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Routing: how do I redirect to a url when a parameter is missing?

I have routes like this:
example.com/{category}/articles.
There isn't a route for
example.com/{category}
example.com/
I want to redirect all that traffic to
example.com/{category}/articles with a default value.
I've read that I can use default values with a RouteValueDictionary:
routes.MapPageRoute("ArticlesAll", "{category}/articles/", "~/ArticlesPage.aspx", false, new RouteValueDictionary { { "category", "cats" } });
But that doesn't do any redirecting.
Would I need another route to forward to the one above or is there a more efficient way of doing this?

What you are asking is a bit unclear, but let me offer what I usually do.
If you are trying to make it so that two similar links can be written, for example www.example.com/read/penguin/article/ and www.example.com/read/panda/article. I would just write the whole string / URL with a variable in the middle:
private string destination = "penguin";
private string url = "www.example.com/read/" + destination + "/article/";
void Test () {
destination = "lion";
text.URL = url;
}
Sorry, I may have made gramatical mistakes in my code, but I hope you get the point. I may have completely misunderstood you though, so clarification would be appreciated.

You can setup these two routes:
routes.MapPageRoute("ArticlesAll", "{category}/articles/{*value}", "~/ArticlesPage.aspx");
routes.MapPageRoute("CatchAll", "{*value}", "~/ArticlesPage.aspx");
to catch anything and redirect to your desired page. I just assumed where you want to lead your users, change the aspx part as needed. Keep in mind that you can use {value} to access the whatever the user wrote.
For example if they wanted to navigate to dogs and you wanted to redirect to dogs/articles/ArticlesPage.aspx, you should use:
routes.MapPageRoute("CatchAll", "{*value}", "~/{value}/articles/ArticlesPage.aspx");
EDIT
If you want to actually redirect to the new URL and not just serve up the right page, you can use the CatchAll route to redirect to a page (say Redirect.aspx) that's sole purpose is to parse data from the RouteData object, construct the new URL and Redirect.

How to get the previous url in C#?

How to get the previous url in a MasterPage in C# ?
I'm trying to find the page which is redirected from.
Thanks in advance.

You can get information of the previous url with the UrlReferrer property. This works in MVC and Web forms.
Request.UrlReferrer.AbsoluteUri
Note that in the first page the property Request.UrlReferrer will be null. Also, it will be null if a redirection occurs (e.g. when a user logs into the web page).
This property is based on the HTTP_REFERER variable, so you could use this one instead.
Request.ServerVariables["HTTP_REFERER"]
Since the HTTP_REFERER is a variable sent by the client it might be altered or removed by the request. Also, the variable is not set when the referre url starts with https.
This article mentions a few points why the Request.UrlReferrer can be null.

Usually you use a query string parameter to achieve this: current?previousUrl=/some/11.
This will allow you to access this value from the server-side code using Context.Request.QueryString["previousUrl"] in your master page code-behind.

string urlName = Request.UrlReferrer.ToString();

How to get current request url in Sharepoint 2013 in C#?

How to get current request url
I type in browser:
http://srv-1/sites/1001/Account Documents/Order
but in Page_Load in my controls I get
HttpContext.Current.Request.Url = http://srv-1/_layouts/15/start.aspx
and
Page.Request.RawUrl = Page.Request.RawUrl
I need to retrieve information: /Account Documents/Order

This happens because of Minimal Download Strategy feature. Your url is rewritten by SharePoint.
Easiest solution is to disable this feature, but you can also try to get url via SPUtility.OriginalServerRelativeRequestUrl property or refactor your code not use url, but current library or something else.

SharePoint does it’s own URL rewrites, if you would try that for example for the page
The solution is to use special property on [SPUtility class – SPUtility.OriginalServerRelativeRequestUrl][1]
http://blog.voyta.net/2012/07/09/how-to-get-original-request-url-in-sharepoint/
This property returns the original URL before it was rewritten, which is useful if you need to get the subweb from which an application page was loaded.
As this URL is server-relative, to get full url, you can use:
SPUtility.GetFullUrl(SPContext.Current.Site,
SPUtility.OriginalServerRelativeRequestUrl);
This can be useful if you need to redirect for example to the same URL with some additional parameters.
Thanks

How to get the site url with bookmark in c#

I want to get the absolute url of .net application with bookmark.
So if the request is coming from - www.mysite.com/myapplication/Home/Index#about
then how to get the above url in .net application?
I tried HttpContext.Request.Url.AbsoluteUri but it is returning - www.mysite.com/myapplication/Home/Index but how to get bookmark ("#about") part of url?

It is not possible: anything after the # is not sent by the browser, it is only used inside the loaded page for navigating to anchors.

The browser doesn't transmit that part to server.you should use java Script like this answer

Try this,
assuming that the variable url holds your actual url
string[] bookmark = url.Split('#');
and the bookmark[1] will hold the Value of your bookmark, also.
string bookmark = url.Replace(HttpContext.Request.Url.AbsoluteUri(url), string.Empty);

Read ID from URL and create a new one

On Page_Load, I have to run code to check a url format. The url can be of the following formats:
http://www.xyzabc.com/DisplayProduct?ID=230 or
http://www.xyzabc.com/DisplayProduct?ID=230&blahblah or
http://www.xyzabc.com/S(yya4h4rf4gjh5eo4uazix2t055)X(1))/DisplayProduct?ID=230
Whenever the url has an ID, I want to create a url in the following format:
http://www.xyzabc.com/DisplayProduct?ID=<the id picked from the url>
Since the code will be run for every page (1500+) in the site, how can i write the best optimized code?

Make use of the URLRouting or make use of HTTPHandler/HTTPModule if possible............
here is link on msdn for this : URL Rewriting in ASP.NET

something like this:
int ID = 0;
int.TryParse(Request.QueryString["ID"], out ID);
if (ID > 0)
{
Response.Redirect(String.Format("http://www.xyzabc.com/DisplayProduct?ID={0}", ID));
}

you can do it by creating custom httpmodule to handle request and rewrite url
see this URL Rewriting in ASP.NET

You can use HttpModules for this, by using
context.BeginRequest += context_BeginRequest;
and in context_BeginRequest you do something like:
context.Response.Redirect(..)
This would be somewhat very "raw" solution, where you have multiple URL rewriting engines for ASP.NET, just check the Internet / SO for these....

Response.Redirect("http://www.xyzabc.com/DisplayProduct?ID=" + Request.QueryString["ID"].ToString().Trim());

Use URL Rewriting Libaray and configure it in web.config to avoid extra code.