Spotify.NET/SpotifyAPI.Web/Util/URIExtension.cs

using System.Web;
using System.Linq;
using System;
using System.Collections.Generic;
using System.Collections.Specialized;

namespace SpotifyAPI.Web
{
  public static class URIExtensions
  {
    public static Uri ApplyParameters(this Uri uri, IDictionary<string, string> parameters)
    {
      Ensure.ArgumentNotNull(uri, nameof(uri));

      if (parameters == null || !parameters.Any())
      {
        return uri;
      }

      var newParameters = new Dictionary<string, string>();
      NameValueCollection existingParameters = HttpUtility.ParseQueryString(uri.Query);
      foreach (string key in existingParameters.AllKeys)
      {
        newParameters.Add(key, existingParameters[key]);
      }
      foreach (KeyValuePair<string, string> parameter in parameters)
      {
        newParameters.Add(parameter.Key, HttpUtility.UrlEncode(parameter.Value));
      }

      var queryString = string.Join("&", newParameters.Select((parameter) => $"{parameter.Key}={parameter.Value}"));
      var query = string.IsNullOrEmpty(queryString) ? null : queryString;

      var uriBuilder = new UriBuilder(uri)
      {
        Query = query
      };

      return uriBuilder.Uri;
    }
  }
}