Error while consuming webservice in c#

Question

I am making a web service in c# , to consume its data via javascript (without being an application made in Asp.net or any technology . NET ,I want to use html and javascript only).

My web service was configured like this:

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.ComponentModel.ToolboxItem(false)]
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
[System.Web.Script.Services.ScriptService]
public class PhoneGap : System.Web.Services.WebService
{


    [WebMethod]
    public string HelloWorld()
    {
        return "Hello World";
    }
}

And my ajax was like this :

function GetData() {
    $.ajax({
        type: 'POST'
        //Caminho do WebService + / + nome do metodo
        , url: "http://localhost:3458/PhoneGap.asmx/HelloWorld"
         , contentType: 'application/json; charset=utf-8'
            , dataType: 'json'
            //Abaixo adicione as variáveis caso haja alguma.
            , data: ''
            , success: function (data, status) {
                //Caso não ocorra nenhum tipo de erro:
                $('.valor').text(data.d);
            }
            , error: function (xmlHttpRequest, status, err) {
                //Caso ocorra algum erro:
                $('.valor').html('Ocorreu um erro');
            }
    });
}

When I am going to take the test to consume the web service Hello Word is giving error(Note: Ententei thinks if I could use the web service locally mind for testing but I did not find anything using html and ajax apeans)

Error:

Failed to load http://localhost:3458/Phonegap.asmx/Helloworld: Response to preflight request doesn’t pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested Resource. Origin 'null' is therefore not allowed access.

Answer 1

In order for the call to work from a different source, you must set up Cross-Origin Resource Sharing (CORS). For an ASP.Net project with ASMX, the following section on the web.Config can solve your problem:

<system.webServer>
    <httpProtocol>
      <customHeaders>
        <add name="Access-Control-Allow-Headers" value="accept, content-type" />
        <add name="Access-Control-Allow-Origin" value="*"/>
        <add name="Access-Control-Allow-Methods" value="POST, GET, OPTIONS" />
     </customHeaders>
</httpProtocol>

However, in order to be able to call the ASMX Webservice method from Javascript, the Webservice class must contain the [Scriptservice] attribute, thus:

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.ComponentModel.ToolboxItem(false)]
[ScriptService] // AQUI
public class WebService : System.Web.Services.WebService

Only one observing: is it really necessary to use ASMX for this project? ASMX webservices are a legacy technology and should not be considered in new projects. I recommend a search on ASP.Net Web API or even WCF.