Imagine that you've got an ASP.NET page that is generating an email message that needs to include links back to the website. Perhaps you're writing the next greatest online message board application and when someone replies to a thread you want to send out emails to the other thread participants indicating that a new message has been posted along with a link to view the just-posted message. You know that the URL to view a particular thread is, say, ~/Threads/View.aspx?ThreadId=threadId. But how do you turn that relative URL into an absolute URL like http://www.yourserver.com/Threads/View.aspx?ThreadId=threadId?
The simplest answer is to hard code the domain name into the URL when generating the email's content, but that approach is naive and wouldn't work if you were building an application that others could buy and install on their servers. It also would cause a headache if you change your domain name. Finally, it's a headache because oftentimes an ASP.NET application has different URLs depending on the environment. From the development environment the URL might be http://localhost:somePortNumber/MyMessageboard/Threads/View.aspx?ThreadId=threadId, whereas on the staging environment it might be http://stagingserver/MB/Threads/View.aspx?ThreadId=threadId. Ideally, the email generated from a particular environment would contain the appropriate URL back to said environment.
The good news is that generating a fully qualified URL in ASP.NET is quite simple. The following code will pick up the correct HTTP or HTTPS setting, the correct server name - be it localhost, localhost:portNumber, serverInIntranet, an IP address, or a domain name (www.example.com) - and the applicaiton path, if any. Here's the code in both C# and VB:
// C#
string relativeUrl = "~/Threads/View.aspx?ThreadId=" + threadId;
string fullyQualifiedUrl = Request.Url.GetLeftPart(UriPartial.Authority) + Page.ResolveUrl(relativeUrl);
' VB
Dim relativeUrl As String = "~/Threads/View.aspx?ThreadId=" + threadId
Dim fullyQualifiedUrl As String = Request.Url.GetLeftPart(UriPartial.Authority) & Page.ResolveUrl(relativeUrl)
That's it! Pretty easy.
The Request.Url property returns the fully qualified URL of the currently requesting page. The GetLeftPart(UriPartial.Authority) method returns just the protocol and domain name, which is then concatenated with the resolved relative URL to give us the fully qualified URL for the relative URL.
For more on Request.Url, along with a look at the many other path-related properties available of the HttpRequest class, see Rick Strahl's blog entry, Making Sense of ASP.NET Paths.