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WebClient.OpenReadAsync Method (Uri, Object)

Opens a readable stream containing the specified resource. This method does not block the calling thread.

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

[HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)]
public void OpenReadAsync(
	Uri address,
	Object userToken
)

Parameters

address
Type: System.Uri

The URI of the resource to retrieve.

userToken
Type: System.Object

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

ExceptionCondition
ArgumentNullException

The address parameter is null.

WebException

The URI formed by combining BaseAddress and address is invalid.

-or-

An error occurred while downloading the resource.

-or-

An error occurred while opening the stream.

This method retrieves a Stream instance used to access the resource specified by the address parameter. The stream is obtained using thread resources that are automatically allocated from the thread pool. To receive notification when the stream is available, add an event handler to the OpenReadCompleted event.

NoteNote

You must call Stream.Close when you are finished with the Stream to avoid running out of system resources.

This method does not block the calling thread while the stream is opening. To block while waiting for the stream, use the OpenRead method.

Asynchronous operations that have not completed can be canceled using the CancelAsync method.

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 resource. If the QueryString property is not null, 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.

NoteNote

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

NoteNote

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: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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