c# - Parse and modify a query string in .NET Core

Question

I am given an absolute URI that contains a query string. I'm looking to safely append a value to the query string, and change an existing parameter.

I would prefer not to tack on &foo=bar, or use regular expressions, URI escaping is tricky. Rather I want to use a built-in mechanism that I know will do this correctly and handle the escaping.

I've found a ton of answers that all use HttpUtility. However this being ASP.NET Core, there is no more System.Web assembly anymore, thus no more HttpUtility.

What is the appropriate way to do this in ASP.NET Core while targeting the core runtime?

Answer 1

If you are using ASP.NET Core 1 or 2, you can do this with Microsoft.AspNetCore.WebUtilities.QueryHelpers in the Microsoft.AspNetCore.WebUtilities package.

If you are using ASP.NET Core 3.0 or greater, WebUtilities is now part of the ASP.NET SDK and does not require a separate nuget package reference.

To parse it into a dictionary:

var uri = new Uri(context.RedirectUri);
var queryDictionary = Microsoft.AspNetCore.WebUtilities.QueryHelpers.ParseQuery(uri.Query);

Note that unlike ParseQueryString in System.Web, this returns a dictionary of type IDictionary<string, string[]> in ASP.NET Core 1.x, or IDictionary<string, StringValues> in ASP.NET Core 2.x or greater, so the value is a collection of strings. This is how the dictionary handles multiple query string parameters with the same name.

If you want to add a parameter on to the query string, you can use another method on QueryHelpers:

var parametersToAdd = new System.Collections.Generic.Dictionary<string, string> { { "resource", "foo" } };
var someUrl = "http://www.google.com";
var newUri = Microsoft.AspNetCore.WebUtilities.QueryHelpers.AddQueryString(someUrl, parametersToAdd);

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Using .net core 2.2 you can get the query string using

var request = HttpContext.Request;
var query = request.Query;
foreach (var item in query){
   Debug.WriteLine(item) 
}

You will get a collection of key:value pairs - like this

[0] {[companyName, ]}
[1] {[shop, ]}
[2] {[breath, ]}
[3] {[hand, ]}
[4] {[eye, ]}
[5] {[firstAid, ]}
[6] {[eyeCleaner, ]}