How to set content-md5 header in GET method using HttpClient?

Question

I have the following code to set content-md5 in my GET method request using HttpClient

httpClient.DefaultRequestHeaders.TryAddWithoutValidation("content-md5", "value");

I cannot use HttpRequestMessage content to set it because it's not a POST method. When using Postman it works like a charm but fails when using HttpClient.GetAsync. Client request a hmac to the server as follows

{ "content_to_hash": "my content" }

The server will give response like this

{ "content_md5": "88af7ceab9fdafb76xxxxx", "date": "Sat, 02 May 2020 00:13:16 +0700", "hmac_value": "WfHgFyT792IENmK8Mqz9LysmP8ftOP00qA=" }

Now I have to access a GET request using that hmac where it's the problem because I cannot set in httpClient GET request header.

Here's the image

Answer 1

From reading the HttpClient and related source code, there's no way you can get around this and add the header to the actual request object headers. There is an internal list of invalid headers, which includes any Content-* headers. It has to be on a content object.

Therefore, my suggest solution is to create your own content object:

public class NoContentMd5 : HttpContent
{
    protected override Task SerializeToStreamAsync(Stream stream, TransportContext context)
    {
        return Task.CompletedTask;
    }

    protected override bool TryComputeLength(out long length)
    {
        length = 0;
        return false;
    }

    public NoContentMd5(byte[] contentMd5)
    {
        this.Headers.ContentMD5 = contentMd5;
    }

    public NoContentMd5(string contentMd5)
    {
        this.Headers.TryAddWithoutValidation("Content-MD5", contentMd5);
    }
}

This will add the Content-MD5 header with a value of your choosing, but the request won't contain a body.

The next problem you'll encounter is that you're trying to make a GET request with content, which isn't supported by the helper client.GetAsync(...) method. You'll have to make your own request object and use client.SendAsync(...) instead:

HttpClient client = new HttpClient();
var request = new HttpRequestMessage(HttpMethod.Get, "https://localhost/my/test/uri");
request.Content = new NoContentMd5("d41d8cd98f00b204e9800998ecf8427e ");
var result = await client.SendAsync(request);

Note that if you have your Content-MD5 hash as bytes, I've also added a constructor to NoContentMd5 for byte[] too.

The only potential issue with this is that it includes a Content-Length: 0 header. Hopefully that's OK with the API you're working with.

There's an alternative solution described in this answer to question with a similar issue. I'd argue against using it since is vulnerable to changes in the implementation details of HttpRequestHeaders (because it uses reflection, so if MS change the code, it might break) .

Answer 2

Aside from the fact that it's not considered good practice to send a body with GET request (see HTTP GET with request body), you can try this:

using (var content = new StringContent(string.Empty))
using (var request = new HttpRequestMessage
{
    Method = HttpMethod.Get,
    RequestUri = new Uri("http://localhost"),
    Content = content
})
{
    request.Headers.TryAddWithoutValidation("content-md5", "value");;
    using (var response = await httpClient.SendAsync(request))
    {
        response.EnsureSuccessStatusCode();
    }
}

UPDATE:

The proper way would be to set the ContentMD5 property on the HttpContentHeaders, for example:

content.Headers.ContentMD5 = Convert.FromBase64String(hashString);

But as you pointed out in the comments, trying to send content in a GET request causes an error.