Post parameters during asmx web service call

Question

I am trying to make a web service call to following address: http://www.webservicex.net/CurrencyConvertor.asmx?op=ConversionRate

However, I get WebException : Protocol Error. So, I need some help to find where I went wrong. Thanks.

protected void Page_Load(object sender, EventArgs e)
{
    try
    {
        string FromCurrency = "GBP";
        string ToCurrency = "ALL";

        string postString = string.Format("FromCurrency={0}&ToCurrency={1}", FromCurrency, ToCurrency);
        const string contentType = "application/x-www-form-urlencoded";
        System.Net.ServicePointManager.Expect100Continue = false;

        // Creates an HttpWebRequest for the specified URL. 
        HttpWebRequest req = (HttpWebRequest)WebRequest.Create("http://www.webservicex.net/CurrencyConvertor.asmx?op=ConversionRate");
        req.ContentType = "text/xml;charset=\"utf-8\"";
        req.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
        req.Method = "POST";
        req.ContentLength = postString.Length;
        req.Referer = "http://webservicex.net";

            StreamWriter requestWriter = new StreamWriter(req.GetRequestStream());
            requestWriter.Write(postString);
            requestWriter.Close();

            StreamReader responseReader = new StreamReader(req.GetResponse().GetResponseStream());
            string responseData = responseReader.ReadToEnd();

            responseReader.Close();
            req.GetResponse().Close();
    }

    catch (WebException ex)
    {
        Response.Write("\r\nWebException Raised. The following error occured : {0}" + ex.Status);
    }
    catch (Exception exc)
    {
        Response.Write("\nThe following Exception was raised : {0}" + exc.Message);
    }
}

Answer 1

I guess you are getting protocol exception because service is SOAP based. It would be easier to add service reference to that service from http://www.webservicex.net/CurrencyConvertor.asmx?WSDL. And then you could use that service as following:

static void Main(string[] args)
    {
        ServiceReference1.CurrencyConvertorSoapClient client = new ServiceReference1.CurrencyConvertorSoapClient("CurrencyConvertorSoap");
        client.Open();
        double conversionResult = client.ConversionRate(ServiceReference1.Currency.AED, ServiceReference1.Currency.ANG);
        Console.WriteLine("{0} to {1} conversion rate is {2}", ServiceReference1.Currency.AED, ServiceReference1.Currency.ANG, conversionResult);
        client.Close();
        Console.Read();
    }

[Edit] Sorry for misunderstanding. Here is how to call SOAP services without adding service reference:

static void Main(string[] args)
    {
        HttpWebRequest request = CreateWebRequest();
        //Create xml document for soap envelope
        XmlDocument soapEnvelopeXml = new XmlDocument();
       //string variable for storing body of xml document 
       //with parameters for  FromCurrency and ToCurrency
        string soapEnvelope =
            @"<s:Envelope xmlns:s=""http://schemas.xmlsoap.org/soap/envelope/"">
                <s:Body xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema"">
                    <ConversionRate xmlns=""http://www.webserviceX.NET/"">
                        <FromCurrency>{0}</FromCurrency>
                        <ToCurrency>{1}</ToCurrency>
                    </ConversionRate>
                </s:Body>
              </s:Envelope>";
        //add your desired currency types
        soapEnvelopeXml.LoadXml(string.Format(soapEnvelope, "USD", "EUR"));
        //add your xml to request
        using (Stream stream = request.GetRequestStream())
        {
            soapEnvelopeXml.Save(stream);
        }
        //call soap service
        using (WebResponse response = request.GetResponse())
        {
            using (StreamReader rd = new StreamReader(response.GetResponseStream()))
            {
                string soapResult = rd.ReadToEnd(); //read the xml result
                //you can handle xml and get the conversion result, but here 
                //I'm outputting raw xml.
                Console.WriteLine(soapResult);
            }
        }

        Console.Read();
    }

    public static HttpWebRequest CreateWebRequest()
    {
        //create web request for calling soap service
        HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(@"http://www.webservicex.net/CurrencyConvertor.asmx");
        //add soap header
        webRequest.Headers.Add(@"SOAP:Action");
        //content type should be text/xml
        webRequest.ContentType = "text/xml; charset=\"utf-8\"";
        //response also will be in xml, so request should accept it
        webRequest.Accept = "text/xml";
        webRequest.Method = "POST";
        return webRequest;
    }

As you see, you should use xml for sending request. I added comments where they are necessary.