Now with .NET Core 3, Microsoft released a new .NET Core HTTP client.

Reference: https://docs.microsoft.com/en-us/aspnet/core/fundamentals/http-requests?view=aspnetcore-3.1

With the new client, to call a WEB API is much easier.

For example, to make an HTTP GET request:

            var request = new HttpRequestMessage(HttpMethod.Get, url)
            {
                Content = new FormUrlEncodedContent(new Dictionary<string, string>())
            };

            request.Headers.Add("accept", "application/json");

            var response = await _client.SendAsync(request);
            if (response.IsSuccessStatusCode)
            {
                return await response.Content.ReadAsStringAsync();
            }
            else
            {
                throw new WebException($"The remote server returned unexpcted status code: {response.StatusCode} - {response.ReasonPhrase}.");
            }

Typically, when we call a Web API, we don't need to submit a binary file with it. But with the new client, how can we submit files to the server?

As you can see, an HTTP POST request with a file is different from form URL encoded content. The form was divided by some boundaries. And the name of the boundary was specified in the HTTP header: content type.

To submit a file from .NET Core HTTP client, use MultipartFormDataContent so the framework can handle the multipart content.

var formData = new MultipartFormDataContent();

To add your file to it, simply add a new stream content.

formData.Add(new StreamContent(fileStream), name: "file", fileName: "file");

Also, you can add other content like URL encoded content to it.

And finally, submit it.

            var formData = new MultipartFormDataContent();
            formData.Add(new StreamContent(fileStream), "file", "file");
            var request = new HttpRequestMessage(HttpMethod.Post, url)
            {
                Content = formData
            };

            request.Headers.Add("accept", "application/json");

            var response = await _client.SendAsync(request);
            if (response.IsSuccessStatusCode)
            {
                return await response.Content.ReadAsStringAsync();
            }
            else
            {
                throw new WebException($"The remote server returned unexpcted status code: {response.StatusCode} - {response.ReasonPhrase}.");
            }

And your file is uploaded to the server.