How do I get the site root URL?

For WebForms, this code will return the absolute path of the application root, regardless of how nested the application may be:

HttpContext.Current.Request.Url.GetLeftPart(UriPartial.Authority) + ResolveUrl("~/")

The first part of the above returns the scheme and domain name of the application (http://localhost) without a trailing slash. The ResolveUrl code returns a relative path to the application root (/MyApplicationRoot/). By combining them together, you get the absolute path of the web forms application.

Using MVC:

HttpContext.Current.Request.Url.GetLeftPart(UriPartial.Authority) + Url.Content("~/")

or, if you are trying to use it directly in a Razor view:

@HttpContext.Current.Request.Url.GetLeftPart(UriPartial.Authority)@Url.Content("~/")