NameValueCollection to URL Query?

I know i can do this

var nv = HttpUtility.ParseQueryString(req.RawUrl);

But is there a way to convert this back to a url?

var newUrl = HttpUtility.Something("/page", nv);

Simply calling ToString() on the NameValueCollection will return the name value pairs in a name1=value1&name2=value2 querystring ready format. Note that NameValueCollection types don't actually support this and it's misleading to suggest this, but the behavior works here due to the internal type that's actually returned, as explained below.

Thanks to @mjwills for pointing out that the HttpUtility.ParseQueryString method actually returns an internal HttpValueCollection object rather than a regular NameValueCollection (despite the documentation specifying NameValueCollection). The HttpValueCollection automatically encodes the querystring when using ToString(), so there's no need to write a routine that loops through the collection and uses the UrlEncode method. The desired result is already returned.

With the result in hand, you can then append it to the URL and redirect:

var nameValues = HttpUtility.ParseQueryString(Request.QueryString.ToString());
string url = Request.Url.AbsolutePath + "?" + nameValues.ToString();
Response.Redirect(url);

Currently the only way to use a HttpValueCollection is by using the ParseQueryString method shown above (other than reflection, of course). It looks like this won't change since the Connect issue requesting this class be made public has been closed with a status of "won't fix."

As an aside, you can call the Add, Set, and Remove methods on nameValues to modify any of the querystring items before appending it. If you're interested in that see my response to another question.

string q = String.Join("&",
             nvc.AllKeys.Select(a => a + "=" + HttpUtility.UrlEncode(nvc[a])));

Make an extension method that uses a couple of loops. I prefer this solution because it's readable (no linq), doesn't require System.Web.HttpUtility, and it supports duplicate keys.

public static string ToQueryString(this NameValueCollection nvc)
{
    if (nvc == null) return string.Empty;

    StringBuilder sb = new StringBuilder();

    foreach (string key in nvc.Keys)
    {
        if (string.IsNullOrWhiteSpace(key)) continue;

        string[] values = nvc.GetValues(key);
        if (values == null) continue;

        foreach (string value in values)
        {
            sb.Append(sb.Length == 0 ? "?" : "&");
            sb.AppendFormat("{0}={1}", Uri.EscapeDataString(key), Uri.EscapeDataString(value));
        }
    }

    return sb.ToString();
}

Example

var queryParams = new NameValueCollection()
{
    { "order_id", "0000" },
    { "item_id", "1111" },
    { "item_id", "2222" },
    { null, "skip entry with null key" },
    { "needs escaping", "special chars ? = &" },
    { "skip entry with null value", null }
};

Console.WriteLine(queryParams.ToQueryString());

Output

?order_id=0000&item_id=1111&item_id=2222&needs%20escaping=special%20chars%20%3F%20%3D%20%26

This should work without too much code:

NameValueCollection nameValues = HttpUtility.ParseQueryString(String.Empty);
nameValues.Add(Request.QueryString);
// modify nameValues if desired
var newUrl = "/page?" + nameValues;

The idea is to use HttpUtility.ParseQueryString to generate an empty collection of type HttpValueCollection. This class is a subclass of NameValueCollection that is marked as internal so that your code cannot easily create an instance of it.

The nice thing about HttpValueCollection is that the ToString method takes care of the encoding for you. By leveraging the NameValueCollection.Add(NameValueCollection) method, you can add the existing query string parameters to your newly created object without having to first convert the Request.QueryString collection into a url-encoded string, then parsing it back into a collection.

This technique can be exposed as an extension method as well:

public static string ToQueryString(this NameValueCollection nameValueCollection)
{
	NameValueCollection httpValueCollection = HttpUtility.ParseQueryString(String.Empty);
	httpValueCollection.Add(nameValueCollection);
	return httpValueCollection.ToString();
}

Actually, you should encode the key too, not just value.

string q = String.Join("&",
nvc.AllKeys.Select(a => $"{HttpUtility.UrlEncode(a)}={HttpUtility.UrlEncode(nvc[a])}"));

Because a NameValueCollection can have multiple values for the same key, if you are concerned with the format of the querystring (since it will be returned as comma-separated values rather than "array notation") you may consider the following.

