How do I turn a relative URL into a full URL?

This is probably explained more easily with an example. I'm trying to find a way of turning a relative URL, e.g. "/Foo.aspx" or "~/Foo.aspx" into a full URL, e.g. http://localhost/Foo.aspx. That way when I deploy to test or stage, where the domain under which the site runs is different, I will get http://test/Foo.aspx and http://stage/Foo.aspx.

Any ideas?

Have a play with this (modified from here)

public string ConvertRelativeUrlToAbsoluteUrl(string relativeUrl) {
    return string.Format("http{0}://{1}{2}",
        (Request.IsSecureConnection) ? "s" : "", 
        Request.Url.Host,
        Page.ResolveUrl(relativeUrl)
    );
}

This one's been beat to death but I thought I'd post my own solution which I think is cleaner than many of the other answers.

public static string AbsoluteAction(this UrlHelper url, string actionName, string controllerName, object routeValues)
{
    return url.Action(actionName, controllerName, routeValues, url.RequestContext.HttpContext.Request.Url.Scheme);
}

public static string AbsoluteContent(this UrlHelper url, string path)
{
    Uri uri = new Uri(path, UriKind.RelativeOrAbsolute);

    //If the URI is not already absolute, rebuild it based on the current request.
    if (!uri.IsAbsoluteUri)
    {
        Uri requestUrl = url.RequestContext.HttpContext.Request.Url;
        UriBuilder builder = new UriBuilder(requestUrl.Scheme, requestUrl.Host, requestUrl.Port);

        builder.Path = VirtualPathUtility.ToAbsolute(path);
        uri = builder.Uri;
    }

    return uri.ToString();
}

You just need to create a new URI using the page.request.url and then get the AbsoluteUri of that:

New System.Uri(Page.Request.Url, "Foo.aspx").AbsoluteUri

This is my helper function to do this

public string GetFullUrl(string relativeUrl) {
    string root = Request.Url.GetLeftPart(UriPartial.Authority);
    return root + Page.ResolveUrl("~/" + relativeUrl) ;
}

I thought I'd share my approach to doing this in ASP.NET MVC using the Uri class and some extension magic.

public static class UrlHelperExtensions
{
    public static string AbsolutePath(this UrlHelper urlHelper, 
                                      string relativePath)
    {
        return new Uri(urlHelper.RequestContext.HttpContext.Request.Url,
                       relativePath).ToString();
    }
}

You can then output an absolute path using:

// gives absolute path, e.g. https://example.com/customers
Url.AbsolutePath(Url.Action("Index", "Customers"));

It looks a little ugly having the nested method calls so I prefer to further extend UrlHelper with common action methods so that I can do:

// gives absolute path, e.g. https://example.com/customers
Url.AbsoluteAction("Index", "Customers");

or

Url.AbsoluteAction("Details", "Customers", new{id = 123});

The full extension class is as follows:

public static class UrlHelperExtensions
{
    public static string AbsolutePath(this UrlHelper urlHelper, 
                                      string relativePath)
    {
        return new Uri(urlHelper.RequestContext.HttpContext.Request.Url,
                       relativePath).ToString();
    }

    public static string AbsoluteAction(this UrlHelper urlHelper, 
                                        string actionName, 
                                        string controllerName)
    {
        return AbsolutePath(urlHelper, 
                            urlHelper.Action(actionName, controllerName));
    }

    public static string AbsoluteAction(this UrlHelper urlHelper, 
                                        string actionName, 
                                        string controllerName, 
                                        object routeValues)
    {
        return AbsolutePath(urlHelper, 
                            urlHelper.Action(actionName, 
                                             controllerName, routeValues));
    }
}

Use the .NET Uri class to combine your relative path and the hostname.
http://msdn.microsoft.com/en-us/library/system.uri.aspx

This is the helper function that I created to do the conversion.

