Articolo
11/18/2011

WebHeaderCollection Class

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Contains protocol headers associated with a request or response.

Inheritance Hierarchy

System.Object
System.Net.WebHeaderCollection

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

Syntax

'Declaration
<DefaultMemberAttribute("Item")> _
Public NotInheritable Class WebHeaderCollection _
    Implements IEnumerable

[DefaultMemberAttribute("Item")]
public sealed class WebHeaderCollection : IEnumerable

The WebHeaderCollection type exposes the following members.

Constructors

	Name	Description
	WebHeaderCollection	Initializes a new instance of the WebHeaderCollection class.

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Properties

	Name	Description
	AllKeys	Gets the collection of header name/value pairs associated with the request.
	Count	Gets the number of headers in the collection.
	Item[HttpRequestHeader]	Gets or sets the specified request header.
	Item[String]	Gets or sets the specified request header.

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Methods

	Name	Description
	Equals(Object)	Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
	Finalize	Allows an object to try to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.)
	GetHashCode	Serves as a hash function for a particular type. (Inherited from Object.)
	GetType	Gets the Type of the current instance. (Inherited from Object.)
	MemberwiseClone	Creates a shallow copy of the current Object. (Inherited from Object.)
	ToString	Returns a string that represents the current object. (Inherited from Object.) In Silverlight for Windows Phone, this member is overridden by ToString(). In XNA Framework, this member is overridden by ToString().

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Extension Methods

	Name	Description
	AsQueryable	Converts an IEnumerable to an IQueryable. (Defined by Queryable.)
	Cast<TResult>	Converts the elements of an IEnumerable to the specified type. (Defined by Enumerable.)
	OfType<TResult>	Filters the elements of an IEnumerable based on a specified type. (Defined by Enumerable.)

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Explicit Interface Implementations

	Name	Description
	IEnumerable.GetEnumerator	Returns an enumerator that can iterate through the WebHeaderCollection instance.

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Remarks

The WebHeaderCollection class is generally accessed through WebRequest.Headers property.

The Item property can be used to get the value of a header stored in a WebHeaderCollection object. The Item[HttpRequestHeader] property can be used to get the value of common headers defined in the HttpRequestHeader enumeration. The Item[String] property can be used to get the value of custom headers and those headers not defined in the HttpRequestHeader enumeration.

The Item property can be used to set custom headers in a WebHeaderCollection object for use by the WebClient or HttpWebRequest objects.

Some common headers are considered restricted and are either exposed directly (such as Content-Type) or protected by the system and cannot be set or changed in a WebHeaderCollection object. Any attempt to set one of these restricted headers in the WebHeaderCollection object associated with a HttpWebRequest object throws an exception. Any attempt to set one of these restricted headers in the WebHeaderCollection object associated with a WebClient object will throw an exception later when attempting to send the WebClient request.

Restricted headers include, but are not limited to the following:

Accept
Accept-Charset
Accept-Encoding
Accept-Language
Accept-Ranges
Age
Allow
Allowed
Connect
Connection
Content-Length
Content-Location
Content-Range
Content-Transfer-Encoding
Content-Type
Cookie
Date
Delete
ETag
Expect
Get
Head
Host
Keep-Alive
Last-Modified
Location
Max-Forwards
Options
Post
Proxy-Authenticate
Proxy-Authorization
Proxy-Connection
Public
Put
Range
Referer
Request-Range
TE
Trace
Trailer
Transfer-Encoding
Upgrade
User-Agent
Via
Warning
Uri
X-Flash-Version
Retry-After
Server
Vary
WWW-Authenticate

Many of these restricted headers are set by the web browser that hosts the Silverlight application.

In addition, some headers are also restricted when the Method property is set to an HTTP GET protocol verb. These restricted headers include, but are not limited to the following:

Content-Encoding
Content-Language
Content-Location
Content-MD5
Content-Range
Content-Type
Expires
Last-Modified

In addition, some headers are also restricted when the Method property is set to an HTTP POST protocol verb. These restricted headers include, but are not limited to the following:

Content-Location
Content-Range
Last-Modified

For security reasons, the Silverlight runtime restricts sending specific headers to a cross-domain site unless the header is allowed by the security policy applicable to the target cross-domain site. This restriction applies to resources from locations other than the site of origin. The Authorization header can be set using the Headers property. However to set the credentials properly, the cross-domain policy applicable to the target must have the http-request-headers set to allow the Authorization header to be sent. For more information, see Network Security Access Restrictions in Silverlight and URL Access Restrictions in Silverlight.

You should not assume that the header values will remain unchanged, because Web servers and caches may change or add headers to a Web request.

The Mozilla FireFox browser does not currently set the Referer header on HTTP GET requests. The Internet Explorer and Safari web browsers do properly set the Referer header on HTTP GET requests. As a result, a Silverlight application hosted in Firefox can't support the Referer header being set on any GET requests applicable to browser HTTP handling for BrowserHttp. The Referer header is properly set on HTTP POST requests by Firefox, Internet Explorer, and Safari applicable to browser HTTP handling for BrowserHttp.

The Referer header is properly set on all HTTP requests applicable to client HTTP handling for ClientHttp regardless of the web browser.

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

Thread Safety

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

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