How do I use HttpClient PostAsync parameters properly?

Question

So I am working on writing an extension class for my project using HttpClient since I am moving over from HttpWebRequest.

When doing the POST request, how do I send a normal string as a parameter? No json or anything just a simple string.

And this is what it looks like so far.

static class HttpClientExtension
    {
        static HttpClient client = new HttpClient();
        public static string GetHttpResponse(string URL)
        {
            string fail = "Fail";
            client.BaseAddress = new Uri(URL);
            HttpResponseMessage Response = client.GetAsync(URL).GetAwaiter().GetResult();
            if (Response.IsSuccessStatusCode)
                return Response.Content.ReadAsStringAsync().GetAwaiter().GetResult();
            else
                return fail;
        }

        public static string PostRequest(string URI, string PostParams)
        {
            client.PostAsync(URI, new StringContent(PostParams));
            HttpResponseMessage response = client.GetAsync(URI).GetAwaiter().GetResult();
            string content = response.Content.ReadAsStringAsync().GetAwaiter().GetResult();
            return content;
        }
    }

If you look at this like

client.PostAsync(URI, new StringContent(PostParams));

You can see that I just tried creating new StringContent and passing a string into it and the response returned 404 page not found. How do I properly use Post.Async(); do I send a string or byte array? Because with HttpWebRequest you would do it like this

public static void SetPost(this HttpWebRequest request, string postdata)
        {
            request.Method = "POST";
            byte[] bytes = Encoding.UTF8.GetBytes(postdata);

            using (Stream requestStream = request.GetRequestStream())
                requestStream.Write(bytes, 0, bytes.Length);
        }

Answer 1

In the PostRequest the following is done..

client.PostAsync(URI, new StringContent(PostParams));
HttpResponseMessage response = client.GetAsync(URI).GetAwaiter().GetResult();

Which does not capture the response of the POST.

Refactor to

public static string PostRequest(string URI, string PostParams) {            
    var response = client.PostAsync(URI, new StringContent(PostParams)).GetAwaiter().GetResult();
    var content = response.Content.ReadAsStringAsync().GetAwaiter().GetResult();
    return content;
}

HttpClient is primarily meant to be used async so consider refactoring to

public static async Task<string> PostRequestAsync(string URI, string PostParams) {            
    var response = await client.PostAsync(URI, new StringContent(PostParams));
    var content = await response.Content.ReadAsStringAsync();
    return content;
}

Answer 2

You need prepare object and then you will serialize the object using Newtonsoft.Json. After that you will prepare byte content from the buffer. We are using api url api/auth/login and it is not full api url as we used dependency injection and configure base address in startup, see the second code.

public async void Login(string username, string password)
    {
        LoginDTO login = new LoginDTO();
        login.Email = username;
        login.Password = password;
        var myContent = JsonConvert.SerializeObject(login);
        var buffer = System.Text.Encoding.UTF8.GetBytes(myContent);
        var byteContent = new ByteArrayContent(buffer);
        byteContent.Headers.ContentType = new MediaTypeHeaderValue("application/json");

        var response = await httpClient.PostAsync("api/auth/login", byteContent);
        var contents = await response.Content.ReadAsStringAsync();
    }

services.AddHttpClient<IAuthService, AuthService>(client =>
        {
            client.BaseAddress = new Uri("https://localhost:44354/");
        });

.NET 5 Solution

In .NET 5, There is new class JsonContent and you can implement this easily

LoginDTO login = new LoginDTO();
login.Email = username;
login.Password = password;

JsonContent content = JsonContent.Create(login);
var url = "http://...";
HttpResponseMessage response = await httpClient.PostAsync(url, content);