到目前为止，我找到了简单的解决方案(一行程序，没有错误检查，没有等待响应):

(new WebClient()).UploadStringAsync(new Uri(Address), dataString);‏

请谨慎使用!

当使用Windows.Web.Http命名空间时，对于POST而不是FormUrlEncodedContent，我们编写HttpFormUrlEncodedContent。同样，响应类型为HttpResponseMessage。其余的就像Evan Mulawski写的那样。

这里有一些非常好的答案。让我发布一种不同的方法来设置你的头部与WebClient()。我还将向您展示如何设置API键。

        var client = new WebClient();
        string credentials = Convert.ToBase64String(Encoding.ASCII.GetBytes(userName + ":" + passWord));
        client.Headers[HttpRequestHeader.Authorization] = $"Basic {credentials}";
        //If you have your data stored in an object serialize it into json to pass to the webclient with Newtonsoft's JsonConvert
        var encodedJson = JsonConvert.SerializeObject(newAccount);

        client.Headers.Add($"x-api-key:{ApiKey}");
        client.Headers.Add("Content-Type:application/json");
        try
        {
            var response = client.UploadString($"{apiurl}", encodedJson);
            //if you have a model to deserialize the json into Newtonsoft will help bind the data to the model, this is an extremely useful trick for GET calls when you have a lot of data, you can strongly type a model and dump it into an instance of that class.
            Response response1 = JsonConvert.DeserializeObject<Response>(response);

还有另一种做法:

using (HttpClient httpClient = new HttpClient())
using (MultipartFormDataContent form = new MultipartFormDataContent())
{
    form.Add(new StringContent(param1), "param1");
    form.Add(new StringContent(param2), "param2");
    using (HttpResponseMessage response = await httpClient.PostAsync(url, form))
    {
        response.EnsureSuccessStatusCode();
        string res = await response.Content.ReadAsStringAsync();
        return res;
    }
}

通过这种方式，您可以轻松地发布一个流。

在. net Core中，你可以用下面的代码进行POST调用。在这里，我为这段代码添加了一些额外的特性，这样您就可以让您的代码在代理后工作，如果有网络凭据的话。

在这里我还提到，您可以更改消息的编码。

HttpClient client = GetHttpClient(_config);

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

client.BaseAddress = new Uri(baseAddress);

Encoding encoding = Encoding.UTF8;

var result = await client.PostAsync(url, new StringContent(body, encoding, "application/json")).ConfigureAwait(false);
if (result.IsSuccessStatusCode)
{
    return new RequestResponse { severity = "Success", httpResponse = result.Content.ReadAsStringAsync().Result, StatusCode = result.StatusCode };
}
else
{
    return new RequestResponse { severity = "failure", httpResponse = result.Content.ReadAsStringAsync().Result, StatusCode = result.StatusCode };
}


public HttpClient GetHttpClient(IConfiguration _config)
{
    bool ProxyEnable = Convert.ToBoolean(_config["GlobalSettings:ProxyEnable"]);

    HttpClient client = null;
    if (!ProxyEnable)
    {
        client = new HttpClient();
    }
    else
    {
        string ProxyURL = _config["GlobalSettings:ProxyURL"];
        string ProxyUserName = _config["GlobalSettings:ProxyUserName"];
        string ProxyPassword = _config["GlobalSettings:ProxyPassword"];
        string[] ExceptionURL = _config["GlobalSettings:ExceptionURL"].Split(';');
        bool BypassProxyOnLocal = Convert.ToBoolean(_config["GlobalSettings:BypassProxyOnLocal"]);
        bool UseDefaultCredentials = Convert.ToBoolean(_config["GlobalSettings:UseDefaultCredentials"]);

        WebProxy proxy = new WebProxy
        {
            Address = new Uri(ProxyURL),
            BypassProxyOnLocal = BypassProxyOnLocal,
            UseDefaultCredentials = UseDefaultCredentials,
            BypassList = ExceptionURL,
            Credentials = new NetworkCredential(ProxyUserName, ProxyPassword)
        };

        HttpClientHandler handler = new HttpClientHandler { Proxy = proxy };
        client = new HttpClient(handler, true);
    }
    return client;
}

MSDN有一个样本。

using System;
using System.IO;
using System.Net;
using System.Text;

namespace Examples.System.Net
{
    public class WebRequestPostExample
    {
        public static void Main()
        {
            // Create a request using a URL that can receive a post. 
            WebRequest request = WebRequest.Create("http://www.contoso.com/PostAccepter.aspx");
            // Set the Method property of the request to POST.
            request.Method = "POST";
            // Create POST data and convert it to a byte array.
            string postData = "This is a test that posts this string to a Web server.";
            byte[] byteArray = Encoding.UTF8.GetBytes(postData);
            // Set the ContentType property of the WebRequest.
            request.ContentType = "application/x-www-form-urlencoded";
            // Set the ContentLength property of the WebRequest.
            request.ContentLength = byteArray.Length;
            // Get the request stream.
            Stream dataStream = request.GetRequestStream();
            // Write the data to the request stream.
            dataStream.Write(byteArray, 0, byteArray.Length);
            // Close the Stream object.
            dataStream.Close();
            // Get the response.
            WebResponse response = request.GetResponse();
            // Display the status.
            Console.WriteLine(((HttpWebResponse)response).StatusDescription);
            // Get the stream containing content returned by the server.
            dataStream = response.GetResponseStream();
            // Open the stream using a StreamReader for easy access.
            StreamReader reader = new StreamReader(dataStream);
            // Read the content.
            string responseFromServer = reader.ReadToEnd();
            // Display the content.
            Console.WriteLine(responseFromServer);
            // Clean up the streams.
            reader.Close();
            dataStream.Close();
            response.Close();
        }
    }
}