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How to maintain the right URL in C#/ASP.NET?

I am given a code and on one of its pages which shows a "search result" after showing different items, it allows user to click on one of records and it is expected to bring up a page so that specific selected record can be modified.
However, when it is trying to bring up the page I get (by IE) "This page cannot be displayed".
It is obvious the URL is wrong because first I see something http://www.Something.org/Search.aspx then it turns into http://localhost:61123/ProductPage.aspx
I did search in the code and found the following line which I think it is the cause. Now, question I have to ask:
What should I do to avoid using a static URL and make it dynamic so it always would be pointing to the right domain?
string url = string.Format("http://localhost:61123/ProductPage.aspx?BC={0}&From={1}", barCode, "Search");
Response.Redirect(url);
Thanks.

Use HttpContext.Current.Request.Url in your controller to see the URL. Url contains many things including Host which is what you're looking for.
By the way, if you're using the latest .Net 4.6+ you can create the string like so:
string url = $"{HttpContext.Current.Request.Url.Host}/ProductPage.aspx?BC={barCode}&From={"Search"}";
Or you can use string.Format
string host = HttpContext.Current.Request.Url.Host;
string url = string.Format("{0}/ProductPage.aspx?BC={1}&From={2}"), host, barCode, "Search";

You can store the Host segment in your AppSettings section of your Web.Config file (per config / environment like so)
Debug / Development Web.Config
Production / Release Web.Config (with config override to replace the localhost value with something.org host)
and then use it in your code like so.
// Creates a URI using the HostUrlSegment set in the current web.config
Uri hostUri = new Uri(ConfigurationManager.AppSettings.Get("HostUrlSegment"));
// does something like Path.Combine(..) to construct a proper Url with the hostName
// and the other url segments. The $ is a new C# construct to do string interpolation
// (makes for readable code)
Uri fullUri = new Uri(hostUri, $"ProductPage.aspx?BC={barCode}&From=Search");
// fullUrl.AbosoluteUri will contain the proper Url
Response.Redirect(fullUri.AbsoluteUri);
The Uri class has a lot of useful properties and methods to give you Relative Url, AbsoluteUrl, your Url Fragments, Host name etc etc.

This should do it.
string url = string.Format("ProductPage.aspx?BC={0}&From={1}", barCode, "Search");
Response.Redirect(url);
If you are using .Net 4.6+ you can also use this string interpolation version
string url = $"ProductPage.aspx?BC={barcode}&From=Search";
Response.Redirect(url);
You should just be able to omit the hostname to stay on the current domain.

strip off prefix of URL's

I have got the following log of URL strings. The logs contain millions of records.
www.example.com/p1?q=k
example.com/p1?q=k
http://example.com/p1?q=k
https://example.com/p1?q=k
http://www.example.com/p1?q=k
I used the C# Uri class but it throws an excepition for format of type "example.com/p1?q=K"
I was wondering if there is a generally/standard accepted method for dealing with such different types of URL to get websitename & the relative URL.
P.S: I could strip off http:// & https:// by using a regex or string comparision, but curious to know if there are any elegant solutions

If you try it with your existing example it will not work.. however you can play around with this and do some appending code where needed which means you will need to create a few variables to store the http://, https://, and www.
System.Uri uriPre = new Uri ("http://www.example.com/p1?q=k");
string uriString = uriPre.Host + uriPre.PathAndQuery;
uriString = uriString.Replace("www.", "");
yields
"example.com/p1?q=k"
the rest of the coding you will have to figure out because only you would know when to utilize the different protocols base on the example I've provided
to expand on Alexei Levenkov answer here is an example that you can use to try to create a new Uri.
Uri tempValue;
var uriPre = new Uri(string.Empty, UriKind.Relative);
if (Uri.TryCreate("example.com/p1?q=k", UriKind.Relative, out tempValue))
{
// do something or retrun tempValue;
}

Uri it the class that is designed to deal with Uris
var noSchemaRelativeUri = new Uri("example.com/foo", UriKind.Relative);
Either UriBuilder or Uri(Uri base, Uri relative) can be used to construct absolute Uri.
To pick between relative and aboslute you can use Uri.TryCreate.
Note. "www.example.com" and "example.com" strictly speaking are unrelated domain names, converting one to another is not guaranteed to always produce registered domain name (also indeed most sites register both and do some sort of redirect between).

