How to build a query string for a URL in C#?

0 votes
asked May 6, 2009 by boaz

A common task when calling web resources from a code is building a query string to including all the necessary parameters. While by all means no rocket science, there are some nifty details you need to take care of like, appending an & if not the first parameter, encoding the parameters etc.

The code to do it is very simple, but a bit tedious:

StringBuilder SB = new StringBuilder();
if (NeedsToAddParameter A) 
{ 
  SB.Append("A="); SB.Append(HttpUtility.UrlEncode("TheValueOfA")); 
}

if (NeedsToAddParameter B) 
{
  if (SB.Length>0) SB.Append("&"); 
  SB.Append("B="); SB.Append(HttpUtility.UrlEncode("TheValueOfB")); }
}

This is such a common task one would expect a utility class to exist that makes it more elegant and readable. Scanning MSDN, I failed to find one—which brings me to the following question:

What is the most elegant clean way you know of doing the above?

29 Answers

0 votes
answered May 6, 2009 by nick-allen

Untested, but I think something along these lines would work quite nicely

public class QueryString
{
    private Dictionary<string,string> _Params = new Dictionary<string,string>();

    public overide ToString()
    {
        List<string> returnParams = new List<string>();

        foreach (KeyValuePair param in _Params)
        {
            returnParams.Add(String.Format("{0}={1}", param.Key, param.Value));
        }

        // return String.Format("?{0}", String.Join("&", returnParams.ToArray())); 

        // credit annakata
        return "?" + String.Join("&", returnParams.ToArray());
    }

    public void Add(string key, string value)
    {
        _Params.Add(key, HttpUtility.UrlEncode(value));
    }
}

QueryString query = new QueryString();

query.Add("param1", "value1");
query.Add("param2", "value2");

return query.ToString();
0 votes
answered May 6, 2009 by annakata

If you look under the hood the QueryString property is a NameValueCollection. When I've done similar things I've usually been interested in serialising AND deserialising so my suggestion is to build a NameValueCollection up and then pass to:

using System.Web;
using System.Collections.Specialized;

private string ToQueryString(NameValueCollection nvc)
{
    var array = (from key in nvc.AllKeys
        from value in nvc.GetValues(key)
        select string.Format("{0}={1}", HttpUtility.UrlEncode(key), HttpUtility.UrlEncode(value)))
        .ToArray();
    return "?" + string.Join("&", array);
}

Possibly I could've formatted that better :)

I imagine there's a super elegant way to do this in LINQ too...

0 votes
answered May 6, 2009 by martin-harris

A quick extension method based version:

class Program
{
    static void Main(string[] args)
    {
        var parameters = new List<KeyValuePair<string, string>>
                             {
                                 new KeyValuePair<string, string>("A", "AValue"),
                                 new KeyValuePair<string, string>("B", "BValue")
                             };

        string output = "?" + string.Join("&", parameters.ConvertAll(param => param.ToQueryString()).ToArray());
    }
}

public static class KeyValueExtensions
{
    public static string ToQueryString(this KeyValuePair<string, string> obj)
    {
        return obj.Key + "=" + HttpUtility.UrlEncode(obj.Value);
    }
}

You could use a where clause to select which parameters get added to the string.

0 votes
answered May 6, 2009 by igal-tabachnik

I answered a similar question a while ago. Basically, the best way would be to use the class HttpValueCollection, which ASP.NET's Request.QueryString property actually is, unfortunately it is internal in the .NET framework. You could use Reflector to grab it (and place it into your Utils class). This way you could manipulate the query string like a NameValueCollection, but with all the url encoding/decoding issues taken care for you.

HttpValueCollection extends NameValueCollection, and has a constructor that takes an encoded query string (ampersands and question marks included), and it overrides a ToString() method to later rebuild the query string from the underlying collection.

Example:

  var coll = new HttpValueCollection();

  coll["userId"] = "50";
  coll["paramA"] = "A";
  coll["paramB"] = "B";      

  string query = coll.ToString(true); // true means use urlencode

  Console.WriteLine(query); // prints: userId=50&paramA=A&paramB=B
0 votes
answered May 6, 2009 by lukeh

How about creating extension methods that allow you to add the parameters in a fluent style like this?

string a = "http://www.somedomain.com/somepage.html"
    .AddQueryParam("A", "TheValueOfA")
    .AddQueryParam("B", "TheValueOfB")
    .AddQueryParam("Z", "TheValueOfZ");

string b = new StringBuilder("http://www.somedomain.com/anotherpage.html")
    .AddQueryParam("A", "TheValueOfA")
    .AddQueryParam("B", "TheValueOfB")
    .AddQueryParam("Z", "TheValueOfZ")
    .ToString(); 

Here's the overload that uses a string:

public static string AddQueryParam(
    this string source, string key, string value)
{
    string delim;
    if ((source == null) || !source.Contains("?"))
    {
        delim = "?";
    }
    else if (source.EndsWith("?") || source.EndsWith("&"))
    {
        delim = string.Empty;
    }
    else
    {
        delim = "&";
    }

    return source + delim + HttpUtility.UrlEncode(key)
        + "=" + HttpUtility.UrlEncode(value);
}

