Asp.Net Absolute Path of a URL

I have used this in the past:

// Gets the base url in the following format: 
// "http(s)://domain(:port)/AppPath"
HttpContext.Current.Request.Url.Scheme 
    + "://"
    + HttpContext.Current.Request.Url.Authority 
    + HttpContext.Current.Request.ApplicationPath;

Using string interpolation:

string redirectUri = $"{this.Request.Url.Scheme}://{this.Request.Url.Authority}{this.Request.ApplicationPath}account/signedout";

Substitute 'this' for 'HttpContext' or 'HttpContext.Current' based on context.

Old post but here is another slightly less verbose method

var baseUri = new Uri(HttpContext.Current.Request.Url, "/");