C#: How to get a value from a query string containing another url with multiple query params?

Question

When a url containing a query params that has a URL containing query params like this: https://example.com/login/login.ashx?redirect_url=https://example.com?test=11&test2=22&test3=33

How can I then get https://example.com?test=11&test2=22&test3=33from that Url?

I've tried this with no luck

redirectUrl = HttpUtility.ParseQueryString(HttpUtility.UrlEncode(context.Request.Url.Query)).Get("redirect_url");.

This results in: https://example.com?test=11 and I want https://example.com?test=11&test2=22&test3=33

&test2=22&test3=33 belongs to the login.ashx URL, not the redirect_url value, because the & characters are not properly encoded to be part of the redirect_url value. The login URL was constructed incorrectly, so garbage in, garbage out. The only way to get what you want is to fix how the login URL is constructed in the first place. — madreflection
– madreflection, Commented Jan 13, 2023 at 7:39
So your question should show how you're constructing the login URL and ask what you're doing wrong there. — madreflection
– madreflection, Commented Jan 13, 2023 at 7:44
@madreflection You are totally right, and I feel stupid for not seeing this. The fix was this: $"{ResolveUrl("~/")}login/login.ashx?redirect_url={HttpUtility.UrlEncode(Request.Url.AbsoluteUri)}" — Sander B
– Sander B, Commented Jan 13, 2023 at 8:46

Kliment Nechaev · Accepted Answer · 2023-01-13 08:00:33Z

2

Any url parameter that contains url string separators should be UrlEncode'd first.

So you need to System.Net.WebUtility.UrlEncode(string value) your parameter url first.

var urlEncodedParameter = System.Net.WebUtility.UrlEncode("https://example.com?test=11&test2=22&test3=33");

then create url

var url = $"https://example.com/login/login.ashx?redirect_url={urlEncodedParameter}";

Then redirect to that. And you will not need to parse that parameter on the controller. It will work on the endpoint with signature like this

public IActionResult Login(string redirect_url)

answered Jan 13, 2023 at 8:00

Kliment Nechaev

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aProger · Accepted Answer · 2023-01-14 06:35:55Z

1

First of all you need to get full url from browser:

string full_url = HttpContext.Request.GetDisplayUrl().ToString();

Then you need to check that full url for existing "redirect_url" in it and if it exists you can use Substring(int position) method. But before we need to know position of "redirect_url", so we can use LastIndexOf() method:

if (full_url.Contains("redirect_url"))
{
    return full_url.Substring(full_url.LastIndexOf("redirect_url"));
}

If there is no "redirect_url" in your full_url we can simply return empty string like this:

return string.Empty;

edited Jan 14, 2023 at 6:35

answered Jan 13, 2023 at 7:57

aProger

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