WebClient::DownloadFileAsync Method (Uri^, String^, Object^)


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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)]
void DownloadFileAsync(
	Uri^ address,
	String^ fileName,
	Object^ userToken


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.

Type: System::Object^

A user-defined object that is passed to the method invoked when the asynchronous operation completes.

Exception Condition

The address parameter is null.


The fileName parameter is null.


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 in the .NET Framework.

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.

.NET Framework
Available since 2.0
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