Preconditions are specified by the client in the request-header fields and are evaluated on the Web server. For example, by using preconditions, clients can prevent a requested method (for example, GET) from being applied to a resource other than the one intended. For more information, see section 14, "Header Field Definitions," in the tp://go.microsoft.com/fwlink/?LinkID=73147 specification on the World Wide Web Consortium (W3C) Web site.

The DefaultHttpHandler.OnExecuteUrlPreconditionFailure method is called when preconditions prevent the System.Web.DefaultHttpHandler object from processing a request. A deriving class might override the DefaultHttpHandler.OnExecuteUrlPreconditionFailure method to throw an exception when the preconditions fail. The System.Web.DefaultHttpHandler does not throw an exception in the DefaultHttpHandler.OnExecuteUrlPreconditionFailure method.

Any of the following will cause the preconditions to fail:

• Calling erload:System.Web.HttpResponse.Write.

• Calling HttpResponse.AddHeader(string, string).

• Calling HttpResponse.Flush.

• Changing the cache policy for the response.

• Adding a System.IO.Stream object as a filter to the HttpResponse.Filter property.