User-Agent from HttpClient.DefaultRequestHeaders is not set properly in FlurlClient

[tranb3r]

User-Agent from HttpClient.DefaultRequestHeaders is not set properly in FlurlClient, when User-Agent is composed of multiple ProductInfo.

Example:

const string userAgent = "Mozilla/5.0 (X11; Linux i686; rv:109.0) Gecko/20100101 Firefox/120.0";
var httpClient = new HttpClient();
httpClient.DefaultRequestHeaders.Add("User-Agent", userAgent);
var flurlClient = new FlurlClient(httpClient);
foreach (var (name, value) in flurlClient.Headers)
{
    Console.WriteLine(name + ": " + value);
}

will output to console: User-Agent: Mozilla/5.0
instead of User-Agent: Mozilla/5.0 (X11; Linux i686; rv:109.0) Gecko/20100101 Firefox/120.0

So, an important part of the User-Agent is lost.
I guess the logic of headers initialization in FlurlClient constructor is incorrect.
What do you think? Am I missing something?

[tmenier]

Have you tried it with 4.0? It handles DefaultRequestHeaders more explicitly so I think it should work. If it's just the opposite (regression bug from 3 to 4), that might be the only case where I'd let it slow down the 4.0 train.

[tranb3r]

I've been using 4.0 for a verrrrry long time. So the problem is, I do not remember how Flurl managed Headers in 3.x.
Doing a quick test with Flurl.Http 3.2.4, I can see that Headers in FlurlClient are not initialized from httpClient.DefaultRequestHeaders. This seems to be new in 4.0.
So, is it possible in 3.x to properly set a User-Agent? And is there a solution in 4.0? I do not have answers right now but I'll let you know if I manage to figure it out.

[tranb3r]

Ok, here is some code that shows this could be considered a regression in 4.0:

            const string userAgent = "Mozilla/5.0 (X11; Linux i686; rv:109.0) Gecko/20100101 Firefox/120.0";
            var httpClient = new HttpClient();
            httpClient.DefaultRequestHeaders.Add("User-Agent", userAgent);
            var flurlClient = new FlurlClient(httpClient);
            var page = await flurlClient
                .Request("https://httpbingo.org/headers")
                .GetStringAsync();
            Console.WriteLine(page);

With 4.0:

{
  "headers": {
    "User-Agent": [
      "Mozilla/5.0"
    ]
  }
}

With 3.2.4:

{
  "headers": {
    "User-Agent": [
      "Mozilla/5.0 (X11; Linux i686; rv:109.0) Gecko/20100101 Firefox/120.0"
    ]
  }
}

But maybe there is a workaround when using 4.0.

[tranb3r]

Here is how I will workaround this issue for now:

        public static FlurlCLient FixUserAgent(this FlurlClient flurlClient)
        {
            var userAgent = flurlClient.HttpClient.DefaultRequestHeaders.UserAgent.ToString();
            flurlClient.Headers.AddOrReplace("User-Agent", userAgent);
            return flurlClient;
        }

But the issue is probably not specific to the User-Agent header.

[tmenier]

Thanks, I'll look into it ahead of the final release. So much to do...really hoped to ship 4.0 this week but not likely to happen.

[tmenier]

This is fixed in pre7 (if you didn't notice). I don't love the fix but MS doesn't make it easy. Thanks for providing the repro, it was perfect.

[tranb3r]

Thank you Todd for the fix!
I agree, MS implementation of user-agent split/join is a bit weird.

Also, I've tested pre7, it works for me.