Example

var nvc = new NameValueCollection();
nvc.Add("key1", "val1");
nvc.Add("key2", "val2");
nvc.Add("empty", null);
nvc.Add("key2", "val2b");

Turn into: key1=val1&key2[]=val2&empty&key2[]=val2b rather than key1=val1&key2=val2,val2b&empty.

Code

string qs = string.Join("&", 
	// "loop" the keys
	nvc.AllKeys.SelectMany(k => {
		// "loop" the values
		var values = nvc.GetValues(k);
		if(values == null) return new[]{ k };
		return nvc.GetValues(k).Select( (v,i) => 
			// 'gracefully' handle formatting
			// when there's 1 or more values
			string.Format(
				values.Length > 1
					// pick your array format: k[i]=v or k[]=v, etc
					? "{0}[]={1}"
					: "{0}={1}"
				, k, HttpUtility.UrlEncode(v), i)
		);
	})
);

or if you don't like Linq so much...

string qs = nvc.ToQueryString(); // using...

public static class UrlExtensions {
	public static string ToQueryString(this NameValueCollection nvc) {
		return string.Join("&", nvc.GetUrlList());
	}
	
	public static IEnumerable<string> GetUrlList(this NameValueCollection nvc) {
		foreach(var k in nvc.AllKeys) {
			var values = nvc.GetValues(k);
			if(values == null)  { yield return k; continue; }
			for(int i = 0; i < values.Length; i++) {
				yield return
				// 'gracefully' handle formatting
				// when there's 1 or more values
				string.Format(
					values.Length > 1
						// pick your array format: k[i]=v or k[]=v, etc
						? "{0}[]={1}"
						: "{0}={1}"
					, k, HttpUtility.UrlEncode(values[i]), i);
			}
		}
	}
}

As has been pointed out in comments already, with the exception of this answer most of the other answers address the scenario (Request.QueryString is an HttpValueCollection, "not" a NameValueCollection) rather than the literal question.

Update: addressed null value issue from comment.

The short answer is to use .ToString() on the NameValueCollection and combine it with the original url.

However, I'd like to point out a few things:

You cant use HttpUtility.ParseQueryString on Request.RawUrl. The ParseQueryString() method is looking for a value like this: ?var=value&var2=value2.

If you want to get a NameValueCollection of the QueryString parameters just use Request.QueryString().

var nv = Request.QueryString;

To rebuild the URL just use nv.ToString().

string url = String.Format("{0}?{1}", Request.Path, nv.ToString());

If you are trying to parse a url string instead of using the Request object use Uri and the HttpUtility.ParseQueryString method.

Uri uri = new Uri("<THE URL>");
var nv = HttpUtility.ParseQueryString(uri.Query);
string url = String.Format("{0}?{1}", uri.AbsolutePath, nv.ToString());

I always use UriBuilder to convert an url with a querystring back to a valid and properly encoded url.

var url = "http://my-link.com?foo=bar";

var uriBuilder = new UriBuilder(url);
var query = HttpUtility.ParseQueryString(uriBuilder.Query);
query.Add("yep", "foo&bar");

uriBuilder.Query = query.ToString();
var result = uriBuilder.ToString();

// http://my-link.com:80/?foo=bar&yep=foo%26bar

In AspNet Core 2.0 you can use QueryHelpers AddQueryString method.

As @Atchitutchuk suggested, you can use QueryHelpers.AddQueryString in ASP.NET Core:

    public string FormatParameters(NameValueCollection parameters)
    {
        var queryString = "";
        foreach (var key in parameters.AllKeys)
        {
            foreach (var value in parameters.GetValues(key))
            {
                queryString = QueryHelpers.AddQueryString(queryString, key, value);
            }
        };

        return queryString.TrimStart('?');
    }

This did the trick for me:

public ActionResult SetLanguage(string language = "fr_FR")
        {            
            Request.UrlReferrer.TryReadQueryAs(out RouteValueDictionary parameters);            
            parameters["language"] = language;            
            return RedirectToAction("Index", parameters);            
        }

You can use.

var ur = new Uri("/page",UriKind.Relative);

if this nv is of type string you can append to the uri first parameter. Like

var ur2 = new Uri("/page?"+nv.ToString(),UriKind.Relative);