Uri Class

Web addresses are typically expressed using uniform resource identifiers that consist of a very restricted set of characters:

• Upper and lower case ASCII letters from the English alphabet.

• Digits from 0 to 9.

• A small number of other ASCII symbols.

The specifications for URIs are documented in RFC 2396, RFC 2732, RFC 3986, and RFC 3987 published by the Internet Engineering Task Force (IETF).

With the growth of the Internet, there is a growing need to identify resources using languages other than English. Identifiers which facilitate this need and allow non-ASCII characters (characters in the Unicode/ISO 10646 character set) are known as International Resource Identifiers (IRIs). The specifications for IRIs are documented in RFC 3987 published by IETF. Using IRIs allows a URL to contain Unicode characters.

The existing Uri class has been extended in .NET Framework v3.5, 3.0 SP1, and 2.0 SP1 to provide IRI support based on RFC 3987. Current users will not see any change from the .NET Framework 2.0 behavior unless they specifically enable IRI. This ensures application compatibility with prior versions of the .NET Framework.

To enable support for IRI, the following change is required:

• Specify whether you want Internationalized Domain Name (IDN) parsing applied to the domain name and whether IRI parsing rules should be applied. This can be done in the machine.config or in the app.config file. For example, add the following:

  Copy

  <configuration>
    <uri>
    <idn enabled="All" />
    <iriParsing enabled="true" />
    </uri>
  </configuration>
  

Enabling IDN will convert all Unicode labels in a domain name to their Punycode equivalents. Punycode names contain only ASCII characters and always start with the xn-- prefix. The reason for this is to support existing DNS servers on the Internet, since most DNS servers only support ASCII characters (see RFC 3940).

Enabling IRI and IDN affects the value of the Uri.DnsSafeHost property. Enabling IRI and IDN can also change the behavior of the Equals, OriginalString, GetComponents, and IsWellFormedOriginalString methods.

There are three possible values for IDN depending on the DNS servers that are used:

• idn enabled = All

  This value will convert any Unicode domain names to their Punycode equivalents (IDN names).

• idn enabled = AllExceptIntranet

  This value will convert all Unicode domain names not on the local Intranet to use the Punycode equivalents (IDN names). In this case to handle international names on the local Intranet, the DNS servers that are used for the Intranet should support Unicode name resolution.

• idn enabled = None

  This value will not convert any Unicode domain names to use Punycode. This is the default value which is consistent with the .NET Framework 2.0 behaviour.

Enabling IRI parsing (iriParsing enabled = true) will do normalization and character checking according to the latest IRI rules in RFC 3986 and RFC 3987. The default value is false and will do normalization and character checking according to RFC 2396 and RFC 2732 (for IPv6 literals).

IRI and IDN processing in the Uri class can also be controlled using the System.Configuration.IriParsingElement, System.Configuration.IdnElement, and System.Configuration.UriSection configuration setting classes. The System.Configuration.IriParsingElement setting enables or disables IRI processing in the Uri class. The System.Configuration.IdnElement setting enables or disables IDN processing in the Uri class. The System.Configuration.IriParsingElement setting also indirectly controls IDN. IRI processing must be enabled for IDN processing to be possible. If IRI processing is disabled, then IDN processing will be set to the default setting where the .NET Framework 2.0 behavior is used for compatibility and IDN names are not used.

The configuration setting for the System.Configuration.IriParsingElement and System.Configuration.IdnElement will be read once when the first System.Uri class is constructed. Changes to configuration settings after that time are ignored.

The System.GenericUriParser class has also been extended to allow creating a customizable parser that supports IRI and IDN. The behavior of a System.GenericUriParser object is specified by passing a bitwise combination of the values available in the System.GenericUriParserOptions enumeration to the System.GenericUriParser constructor. The GenericUriParserOptions.IriParsing type indicates the parser supports the parsing rules specified in RFC 3987 for International Resource Identifiers (IRI). Whether IRI is used is dictated by the configuration values previously discussed.

The GenericUriParserOptions.Idn type indicates the parser supports Internationalized Domain Name (IDN) parsing (IDN) of host names. Whether IDN is used is dictated by the configuration values previously discussed.

If you use a Web.config file that contains URIs to initialize your application, additional time is required to process the URIs if their scheme identifiers are nonstandard. In such a case, initialize the affected parts of your application when the URIs are needed, not at start time.