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WebClient.UploadValuesAsync Method (Uri, String, NameValueCollection)

Uploads the data in the specified name/value collection to the resource identified by the specified URI, using the specified method. This method does not block the calling thread.

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

[HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)]
public void UploadValuesAsync(
	Uri address,
	string method,
	NameValueCollection data


Type: System.Uri

The URI of the resource to receive the collection. This URI must identify a resource that can accept a request sent with the method method.

Type: System.String

The method used to send the string to the resource. If null, the default is POST for http and STOR for ftp.

Type: System.Collections.Specialized.NameValueCollection

The NameValueCollection to send to the resource.


The address parameter is null.


The data parameter is null.


The URI formed by combining BaseAddress and address is invalid.


There was no response from the server hosting the resource.


method cannot be used to send content.

This method sends the data contained in a NameValueCollection to the address resource. Use this method to send form data to a URI for processing. The data is sent using the form-urlencoded media type; the Content-Type header value must be set to "application/x-www-form-urlencoded". The header is set correctly by default. The UploadValuesAsync methods throw a WebException if you call this method with a different Content-Type header value set in the Headers collection.

If the method method is not understood by the server, the underlying protocol classes determine what occurs. Typically, a WebException is thrown with the Status property set to indicate the error.

The NameValueCollection is sent asynchronously using thread resources that are automatically allocated from the thread pool. To receive notification when the upload operation completes, add an event handler to the UploadValuesCompleted event.

This method does not block the calling thread while the string is being sent. To send a string and block while waiting for the server's response, use one of the UploadValues methods.

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 contain 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 empty, it is appended to address.


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


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: 4.6, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

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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