BeginAuthenticateAsClient Method (NetworkCredential, String, ProtectionLevel, TokenImpersonationLevel, AsyncCallback, Object)

NegotiateStream.BeginAuthenticateAsClient Method (NetworkCredential, String, ProtectionLevel, TokenImpersonationLevel, AsyncCallback, Object)


Called by clients to begin an asynchronous operation to authenticate the client, and optionally the server, in a client-server connection. The authentication process uses the specified credentials and authentication options. This method does not block.

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

[HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)]
public virtual IAsyncResult BeginAuthenticateAsClient(
	NetworkCredential credential,
	string targetName,
	ProtectionLevel requiredProtectionLevel,
	TokenImpersonationLevel allowedImpersonationLevel,
	AsyncCallback asyncCallback,
	object asyncState


Type: System.Net.NetworkCredential

The NetworkCredential that is used to establish the identity of the client.

Type: System.String

The Service Principal Name (SPN) that uniquely identifies the server to authenticate.

Type: System.Net.Security.ProtectionLevel

One of the ProtectionLevel values, indicating the security services for the stream.

Type: System.Security.Principal.TokenImpersonationLevel

One of the TokenImpersonationLevel values, indicating how the server can use the client's credentials to access resources.

Type: System.AsyncCallback

An AsyncCallback delegate that references the method to invoke when the authentication is complete.

Type: System.Object

A user-defined object containing information about the write operation. This object is passed to the asyncCallback delegate when the operation completes.

Return Value

Type: System.IAsyncResult

An IAsyncResult object indicating the status of the asynchronous operation.

Exception Condition

credential is null.

- or -

targetName is null.


The authentication failed. You can use this object to retry the authentication.


The authentication failed. You can use this object to retry the authentication.


This object has been closed.


Authentication has already occurred.

- or -

This stream was used previously to attempt authentication as the server. You cannot use the stream to retry authentication as the client.

Use the requiredProtectionLevel parameter to request security services for data transmitted using the authenticated stream. For example, to have the data encrypted and signed, specify the EncryptAndSign value. Successful authentication does not guarantee that the requested ProtectionLevel has been granted. You must check the IsEncrypted and IsSigned properties to determine what security services are used by the NegotiateStream.

This method is asynchronous and does not block while the operation completes. To block until the operation completes, use one of the AuthenticateAsClient method overloads.

The asynchronous authentication operation must be completed by calling the EndAuthenticateAsClient method. Typically, the method is invoked by the asyncCallback delegate. For detailed information about using the asynchronous programming model, see Calling Synchronous Methods Asynchronously

If the authentication fails, you receive an AuthenticationException or an InvalidCredentialException. In this case, you can retry the authentication with a different credential.

.NET Framework
Available since 2.0
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