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.Add("key1", "value1");
queryString.Add("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/dotnet/api/microsoft.aspnetcore.webutilities.queryhelpers

Sample Code:

const string url = "https://customer-information.azure-api.net/customers/search/taxnbr";
var param = new Dictionary<string, string>() { { "CIKey", "123456789" } };

var newUrl = new Uri(QueryHelpers.AddQueryString(url, param));