How to efficiently remove a query string by Key from a Url?

    var queryString = "hello=hi&xpid=4578";
    var qs = System.Web.HttpUtility.ParseQueryString(queryString);
    qs.Remove("xpid");
    var newQuerystring = qs.ToString();

This still works in .NET 5.

There's a useful class called UriBuilder in the System namespace. We can use it along with a couple of extension methods to do the following:

Uri u = new Uri("http://example.com?key1=value1&key2=value2");
u = u.DropQueryItem("key1");

Or like this:

Uri u = new Uri("http://example.com?key1=value1&key2=value2");
UriBuilder b = new UriBuilder(u);
b.RemoveQueryItem("key1");
u = b.Uri;

The extension methods:

using System;
using System.Collections.Specialized;
using System.Text;
using System.Text.RegularExpressions;

public static class UriExtensions
{
    public static Uri DropQueryItem(this Uri u, string key)
    {
        UriBuilder b = new UriBuilder(u);
        b.RemoveQueryItem(key);
        return b.Uri;
    }
}
public static class UriBuilderExtensions
{
    private static string _ParseQueryPattern = @"(?<key>[^&=]+)={0,1}(?<value>[^&]*)";
    private static Regex _ParseQueryRegex = null;

    private static Regex ParseQueryRegex
    {
        get
        {
            if (_ParseQueryRegex == null)
            {
                _ParseQueryRegex = new Regex(_ParseQueryPattern, RegexOptions.Compiled | RegexOptions.Singleline);
            }
            return _ParseQueryRegex;

        }
    }

    public static void SetQueryItem(this UriBuilder b, string key, string value)
    {
        NameValueCollection parms = ParseQueryString(b.Query);
        parms[key] = value;
        b.Query = RenderQuery(parms);
    }

    public static void RemoveQueryItem(this UriBuilder b, string key)
    {
        NameValueCollection parms = ParseQueryString(b.Query);
        parms.Remove(key);
        b.Query = RenderQuery(parms);
    }       
    private static string RenderQuery(NameValueCollection parms)
    {
        StringBuilder sb = new StringBuilder();
        for (int i=0; i<parms.Count; i++)
        {
            string key = parms.Keys[i];
            sb.Append(key + "=" + parms[key]);
            if (i < parms.Count - 1)
            {
                sb.Append("&");
            }
        }
        return sb.ToString();
    }
    public static NameValueCollection ParseQueryString(string query, bool caseSensitive = true)
    {
        NameValueCollection pairs = new NameValueCollection(caseSensitive ? StringComparer.Ordinal : StringComparer.OrdinalIgnoreCase);

        string q = query.Trim().TrimStart(new char[] {'?'});
        MatchCollection matches = ParseQueryRegex.Matches(q);

        foreach (Match m in matches)
        {
            string key = m.Groups["key"].Value;
            string value = m.Groups["value"].Value;
            if (pairs[key] != null)
            {
                pairs[key] = pairs[key] + "," + value;
            }
            else
            {
                pairs[key] = value;
            }

        }

        return pairs;

    }

}

This works well:

public static string RemoveQueryStringByKey(string url, string key)
{                   
    var uri = new Uri(url);

    // this gets all the query string key value pairs as a collection
    var newQueryString = HttpUtility.ParseQueryString(uri.Query);

    // this removes the key if exists
    newQueryString.Remove(key);

    // this gets the page path from root without QueryString
    string pagePathWithoutQueryString = uri.GetLeftPart(UriPartial.Path);

    return newQueryString.Count > 0
        ? String.Format("{0}?{1}", pagePathWithoutQueryString, newQueryString)
        : pagePathWithoutQueryString;
}

an example:

RemoveQueryStringByKey("https://www.google.co.uk/search?#hl=en&output=search&sclient=psy-ab&q=cookie", "q");

and returns:

https://www.google.co.uk/search?#hl=en&output=search&sclient=psy-ab