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WebClient.DownloadFileAsync Method (Uri, String)

Downloads, to a local file, the resource with the specified URI. This method does not block the calling thread.

Namespace:  System.Net
Assembly:  System (in System.dll)

[<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)>]
member DownloadFileAsync : 
        address:Uri * 
        fileName:string -> unit 


Type: System.Uri
The URI of the resource to download.
Type: System.String
The name of the file to be placed on the local computer.


The address parameter is a null reference (Nothing in Visual Basic).


The fileName parameter is a null reference (Nothing in Visual Basic).


The URI formed by combining BaseAddress and address is invalid.


An error occurred while downloading the resource.


The local file specified by fileName is in use by another thread.

This method downloads the resource at the URI specified by in the address parameter. When the download completes successfully, the downloaded file is named fileName on the local computer. The file is downloaded asynchronously using thread resources that are automatically allocated from the thread pool. To receive notification when the file is available, add an event handler to the DownloadFileCompleted event.

This method does not block the calling thread while the resource is being downloaded. To block while waiting for the download to complete, use one of the DownloadFile methods.

You can use the CancelAsync method to cancel asynchronous operations that have not completed.

If the BaseAddress property is not an empty string ("") and address does not specify an absolute URI, address must be a relative URI that is combined with BaseAddress to form the absolute URI of the requested data. If the QueryString property is not an empty string, it is appended to address.

This method uses the RETR command to download an FTP resource. For an HTTP resource, the GET method is used.


This member outputs trace information when you enable network tracing in your application. For more information, see Network Tracing.

When using this method in an ASP.NET page, you will receive an error if the account that the page executes under does not have permission to access the local file.


The HostProtectionAttribute attribute applied to this type or member has the following Resources property value: ExternalThreading. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes.

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.