//"~/SomeFolder/SomePage.aspx"
public static string GetFullURL(string relativePath)
{
   string sRelative=Page.ResolveUrl(relativePath);
   string sAbsolute=Request.Url.AbsoluteUri.Replace(Request.Url.PathAndQuery,sRelative);
   return sAbsolute;
}

Simply:

url = new Uri(baseUri, url);

In ASP.NET MVC you can use the overloads of HtmlHelper or UrlHelper that take the protocol or host parameters. When either of these paramters are non-empty, the helpers generate an absolute URL. This is an extension method I'm using:

public static MvcHtmlString ActionLinkAbsolute<TViewModel>(
    this HtmlHelper<TViewModel> html, 
    string linkText, 
    string actionName, 
    string controllerName, 
    object routeValues = null,
    object htmlAttributes = null)
{
    var request = html.ViewContext.HttpContext.Request;
    var url = new UriBuilder(request.Url);
    return html.ActionLink(linkText, actionName, controllerName, url.Scheme, url.Host, null, routeValues, htmlAttributes);
}

And use it from a Razor view, e.g.:

 @Html.ActionLinkAbsolute("Click here", "Action", "Controller", new { id = Model.Id }) 

Ancient question, but I thought I'd answer it since many of the answers are incomplete.

public static string ResolveFullUrl(this System.Web.UI.Page page, string relativeUrl)
{
    if (string.IsNullOrEmpty(relativeUrl))
        return relativeUrl;

    if (relativeUrl.StartsWith("/"))
        relativeUrl = relativeUrl.Insert(0, "~");
    if (!relativeUrl.StartsWith("~/"))
        relativeUrl = relativeUrl.Insert(0, "~/");
 
    return $"{page.Request.Url.Scheme}{Uri.SchemeDelimiter}{page.Request.Url.Authority}{VirtualPathUtility.ToAbsolute(relativeUrl)}";
}

This works as an extension of off Page, just like ResolveUrl and ResolveClientUrl for webforms. Feel free to convert it to a HttpResponse extension if you want or need to use it in a non-webforms environment. It correctly handles both http and https, on standard and non-standard ports, and if there is a username/password component. It also doesn't use any hard coded strings (namely ://).

Here's an approach. This doesn't care if the string is relative or absolute, but you must provide a baseUri for it to use.

    /// <summary>
    /// This function turns arbitrary strings containing a 
    /// URI into an appropriate absolute URI.  
    /// </summary>
    /// <param name="input">A relative or absolute URI (as a string)</param>
    /// <param name="baseUri">The base URI to use if the input parameter is relative.</param>
    /// <returns>An absolute URI</returns>
    public static Uri MakeFullUri(string input, Uri baseUri)
    {
        var tmp = new Uri(input, UriKind.RelativeOrAbsolute);
        //if it's absolute, return that
        if (tmp.IsAbsoluteUri)
        {
            return tmp;
        }
        // build relative on top of the base one instead
        return new Uri(baseUri, tmp);
    }

In an ASP.NET context, you could do this:

Uri baseUri = new Uri("http://yahoo.com/folder");
Uri newUri = MakeFullUri("/some/path?abcd=123", baseUri);
//
//newUri will contain http://yahoo.com/some/path?abcd=123
//
Uri newUri2 = MakeFullUri("some/path?abcd=123", baseUri);
//
//newUri2 will contain http://yahoo.com/folder/some/path?abcd=123
//
Uri newUri3 = MakeFullUri("http://google.com", baseUri);
//
//newUri3 will contain http://google.com, and baseUri is not used at all.
//

Modified from other answer for work with localhost and other ports... im using for ex. email links. You can call from any part of app, not only in a page or usercontrol, i put this in Global for not need to pass HttpContext.Current.Request as parameter

            /// <summary>
            ///  Return full URL from virtual relative path like ~/dir/subir/file.html
            ///  usefull in ex. external links
            /// </summary>
            /// <param name="rootVirtualPath"></param>
            /// <returns></returns>
            public static string GetAbsoluteFullURLFromRootVirtualPath(string rootVirtualPath)
            {

                return string.Format("http{0}://{1}{2}{3}",
                    (HttpContext.Current.Request.IsSecureConnection) ? "s" : ""
                    , HttpContext.Current.Request.Url.Host
                    , (HttpContext.Current.Request.Url.IsDefaultPort) ? "" : ":" + HttpContext.Current.Request.Url.Port
                    , VirtualPathUtility.ToAbsolute(rootVirtualPath)
                    );

            }

In ASP.NET MVC, you can use Url.Content(relativePath) to convert into absolute Url