HttpUtility.ParseQueryString will work as long as you are in a web app or don't mind including a dependency on System.Web. Another way to do this is:

NameValueCollection queryParameters = new NameValueCollection();
string[] querySegments = queryString.Split('&');
foreach(string segment in querySegments)
{
   string[] parts = segment.Split('=');
   if (parts.Length > 0)
   {
      string key = parts[0].Trim(new char[] { '?', ' ' });
      string val = parts[1].Trim();

      queryParameters.Add(key, val);
   }
}

A lot of the answers are providing custom examples because of the accepted answer's dependency on System.Web. From the Microsoft.AspNet.WebApi.Client NuGet package there is a UriExtensions.ParseQueryString, method that can also be used:

var uri = new Uri("https://stackoverflow.com/a/22167748?p1=6&p2=7&p3=8");
NameValueCollection query = uri.ParseQueryString();

So if you want to avoid the System.Web dependency and don't want to roll your own, this is a good option.

I wanted to remove the dependency on System.Web so that I could parse the query string of a ClickOnce deployment, while having the prerequisites limited to the "Client-only Framework Subset".

I liked rp's answer. I added some additional logic.

public static NameValueCollection ParseQueryString(string s)
    {
        NameValueCollection nvc = new NameValueCollection();

        // remove anything other than query string from url
        if(s.Contains("?"))
        {
            s = s.Substring(s.IndexOf('?') + 1);
        }

        foreach (string vp in Regex.Split(s, "&"))
        {
            string[] singlePair = Regex.Split(vp, "=");
            if (singlePair.Length == 2)
            {
                nvc.Add(singlePair[0], singlePair[1]);
            }
            else
            {
                // only one key with no value specified in query string
                nvc.Add(singlePair[0], string.Empty);
            }
        }

        return nvc;
    }

To do this without System.Web, without writing it yourself, and without additional NuGet packages:

1. Add a reference to System.Net.Http.Formatting
2. Add using System.Net.Http;

3. Use this code:

   new Uri(uri).ParseQueryString()
   

https://msdn.microsoft.com/en-us/library/system.net.http.uriextensions(v=vs.118).aspx

I needed a function that is a little more versatile than what was provided already when working with OLSC queries.

• Values may contain multiple equal signs
• Decode encoded characters in both name and value
• Capable of running on Client Framework
• Capable of running on Mobile Framework.

Here is my solution:

Public Shared Function ParseQueryString(ByVal uri As Uri) As System.Collections.Specialized.NameValueCollection
    Dim result = New System.Collections.Specialized.NameValueCollection(4)
    Dim query = uri.Query
    If Not String.IsNullOrEmpty(query) Then
        Dim pairs = query.Substring(1).Split("&"c)
        For Each pair In pairs
            Dim parts = pair.Split({"="c}, 2)

            Dim name = System.Uri.UnescapeDataString(parts(0))
            Dim value = If(parts.Length = 1, String.Empty,
                System.Uri.UnescapeDataString(parts(1)))

            result.Add(name, value)
        Next
    End If
    Return result
End Function

It may not be a bad idea to tack <Extension()> on that too to add the capability to Uri itself.

If you want to avoid the dependency on System.Web that is required to use HttpUtility.ParseQueryString, you could use the Uri extension method ParseQueryString found in System.Net.Http.

Make sure to add a reference (if you haven't already) to System.Net.Http in your project.

Note that you have to convert the response body to a valid Uri so that ParseQueryString (in System.Net.Http)works.

string body = "value1=randomvalue1&value2=randomValue2";

// "http://localhost/query?" is added to the string "body" in order to create a valid Uri.
string urlBody = "http://localhost/query?" + body;
NameValueCollection coll = new Uri(urlBody).ParseQueryString();

I just realized that Web API Client has a ParseQueryString extension method that works on a Uri and returns a HttpValueCollection:

var parameters = uri.ParseQueryString();
string foo = parameters["foo"];

If you don't want the System.Web dependency, just paste this source code from HttpUtility class.

I just whipped this together from the source code of Mono. It contains the HttpUtility and all it's dependencies (like IHtmlString, Helpers, HttpEncoder, HttpQSCollection).

