Develop Reference

How to get the parameter from a relative URL string in C#?

What's the most efficient way to get a specific parameter from a relative URL string using C#?
For example, how would you get the value of the ACTION parameter from the following relative URL string:
string url = "/page/example?ACTION=data&FOO=test";
I have already tried using:
var myUri = new Uri(url, UriKind.Relative);
String action = HttpUtility.ParseQueryString(myUri.Query).Get("ACTION");
However, I get the following error:
This operation is not supported for a relative URI.

int idx = url.IndexOf('?');
string query = idx >= 0 ? url.Substring(idx) : "";
HttpUtility.ParseQueryString(query).Get("ACTION");

While many of the URI operations are unavailable for UriKind.Relative (for whatever reason), you can build a fully qualified URI through one of the overloads that takes in a Base URI
Here's an example from the docs on Uri.Query:
Uri baseUri = new Uri ("http://www.contoso.com/");
Uri myUri = new Uri (baseUri, "catalog/shownew.htm?date=today");
Console.WriteLine(myUri.Query); // date=today
You can also get the current base from HttpContext.Current.Request.Url or even just create a mock URI base with "http://localhost" if all you care about is the path components.
So either of the following approaches should also return the QueryString from a relative path:
var path = "catalog/shownew.htm?date=today"
var query1 = new Uri(HttpContext.Current.Request.Url, path).Query;
var query2 = new Uri(new Uri("http://localhost"), path).Query;

C# add scheme to URI

I'm trying to add the HTTP Protocol to this URI "example:8888"
What I've done :
var uriBuilder = new UriBuilder("example:8888")
{
Scheme = Uri.UriSchemeHttp,
};
var uri = uriBuilder.Uri;
The Output is
http:0.0.34.184
What I'm doing wrong :S ?

Your string is being parsed as the URL path, not a hostname.
To force it to parse as a hostname, you need to add a scheme to the string.

'System.UriFormatException' in HyperLink

I have some strings like this:
www.example.com/sdWqaP
twitter.com/sdfks
and want to assign them to a HyperLink
var hyperlink = new Hyperlink
{
NavigateUri = new Uri(url),
TargetName = "_blank",
};
if url starts with http:// it works fine, otherwise throws a UriFormatException.
Update: urls like this www.google.com aren't valid http urls. isn't there a better way than var url = "http://" + "www.google.com"

You can use
var uri = new UriBuilder(s).Uri;
Reference: http://msdn.microsoft.com/en-us/library/y868d5wh(v=vs.110).aspx
public UriBuilder(
string uri
)
// If uri does not specify a scheme, the scheme defaults to "http:".

Scheme (http:// in your case) is mandatory part of Uri string. UriFormatException will be thrown if the scheme specified in uri string is not correctly formed according to Uri.CheckSchemeName() method.
[MSDN : Uri Constructor (String)].
I don't understand well what you mean "better safer way". Appending scheme in uri string is common practice anyway.

Check your URL is valid and then assign to the URL
For validating a URL check the below link
How to check whether a string is a valid HTTP URL?

webclient or httpwebrequest to retrieve hrefs and url

How do I use either webclient or httpwebrequest to do two things:
1)Say after downloading the resource as a string using:
var result = x.DownloadString("http://randomsite.com);
there's a relative url(also query string):
Click here to get your name and age
how do I click(follow) on that link using webclient? after initially loading the resource in result. i was able to use htmlagilitypack to isolate the href but I would now like to follow it in code.
2) If the httpwebrequest does not redirect but instead loads the same page with different parameters how would i use webclient to retrieve the new url that is generated?
i.e if i call
var result = x.DownloadString("http://randomsite.com);
but this actually calls
http://randomsite.com/q?site=default
I then want to retrieve the second url
Thanks in advance

You can construct the url from the link and the link that you just downloaded like this:
Uri baseUri = new Uri("http://randomsite.com");
Uri myUri = new Uri(baseUri, "/q?name=john&age=50");
Console.WriteLine(myUri.ToString()); // gives you http://randomsite.com/q?name=john&age=50
This also works if you base Url has url parameters.
As for the second question, i guess you meant that the request was redirected and you want that url instead? Then the easiest way to do so is to sub-class WebClient described here.
Uri baseUri = new Uri("http://randomsite.com");
using(var client=new WebClient())
{
var result = client.DownloadString(myUri);
//get href via HtmlAgilityPack...
Uri myUri = new Uri(baseUri, "/q?name=john&age=50");
result = client.DownloadString(myUri);
}

.NET URI: How can I change ONE part of a URI?

Often I want to change just one part of a URI and get a new URI object back.
In my current dilemma, I want to append .nyud.net, to use the CoralCDN.
I have a fully qualified URI fullUri. How can I, in effect, do this:
fullUri.Host = fullUri.Host + ".nyud.net";
This needs to work for almost any URL, and the PORT of the request needs to be maintained.
Any help would be much appreciated.

You can use an UriBuilder to modify individual parts of an Uri:
Uri uri = new Uri("http://stackoverflow.com/questions/2163191/");
UriBuilder builder = new UriBuilder(uri);
builder.Host += ".nyud.net";
Uri result = builder.Uri;
// result is "http://stackoverflow.com.nyud.net/questions/2163191/"