priority { get; set; }public BackgroundTransferPriority Priority { get; set; }Public ReadWrite Property Priority As BackgroundTransferPriority
Attributes
See Also

Progress Progress Progress

Gets the current progress of the upload operation.

public : BackgroundDownloadProgress Progress { get; }public BackgroundDownloadProgress Progress { get; }Public ReadOnly Property Progress As BackgroundDownloadProgress
Attributes

Remarks

The value of the Progress property is updated in real time, which means that the value can change while a progress handler is executing. As a best practice, create a local copy of the Progress property at the beginning of your progress handler, and use only that copy in your progress handler, to maintain a consistent view of progress as your handler executes.

See Also

RequestedUri RequestedUri RequestedUri

Gets the URI from which to download the file.

public : Uri RequestedUri { get; }public Uri RequestedUri { get; }Public ReadOnly Property RequestedUri As Uri
Value
Uri Uri Uri

The URI to download from.

Attributes

ResultFile ResultFile ResultFile

Returns the IStorageFile object provided by the caller when creating the DownloadOperation object using CreateDownload.

public : IStorageFile ResultFile { get; }public IStorageFile ResultFile { get; }Public ReadOnly Property ResultFile As IStorageFile
Value
IStorageFile IStorageFile IStorageFile

The returned file object.

Attributes

TransferGroup TransferGroup TransferGroup

Gets the group that this download operation belongs to.

public : BackgroundTransferGroup TransferGroup { get; }public BackgroundTransferGroup TransferGroup { get; }Public ReadOnly Property TransferGroup As BackgroundTransferGroup
Attributes
See Also

Methods

AttachAsync() AttachAsync() AttachAsync()

Returns an asynchronous operation that can be used to monitor progress and completion of the attached download. Calling this method allows an app to attach download operations that were started in a previous app instance.

public : IAsyncOperationWithProgress<DownloadOperation, DownloadOperation> AttachAsync()public IAsyncOperationWithProgress<DownloadOperation, DownloadOperation> AttachAsync()Public Function AttachAsync() As IAsyncOperationWithProgress( Of DownloadOperation, DownloadOperation )
Returns
Attributes

Examples


        function AttachDownload (loadedDownload) {
            try {
                download = loadedDownload;
                promise = download.attachAsync().then(complete, error, progress);
            } catch (err) {
                displayException(err);
            }
        };

Remarks

Exceptions

COMException

Thrown when a feature-specific HRESULT is returned from a method call.

This is the most common exception that is thrown by networking methods. An app should use the HRESULT from the exception to determine the cause of the error. For more information on specific errors, see the Error Codes section below.

AccessDeniedException

Thrown when access is denied to a resource or feature. This exception occurs when an app doesn't have the required network capabilities set in the app manifest for the network operation requested.

InvalidArgumentException

Thrown when one of the arguments that are provided to a method is not valid.

If user-supplied input caused this exception, an app could inform the user and request new input.

ObjectDisposedException

Thrown when an operation is performed on a disposed object.

OutOfMemoryException

Thrown when insufficient memory is available to complete the operation.

While, this method can be called from multiple app instances, developers should not attach callbacks from the primary app instance in a background task. This will cause BackgroundTransferHost.exe to hang.

GetResponseInformation() GetResponseInformation() GetResponseInformation()

Gets the response information.

public : ResponseInformation GetResponseInformation()public ResponseInformation GetResponseInformation()Public Function GetResponseInformation() As ResponseInformation
Returns

Contains the data returned by a server response.

Attributes

Remarks

This method returns an object that contains information about the current server response. When calling this method multiple times, the object returned will be the same as long as there is no new server response information available. Sometimes however, one download consists of multiple requests to a server. If a request gets aborted due to some non-fatal error (e.g. machine lost internet connectivity, cost policy required download to pause, application called Pause(), etc.), Download API will invoke a new request and try to resume/restart the download. The response information from the server for this second request may be different than the one returned by the first request.

Returns null if any of the following conditions is true.

  • The transfer associated with the DownloadOperation is being made using a protocol other than HTTP (e.g., FTP).
  • The DownloadOperation hasn't yet begun.
  • There was a failure before any response from the server was received (e.g., connection couldn't be established).

GetResultStreamAt(UInt64) GetResultStreamAt(UInt64) GetResultStreamAt(UInt64)

Gets the partially downloaded response at the specified position.

public : IInputStream GetResultStreamAt(unsigned __int64 position)public IInputStream GetResultStreamAt(UInt64 position)Public Function GetResultStreamAt(position As UInt64) As IInputStream
Parameters
position
unsigned __int64 UInt64 UInt64

The position at which to start reading.

Returns
Attributes

Pause() Pause() Pause()

Pauses a download operation.

public : void Pause()public void Pause()Public Function Pause() As void
Attributes

Remarks

Calling this method will abort the HTTP request but preserve state for the download and the partially downloaded response. This method will fail when called for operations that haven't been started yet, are already paused, or use POST requests.

See Also

Resume() Resume() Resume()

Resumes a paused download operation.

public : void Resume()public void Resume()Public Function Resume() As void
Attributes

Remarks

This method can be called on a paused download operation to resume the download. The download operation will try to resume the download if possible. Only requests where the server accepts range-requests can resume. Otherwise the download is restarted.

See Also

StartAsync() StartAsync() StartAsync()

Starts an asynchronous download operation.

public : IAsyncOperationWithProgress<DownloadOperation, DownloadOperation> StartAsync()public IAsyncOperationWithProgress<DownloadOperation, DownloadOperation> StartAsync()Public Function StartAsync() As IAsyncOperationWithProgress( Of DownloadOperation, DownloadOperation )
Returns
Attributes

Remarks

Exceptions

COMException

Thrown when a feature-specific HRESULT is returned from a method call.

This is the most common exception that is thrown by networking methods. An app should use the HRESULT from the exception to determine the cause of the error. For more information on specific errors, see the Error Codes section below.

AccessDeniedException

Thrown when access is denied to a resource or feature. This exception occurs when an app doesn't have the required network capabilities set in the app manifest for the network operation requested.

InvalidArgumentException

Thrown when one of the arguments that are provided to a method is not valid.

If user-supplied input caused this exception, an app could inform the user and request new input.

ObjectDisposedException

Thrown when an operation is performed on a disposed object.

OutOfMemoryException

Thrown when insufficient memory is available to complete the operation.

Background transfer doesn't support concurrent downloads of the same Uri. So an app can download http://example.com/myfile.wmv once, or download it again after a previous download completed. An app shouldn't start two downloads of the same Uri concurrently, since this may result in truncated files.

A download operation must be scheduled using one of the BackgroundDownloader.CreateDownload or BackgroundDownloader.CreateDownloadAsync methods before the StartAsync method is called.

Important

Queuing up a large number of transfers on the main UI thread can result in degraded performance of your app's UI, even though the call is awaitable. If you are queuing up a large number of transfers, it is recommended that you call StartAsync on a background worker thread as in the following example.

operation = await Task.Run(() => { return myDownloadOperation.StartAsync(); });
See Also

See Also