The character set provided in ContentType is invalid. Cannot read content as string using an invalid character set.”

ArgumentException: 'gb2312' is not a supported encoding name. For information on defining a custom encoding, see the documentation for the Encoding.RegisterProvider method. Arg_ParamName_Name

//注册字符集
                System.Text.EncodingProvider provider = System.Text.CodePagesEncodingProvider.Instance;
                Encoding.RegisterProvider(provider);

HttpClient client = new HttpClient();
using (MemoryStream ms = new MemoryStream())
{

    byte[] bytes = Encoding.UTF8.GetBytes(paramJson);
    ms.Write(bytes, 0, bytes.Length);

    HttpContent hc = new StreamContent(ms);
    hc.Headers.ContentType = new MediaTypeHeaderValue("text/html");
    hc.Headers.ContentEncoding.Add("utf-8");

    //注册字符集
    System.Text.EncodingProvider provider = System.Text.CodePagesEncodingProvider.Instance;
    Encoding.RegisterProvider(provider);

    HttpResponseMessage resp = await client.PostAsync(url, hc);

    string result = await resp.Content.ReadAsStringAsync();
    //判断结果处理
}