Deriving from WebRequest

아티클
10/17/2014

The WebRequest class is an abstract base class that provides the basic methods and properties for creating a protocol-specific request handler that fits the .NET Framework pluggable protocol model. Applications that use the WebRequest class can request data using any supported protocol without needing to specify the protocol used.

Two criteria must be met in order for a protocol-specific class to be used as a pluggable protocol: The class must implement the IWebRequestCreate interface, and it must register with the WebRequest.RegisterPrefix method. The class must override all the abstract methods and properties of WebRequest to provide the pluggable interface.

WebRequest instances are intended for one-time use; if you want to make another request, create a new WebRequest. WebRequest supports the ISerializable interface to enable developers to serialize a template WebRequest and then reconstruct the template for additional requests.

IWebRequest Create Method

The Create method is responsible for initializing a new instance of the protocol-specific class. When a new WebRequest is created, the WebRequest.Create method matches the requested URI with the URI prefixes registered with the RegisterPrefix method. The Create method of the proper protocol-specific descendant must return an initialized instance of the descendant capable of performing a standard request/response transaction for the protocol without needing any protocol-specific fields modified.

ConnectionGroupName Property

The ConnectionGroupName property is used to name a group of connections to a resource so that multiple requests can be made over a single connection. To implement connection-sharing, you must use a protocol-specific method of pooling and assigning connections. For example, the provided ServicePointManager class implements connection sharing for the HttpWebRequest class. The ServicePointManager class creates a ServicePoint that provides a connection to a specific server for each connection group.

ContentLength Property

The ContentLength property specifies the number of bytes of data that will be sent to the server when uploading data.

Typically the Method property must be set to indicate that an upload is taking place when the ContentLength property is set to a value greater than zero.

ContentType Property

The ContentType property provides any special information that your protocol requires you to send to the server to identify the type of content that you are sending. Typically this is the MIME content type of any data uploaded.

Credentials Property

The Credentials property contains information needed to authenticate the request with the server. You must implement the details of the authentication process for your protocol. The AuthenticationManager class is responsible for authenticating requests and providing an authentication token. The class that provides the credentials used by your protocol must implement the ICredentials interface.

Headers Property

The Headers property contains an arbitrary collection of name/value pairs of metadata associated with the request. Any metadata needed by the protocol that can be expressed as a name/value pair can be included in the Headers property. Typically this information must be set before calling the GetRequestStream or GetResponse methods; once the request has been made, the metadata is considered read-only.

You are not required to use the Headers property to use header metadata. Protocol-specific metadata can be exposed as properties; for example, the HttpWebRequest.UserAgent property exposes the User-Agent HTTP header. When you expose header metadata as a property, you should not allow the same property to be set using the Headers property.

Method Property

The Method property contains the verb or action that the request is asking the server to perform. The default for the Method property must enable a standard request/response action without requiring any protocol-specific properties to be set. For example, the HttpWebResponse method defaults to GET, which requests a resource from a Web server and returns the response.

Typically the ContentLength property must be set to a value greater than zero when the Method property is set to a verb or action that indicates that an upload is taking place.

PreAuthenticate Property

Applications set the PreAuthenticate property to indicate that authentication information should be sent with the initial request rather than waiting for an authentication challenge. The PreAuthenticate property is only meaningful if the protocol supports authentication credentials sent with the initial request.

Proxy Property

The Proxy property contains an IWebProxy interface that is used to access the requested resource. The Proxy property is meaningful only if your protocol supports proxied requests. You must set the default proxy if one is required by your protocol.

In some environments, such as behind a corporate firewall, your protocol might be required to use a proxy. In that case, you must implement the IWebProxy interface to create a proxy class that will work for your protocol.

RequestUri Property

The RequestUri property contains the URI that was passed to the WebRequest.Create method. It is read-only and cannot be changed once the WebRequest has been created. If your protocol supports redirection, the response can come from a resource identified by a different URI. If you need to provide access to the URI that responded, you must provide an additional property containing that URI.

Timeout Property

The Timeout property contains the length of time, in milliseconds, to wait before the request times out and throws an exception. Timeout applies only to synchronous requests made with the GetResponse method; asynchronous requests must use the Abort method to cancel a pending request.

Setting the Timeout property is meaningful only if the protocol-specific class implements a time-out process.

Abort Method

The Abort method cancels a pending asynchronous request to a server. After the request has been canceled, calling GetResponse, BeginGetResponse, EndGetResponse, GetRequestStream, BeginGetRequestStream, or EndGetRequestStream will throw a WebException with the Status property set to RequestCanceled.

BeginGetRequestStream and EndGetRequestStream Methods

The BeginGetRequestStream method starts an asynchronous request for the stream that is used to upload data to the server. The EndGetRequestStream method completes the asynchronous request and returns the requested stream. These methods implement the GetRequestStream method using the standard .NET Framework asynchronous pattern.

BeginGetResponse and EndGetResponse Methods

The BeginGetResponse method starts an asynchronous request to a server. The EndGetResponse method completes the asynchronous request and returns the requested response. These methods implement the GetResponse method using the standard .NET Framework asynchronous pattern.

GetRequestStream Method

The GetRequestStream method returns a stream that is used to write data to the requested server. The stream returned should be a write-only stream that does not seek; it is intended as a one-way stream of data that is written to the server. The stream returns false for the CanRead and CanSeek properties and true for the CanWrite property.

The GetRequestStream method typically opens a connection to the server and, before returning the stream, sends header information that indicates that data is being sent to the server. Because GetRequestStream begins the request, setting any Header properties or the ContentLength property is typically not allowed after calling GetRequestStream.

GetResponse Method

The GetResponse method returns a protocol-specific descendant of the WebResponse class that represents the response from the server. Unless the request has already been initiated by the GetRequestStream method, the GetResponse method creates a connection to the resource identified by RequestUri, sends header information indicating the type of request being made, and then receives the response from the resource.

Once the GetResponse method is called, all properties should be considered read-only. WebRequest instances are intended for one-time use; if you want to make another request, you should create a new WebRequest.

The GetResponse method is responsible for creating an appropriate WebResponse descendant to contain the incoming response.

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