And here's the overload that uses a StringBuilder:

public static StringBuilder AddQueryParam(
    this StringBuilder source, string key, string value)
{
    bool hasQuery = false;
    for (int i = 0; i < source.Length; i++)
    {
        if (source[i] == '?')
        {
            hasQuery = true;
            break;
        }
    }

    string delim;
    if (!hasQuery)
    {
        delim = "?";
    }
    else if ((source[source.Length - 1] == '?')
        || (source[source.Length - 1] == '&'))
    {
        delim = string.Empty;
    }
    else
    {
        delim = "&";
    }

    return source.Append(delim).Append(HttpUtility.UrlEncode(key))
        .Append("=").Append(HttpUtility.UrlEncode(value));
}
0 votes
answered May 6, 2009 by thomas-bratt

Assuming that you want to reduce dependencies to other assemblies and to keep things simple, you can do:

var sb = new System.Text.StringBuilder();

sb.Append("a=" + HttpUtility.UrlEncode("TheValueOfA") + "&");
sb.Append("b=" + HttpUtility.UrlEncode("TheValueOfB") + "&");
sb.Append("c=" + HttpUtility.UrlEncode("TheValueOfC") + "&");
sb.Append("d=" + HttpUtility.UrlEncode("TheValueOfD") + "&");

sb.Remove(sb.Length-1, 1); // Remove the final '&'

string result = sb.ToString();

This works well with loops too. The final ampersand removal needs to go outside of the loop.

Note that the concatenation operator is used to improve readability. The cost of using it compared to the cost of using a StringBuilder is minimal (I think Jeff Atwood posted something on this topic).

0 votes
answered May 6, 2009 by mike-cole

I added the following method to my PageBase class.

protected void Redirect(string url)
    {
        Response.Redirect(url);
    }
protected void Redirect(string url, NameValueCollection querystrings)
    {
        StringBuilder redirectUrl = new StringBuilder(url);

        if (querystrings != null)
        {
            for (int index = 0; index < querystrings.Count; index++)
            {
                if (index == 0)
                {
                    redirectUrl.Append("?");
                }

                redirectUrl.Append(querystrings.Keys[index]);
                redirectUrl.Append("=");
                redirectUrl.Append(HttpUtility.UrlEncode(querystrings[index]));

                if (index < querystrings.Count - 1)
                {
                    redirectUrl.Append("&");
                }
            }
        }

        this.Redirect(redirectUrl.ToString());
    }

To call:

NameValueCollection querystrings = new NameValueCollection();    
querystrings.Add("language", "en");
querystrings.Add("id", "134");
this.Redirect("http://www.mypage.com", querystrings);
0 votes
answered May 9, 2009 by john-bledsoe

You can create a new writeable instance of HttpValueCollection by calling System.Web.HttpUtility.ParseQueryString(string.Empty), and then use it as any NameValueCollection. Once you have added the values you want, you can call ToString on the collection to get a query string, as follows:

NameValueCollection queryString = System.Web.HttpUtility.ParseQueryString(string.Empty);

queryString["key1"] = "value1";
queryString["key2"] = "value2";

return queryString.ToString(); // Returns "key1=value1&key2=value2", all URL-encoded

The HttpValueCollection is internal and so you cannot directly construct an instance. However, once you obtain an instance you can use it like any other NameValueCollection. Since the actual object you are working with is an HttpValueCollection, calling ToString method will call the overridden method on HttpValueCollection, which formats the collection as a URL-encoded query string.

After searching SO and the web for an answer to a similar issue, this is the most simple solution I could find.

.NET Core

If you're working in .NET Core, you can use the Microsoft.AspNetCore.WebUtilities.QueryHelpers class, which simplifies this greatly.

https://docs.microsoft.com/en-us/aspnet/core/api/microsoft.aspnetcore.webutilities.queryhelpers

0 votes
answered May 1, 2011 by jay-douglass
    public static string ToQueryString(this Dictionary<string, string> source)
    {
        return String.Join("&", source.Select(kvp => String.Format("{0}={1}", HttpUtility.UrlEncode(kvp.Key), HttpUtility.UrlEncode(kvp.Value))).ToArray());
    }

    public static string ToQueryString(this NameValueCollection source)
    {
        return String.Join("&", source.Cast<string>().Select(key => String.Format("{0}={1}", HttpUtility.UrlEncode(key), HttpUtility.UrlEncode(source[key]))).ToArray());
    }
0 votes
answered May 2, 2011 by dso

Here's my late entry. I didn't like any of the others for various reasons, so I wrote my own.

This version features:

  • Use of StringBuilder only. No ToArray() calls or other extension methods. It doesn't look as pretty as some of the other responses, but I consider this a core function so efficiency is more important than having "fluent", "one-liner" code which hide inefficiencies.

  • Handles multiple values per key. (Didn't need it myself but just to silence Mauricio ;)

    public string ToQueryString(NameValueCollection nvc)
    {
        StringBuilder sb = new StringBuilder("?");
    
        bool first = true;
    
        foreach (string key in nvc.AllKeys)
        {
            foreach (string value in nvc.GetValues(key))
            {
                if (!first)
                {
                    sb.Append("&");
                }
    
                sb.AppendFormat("{0}={1}", Uri.EscapeDataString(key), Uri.EscapeDataString(value));
    
                first = false;
            }
        }
    
        return sb.ToString();
    }
    

Example Usage

        var queryParams = new NameValueCollection()
        {
            { "x", "1" },
            { "y", "2" },
            { "foo", "bar" },
            { "foo", "baz" },
            { "special chars", "? = &" },
        };

        string url = "http://example.com/stuff" + ToQueryString(queryParams);

        Console.WriteLine(url);

Output

http://example.com/stuff?x=1&y=2&foo=bar&foo=baz&special%20chars=%3F%20%3D%20%26
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