Request headers must contain only ASCII characters while using c# http client?

Question

I am using HttpClient to make a POST call by passing header but at some point of time I am getting an error as:

Request headers must contain only ASCII characters.

With stacktrace as:

at System.Net.Http.HttpConnection.WriteStringAsync(String s)
   at System.Net.Http.HttpConnection.WriteHeadersAsync(HttpHeaders headers, String cookiesFromContainer)
   at System.Net.Http.HttpConnection.SendAsyncCore(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.SendWithNtConnectionAuthAsync(HttpConnection connection, HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.SendWithRetryAsync(HttpRequestMessage request, Boolean doRequestAuth, CancellationToken cancellationToken)
   at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.DiagnosticsHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at System.Net.Http.HttpClient.FinishSendAsyncBuffered(Task`1 sendTask, HttpRequestMessage request, CancellationTokenSource cts, Boolean disposeCts)

Here is my code:

public HttpWrapper(string endpoint, Func<IDictionary<string, string>> NewHeader)
{
    _httpClient = new HttpClient();
    _httpClient.BaseAddress = new Uri(endpoint);

    if (NewHeader != null)
    {
        var headers = NewHeader();
        foreach (var header in headers)
        {
            _httpClient.DefaultRequestHeaders.TryAddWithoutValidation(header.Key, header.Value);
        }
    }
}

Do I need to do something with header.Value to fix this issue? I was reading online so looks like I need to use Utf-8 here but not sure on how to do it properly?

Update

I got header value like this today and it threw same exception since HttpUtility.HtmlEncode didn't do anything on it. Also I am not sure what is this character <0x94>? Any thoughts why it is happening?

Also I am not sure

"In order to use non ASCII characters in URI you need to escape them using the %hexcode syntax (see section 2 of RFC 2396)." from stackoverflow.com/a/5251951/5386938 — user5386938
– user5386938, Commented Nov 11, 2020 at 18:44
What about the accepted answer? Is that correct way as well? @JustinEzequiel — AndyP
– AndyP, Commented Nov 11, 2020 at 19:55

Yigit Yuksel · Accepted Answer · 2020-11-11 18:59:39Z

4

You can try to encode header values to HTML:

_httpClient.DefaultRequestHeaders.TryAddWithoutValidation(header.Key, HttpUtility.HtmlEncode(header.Value));

Your string, Ergänzendes will be "Ergänzendes".

edited Nov 11, 2020 at 18:59

answered Nov 11, 2020 at 18:39

Yigit Yuksel

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10 Comments

AndyP Over a year ago

I tried like this already but it doesn't give me that value -

string str = "Ergänzendes";             string val = Convert.ToBase64String(Encoding.UTF8.GetBytes(str));             Console.WriteLine(val);

back. Is this the right expectation? It just prints this value RXJnw6RuemVuZGVz

AndyP Over a year ago

So internally httpclient will convert this base64 encoded string RXJnw6RuemVuZGVz to the way it came back originally? Is my assumptions correct here?

Yigit Yuksel Over a year ago

Hmm, it's changing the original text. You can try to encode to HMTL your string. The answer is updated.

AndyP Over a year ago

It then prints like this - Ergänzendes. I am not sure if that is the right way?

Yigit Yuksel Over a year ago

Yes, this is the correct way. The browsers render the header text to "Ergänzendes ".

|

Sebastian Inones · Accepted Answer · 2020-11-16 17:21:48Z

3

I hope this may help in your specific scenario or others with a similar situation. In my case, my header was containing non ASCII characters as one of the Headers values was generated by creating a sha256 HMAC. That was a requirement from the service I was trying to send my request. So, what I had to do in order to encode those characters into ASCII was just this:

//hashedSignature is the one containing NON ASCII charcters!
string MessageSignatureValue = System.Text.Encoding.ASCII.GetString(hashedSignature); 
httpClient.DefaultRequestHeaders.Add(MessageSignaturField, MessageSignatureValue);

I have just included the importants pieces of code.

So, System.Text.Encoding.ASCII.GetString() method did the trick!

Again, I hope it helps.

answered Nov 16, 2020 at 17:21

Sebastian Inones

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4 Comments

AndyP Over a year ago

I see. So what's the difference between accepted answer and your suggestion? Any thoughts?

AndyP Over a year ago

You sure this is the right syntax - string MessageSignatureValue = System.Text.Encoding.ASCII.GetString(hashedSignature); ?

Sebastian Inones Over a year ago

Honestly, I have never used HttpUtility. So, I haven't a concrete answer about that. But, from the documentation it states: "Converts a string into an HTML-encoded string." If that works for you there will be no difference, except the in the way I call the method it's telling that you want something encoded to ASCII explicitely. Maybe, it's more esay to understand the purpose. Just a thought. Here you have the link to the ASCIIEncoding

IbrarMumtaz Over a year ago

This answer will convert non-recognised characters to '?' so the outputted header value or name, will have a '?' somewhere in the char sequence where the non-ascii char was detected. This might not be the answer some devs are looking for,.

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