How to get website name from domain name?

I fetch the domain from the URL as follows:
var uri = new Uri("Http://www.google.com");
var host = uri.Host;
//host ="www.google.com"
But I want only google.com in Host,
host = "google.com"

Given the accepted answer I guess the issue was not knowing how to manipulate strings rather than how to deal with uris... but for anyone else who ends up here:
The Uri class does not have this property so you will have to parse it yourself.
Presumably you do not know what the subdomain is before time so a simple replace may not be possible.
This is not trivial since the TLDs are so varied (http://en.wikipedia.org/wiki/List_of_Internet_top-level_domains), and there maybe be multiple parts to the url (eg http://pre.subdomain.domain.co.uk).
You will have to decide exactly what you want to get and how complex you want the solution to be.
simple - do a string replace, see ekad's answer
medium - regex that works most of the time, see Strip protocol and subdomain from a URL
or complex - refer to a list of suffixes in order to figure out what is subdomain and what is domain eg
Get the subdomain from a URL

If host begins with "www.", you can replace "www." with an empty string using String.Replace Method like this:
var uri = new Uri("Http://www.google.com");
var host = uri.Host.ToLower();
if (host.StartsWith("www."))
{
host = host.Replace("www.", "");
}

How do I force a relative URI to use https?

I have a relative URI:
Uri U = new Uri("../Services/Authenticated/VariationsService.svc",
UriKind.Relative);
The problem is that depending on whether the user has typed https:// or http:// into their web browser to get to the silverlight application, it may use either http or https when trying to contact the service.
I want to force the program to use https for connecting to the service eitherway.
Initially I tried this:
Uri U = new Uri("../Services/Authenticated/VariationsService.svc",
UriKind.Relative);
string NU = U.AbsoluteUri;
U = new Uri(NU.Replace("http://", "https://"), UriKind.Absolute);
But it fails at U.AbsoluteUri because it can't convert the relative Uri into an absolute Uri at that stage. So how do I change the Uri Scheme to https?

The relative path has to be converted to absolute first. I do that using the Uri of the excuting Silverlight XAP file.
There might be ways to reduce this a bit (it feels wrong doing string operations with Uris) but it's a start:
// Get the executing XAP Uri
var appUri = App.Current.Host.Source;
// Remove the XAP filename
var appPath = appUri.AbsolutePath.Substring(0, appUri.AbsolutePath.LastIndexOf('/'));
// Construct the required absolute path
var rootPath = string.Format("https://{0}{1}", appUri.DnsSafeHost, appUri.AbsolutePath);
// Make the relative target Uri absolute (relative to the target Uri)
var uri = new Uri(new Uri(rootPath), "../Services/Authenticated/VariationsService.svc");
This does not include transferring a portnumber (which you might want to do in other circumstance). Personally I would put the above code in a helper method that also handles the port (and whatever you want to do differently when running localhost).
Hope this helps.

Instead, you should change your ASPX file which hosts your silverlight, and force user to redirect to SSL only if he/she is logged in using non SSL url. Because ideally it would be perfect if silverlight opens connection only to the same domain and scheme it loaded from.

The protocol is a separate component, so I think that you can just put it in front of your relative address:
Uri U = new Uri("https:../Services/Authenticated/VariationsService.svc", UriKind.Relative);

"Scheme" does not have any meaning in a relative URI. You'll have to convert it to an absolute URI at some point to change the scheme.

Get name of virtual directory?

I'm using Request.ApplicationPath to learn the name of the Virtual Directory in which I'm running. Is there a more reliable way?

Request.ApplicationPath is perfectly reliable way of getting the virtual directory and works always when you have the HttpContext and can ask for the Request data.
For further processing and extracting parts of the path, take a look at the VirtualPathUtility class.

You need to use Request.ApplicationPath. That is what it is designed for.
Editing to go with your comment.
Since you want a 'cleaner' way to handle the slash, I recommend creating a utility function that returns the application path with the logic in it to deal with the slash as you see fit.

Use this function in C#:
public static String GetHost()
{
var request = HttpContext.Current.Request;
return request.Url.Scheme + "://" + request.ServerVariables["HTTP_HOST"] + request.ApplicationPath;
}