Then use HttpUtility.ParseQueryString.

https://gist.github.com/bjorn-ali-goransson/b04a7c44808bb2de8cca3fc9a3762f9c

    private void button1_Click( object sender, EventArgs e )
    {
        string s = @"p1=6&p2=7&p3=8";
        NameValueCollection nvc = new NameValueCollection();

        foreach ( string vp in Regex.Split( s, "&" ) )
        {
            string[] singlePair = Regex.Split( vp, "=" );
            if ( singlePair.Length == 2 )
            {
                nvc.Add( singlePair[ 0 ], singlePair[ 1 ] );    
            }    
        }
    }

Just access Request.QueryString. AllKeys mentioned as another answer just gets you an array of keys.

HttpUtility.ParseQueryString(Request.Url.Query) return is HttpValueCollection (internal class). It inherits from NameValueCollection.

    var qs = HttpUtility.ParseQueryString(Request.Url.Query);
    qs.Remove("foo"); 

    string url = "~/Default.aspx"; 
    if (qs.Count > 0)
       url = url + "?" + qs.ToString();

    Response.Redirect(url); 

Since everyone seems to be pasting his solution.. here's mine :-) I needed this from within a class library without System.Web to fetch id parameters from stored hyperlinks.

Thought I'd share because I find this solution faster and better looking.

public static class Statics
    public static Dictionary<string, string> QueryParse(string url)
    {
        Dictionary<string, string> qDict = new Dictionary<string, string>();
        foreach (string qPair in url.Substring(url.IndexOf('?') + 1).Split('&'))
        {
            string[] qVal = qPair.Split('=');
            qDict.Add(qVal[0], Uri.UnescapeDataString(qVal[1]));
        }
        return qDict;
    }

    public static string QueryGet(string url, string param)
    {
        var qDict = QueryParse(url);
        return qDict[param];
    }
}

Usage:

Statics.QueryGet(url, "id")

Hit up Request.QueryString.Keys for a NameValueCollection of all query string parameters.

To get all Querystring values try this:

    Dim qscoll As NameValueCollection = HttpUtility.ParseQueryString(querystring)

Dim sb As New StringBuilder("<br />")
For Each s As String In qscoll.AllKeys

  Response.Write(s & " - " & qscoll(s) & "<br />")

Next s

        var q = Request.QueryString;
        NameValueCollection qscoll = HttpUtility.ParseQueryString(q.ToString());

Rick Strahl's Westwind Utilities has a UrlEncodingParser that is derived from a NameValueCollection and allows querying & editing the url:

string str = "http://example.com/page1?id=3123&format=json&format=xml";

var query = new UrlEncodingParser(str);

Assert.IsTrue(query["id"] == "3123");

He has blogged about it: A .NET QueryString and Form Data Parser

I translate to C# version of josh-brown in VB

private System.Collections.Specialized.NameValueCollection ParseQueryString(Uri uri)
{
    var result = new System.Collections.Specialized.NameValueCollection(4);
    var query = uri.Query;
    if (!String.IsNullOrEmpty(query))
    {
        var pairs = query.Substring(1).Split("&".ToCharArray());
        foreach (var pair in pairs)
        {
            var parts = pair.Split("=".ToCharArray(), 2);
            var name = System.Uri.UnescapeDataString(parts[0]);
            var value = (parts.Length == 1) ? String.Empty : System.Uri.UnescapeDataString(parts[1]);
            result.Add(name, value);
        }
    }
    return result;
}

let search = window.location.search;

console.log(search);

let qString = search.substring(1);

while(qString.indexOf("+") !== -1)

   qString  = qString.replace("+", "");

let qArray = qString.split("&");

let values = [];

for(let i = 0; i < qArray.length; i++){
   let pos = qArray[i].search("=");
   let keyVal = qArray[i].substring(0, pos);
   let dataVal = qArray[i].substring(pos + 1);
   dataVal = decodeURIComponent(dataVal);
   values[keyVal] = dataVal;
}

This is my code, I think it's very useful:

public String GetQueryString(string ItemToRemoveOrInsert = null, string InsertValue = null )
{
    System.Collections.Specialized.NameValueCollection filtered = new System.Collections.Specialized.NameValueCollection(Request.QueryString);
    if (ItemToRemoveOrInsert != null)
    {
        filtered.Remove(ItemToRemoveOrInsert);
        if (!string.IsNullOrWhiteSpace(InsertValue))
        {
            filtered.Add(ItemToRemoveOrInsert, InsertValue);
        }
    }

    string StrQr = string.Join("&", filtered.AllKeys.Select(key => key + "=" + filtered[key]).ToArray());
    if (!string.IsNullOrWhiteSpace(StrQr)){
        StrQr="?" + StrQr;
    }

    return StrQr;
}