출처 : http://rest.elkstein.org/2008/02/using-rest-in-c-sharp.html
public static class RestApiHelper
{
    #region ContentType
    [Flags]
    public enum ContentType
    {
        None = 0,
        urlencoded = 1 << 0,
        Json = 1 << 1,

        UTF8 = 1 << 256
    }

    public static string ToStringValue(this ContentType type)
    {
        // https://stackoverflow.com/questions/479410/enum-tostring-with-user-friendly-strings#answer-479453
        string rtn = string.Empty;

        if ((type & ContentType.urlencoded) != 0)
            rtn = "application/x-www-form-urlencoded";
        if ((type & ContentType.Json) != 0)
            rtn = "application/json";

        if ((type & ContentType.UTF8) != 0)
            rtn += "; charset=UTF-8";

        return rtn;
    }
    #endregion


    private static void SetRequestHeader(HttpWebRequest req, Dictionary<string, string> headers)
    {
        foreach (KeyValuePair<string, string> kv in headers)
        {
            req.Headers[kv.Key] = kv.Value;
        }
    }

    public static bool HttpGet(string url, Dictionary<string, string> headers, out string returnText)
    {
        // SSL 사용시에 SSL 인증서가 맞지 않더라도 통과하게 하는 부분, 테스트시에만 사용하고 제품에는 나가지 않도록 하는 것이 좋다
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
        ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };

        bool _flag = false;
        HttpWebRequest req = WebRequest.Create(url) as HttpWebRequest;
        SetRequestHeader(req, headers);
        using (HttpWebResponse resp = req.GetResponse() as HttpWebResponse)
        {
            if (resp.StatusCode == HttpStatusCode.OK)
            {
                _flag = true;
            }
            StreamReader reader = new StreamReader(resp.GetResponseStream(), Encoding.GetEncoding("UTF-8"));
            returnText = reader.ReadToEnd();
        }
        return _flag;

    }

    public static bool HttpPost(string url, ContentType type, Dictionary<string, string> headers, Dictionary<string, string> parameters, out string returnText)
    {
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
        ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };

        bool _flag = false;
        HttpWebRequest req = WebRequest.Create(new Uri(url)) as HttpWebRequest;
        SetRequestHeader(req, headers);
        req.Method = "POST";
        req.ContentType = type.ToStringValue();
        // Build a string with all the params, properly encoded.
        // We assume that the arrays paramName and paramVal are
        // of equal length:
        StringBuilder paramz = new StringBuilder();
        foreach (KeyValuePair<string, string> param in parameters)
        {
            paramz.Append(param.Key);
            paramz.Append("=");
            paramz.Append(System.Web.HttpUtility.UrlEncode(param.Value));
            paramz.Append("&");
        }
        // Encode the parameters as form data:
        byte[] formData = UTF8Encoding.UTF8.GetBytes(paramz.ToString());
        req.ContentLength = formData.Length;
        // Send the request:
        using (Stream post = req.GetRequestStream())
        {
            post.Write(formData, 0, formData.Length);
            post.Flush();
        }
        // Pick up the response:
        using (HttpWebResponse resp = req.GetResponse() as HttpWebResponse)
        {
            if (resp.StatusCode == HttpStatusCode.OK)
            {
                _flag = true;
            }
            StreamReader reader = new StreamReader(resp.GetResponseStream(), Encoding.GetEncoding("UTF-8"));
            returnText = reader.ReadToEnd();
        }
        return _flag;
    }

    public static bool HttpPost(string url, ContentType type, Dictionary<string, string> headers, string jsonString, out string returnText)
    {
        ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
        ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };

        bool _flag = false;
        HttpWebRequest req = WebRequest.Create(new Uri(url)) as HttpWebRequest;
        SetRequestHeader(req, headers);
        req.Method = "POST";
        req.ContentType = type.ToStringValue();

        // Encode the parameters as form data:
        byte[] formData = UTF8Encoding.UTF8.GetBytes(jsonString);
        req.ContentLength = formData.Length;
        // Send the request:
        using (Stream post = req.GetRequestStream())
        {
            post.Write(formData, 0, formData.Length);
            post.Flush();
        }
        // Pick up the response:
        using (HttpWebResponse resp = req.GetResponse() as HttpWebResponse)
        {
            if (resp.StatusCode == HttpStatusCode.OK)
            {
                _flag = true;
            }
            StreamReader reader = new StreamReader(resp.GetResponseStream(), Encoding.GetEncoding("UTF-8"));
            returnText = reader.ReadToEnd();
        }
        return _flag;
    }

}
[ASP.NET] WebAPI RESTful Service Client C# 웹API 레스트서비스 클라이언트 소스 POST GET 전송
