Response to preflight: “No 'Access-Control-Allowed-Origin'” despite CORS-enabled

Question

I am having trouble figuring out an issue with an angular 4 preflight request not passing a CORS access-control check: “No 'Access-Control-Allowed-Origin'”. I am able to successfully get data from the db but not able to post/save data. I am using VS code for my front end to access my visual studio 2015 back end. My Web Api controller has an attribute of:

[EnableCors(origins: "*", headers: "*", methods: "*")]

    [HttpPost]
    public async Task<IHttpActionResult> Post([FromBody]Employee employee)
    {
        _repo.Create(employee);
        return Ok();
    }

…but when request is made, I'm getting an error of:

XMLHttpRequest cannot load http://localhost:54429/api/createEmployee/. Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:4200' is therefore not allowed access

In my vs code, my service looks like this:

postEmployeeForm(employee: Employee): Observable<any>{

    let body = JSON.stringify(employee); 
    let headers = new Headers();
    headers.append('Content-Type', 'application/json');
    let options       = new RequestOptions({ headers: headers });

    console.log('posting employee ' , employee);

    return this.http.post("http://localhost:54429/api/employees/", body, options)
                    .map(this.extractData)
                    .catch(this.handleError)


}

405 Response after removing the body and options from the post.

Not sure what I am missing.

web config

<configuration>
  <configSections>
    <!-- For more information on Entity Framework configuration, visit http://go.microsoft.com/fwlink/?LinkID=237468 -->
    <section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
  </configSections>
  <connectionStrings>
    <add name="DefaultConnection" connectionString="Data Source=(LocalDb)\MSSQLLocalDB;AttachDbFilename=|DataDirectory|\aspnet-TBryant.WebAPI-20170303082842.mdf;Initial Catalog=aspnet-TBryant.WebAPI-20170303082842;Integrated Security=True" providerName="System.Data.SqlClient" />
    <!--<add name="SampleEntities" connectionString="metadata=res://*/MyModels.csdl|res://*/MyModels.ssdl|res://*/MyModels.msl;provider=System.Data.SqlClient;provider connection string=&quot;data source=(localdb)\MSSQLLocalDB;initial catalog=Sample;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework&quot;" providerName="System.Data.EntityClient" />-->
    <add name="northwindEntities" connectionString="metadata=res://*/NorthwindModel.csdl|res://*/NorthwindModel.ssdl|res://*/NorthwindModel.msl;provider=System.Data.SqlClient;provider connection string=&quot;data source=(localdb)\MSSQLLocalDB;initial catalog=northwind;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework&quot;" providerName="System.Data.EntityClient" />
    <add name="SampleEntities1" connectionString="metadata=res://*/EmployeeDataModel.csdl|res://*/EmployeeDataModel.ssdl|res://*/EmployeeDataModel.msl;provider=System.Data.SqlClient;provider connection string=&quot;data source=(localdb)\MSSQLLocalDB;initial catalog=Sample;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework&quot;" providerName="System.Data.EntityClient" />
  </connectionStrings>
  <appSettings></appSettings>
  <system.web>
    <authentication mode="None" />
    <compilation debug="true" targetFramework="4.6.1" />
    <httpRuntime targetFramework="4.6.1" />
  </system.web>
  <system.webServer>
    <!--<httpProtocol>
      <customHeaders>
        <add name="Access-Control-Allow-Origin" value="*"/>
        <add name="Access-Control-Allow-Headers" value="Content-Type"/>
        <add name="Access-Control-Allow-Methods" value="GET,POST,PUT,DELTE,OPTIONS"/>
      </customHeaders>
    </httpProtocol>-->
    <modules>
      <remove name="FormsAuthentication" />
    </modules>
    <handlers>
      <remove name="ExtensionlessUrlHandler-Integrated-4.0" />
      <remove name="OPTIONSVerbHandler" />
      <remove name="TRACEVerbHandler" />
      <add name="ExtensionlessUrlHandler-Integrated-4.0" path="*." verb="*" type="System.Web.Handlers.TransferRequestHandler" preCondition="integratedMode,runtimeVersionv4.0" />
    </handlers>
  </system.webServer>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="Microsoft.Owin.Security" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="0.0.0.0-3.0.1.0" newVersion="3.0.1.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="Microsoft.Owin.Security.OAuth" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="0.0.0.0-3.0.1.0" newVersion="3.0.1.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="Microsoft.Owin.Security.Cookies" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="0.0.0.0-3.0.1.0" newVersion="3.0.1.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="Microsoft.Owin" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="0.0.0.0-3.0.1.0" newVersion="3.0.1.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="Newtonsoft.Json" culture="neutral" publicKeyToken="30ad4fe6b2a6aeed" />
        <bindingRedirect oldVersion="0.0.0.0-6.0.0.0" newVersion="6.0.0.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="System.Web.Optimization" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="1.1.0.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="WebGrease" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="0.0.0.0-1.5.2.14234" newVersion="1.5.2.14234" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="System.Web.Helpers" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0-3.0.0.0" newVersion="3.0.0.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="System.Web.Mvc" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0-5.2.3.0" newVersion="5.2.3.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="System.Web.WebPages" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0-3.0.0.0" newVersion="3.0.0.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="System.Web.Cors" publicKeyToken="31bf3856ad364e35" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-5.2.3.0" newVersion="5.2.3.0" />
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
  <entityFramework>
    <defaultConnectionFactory type="System.Data.Entity.Infrastructure.SqlConnectionFactory, EntityFramework" />
    <providers>
      <provider invariantName="System.Data.SqlClient" type="System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer" />
    </providers>
  </entityFramework>
  <system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701" />
      <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:14 /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+" />
    </compilers>
  </system.codedom>
</configuration>

Answer 1

I have managed to reproduce your problem. The solution that worked for me is to replace part of Web.config. Under system.webServer, either add or replace the existing <handlers> block with the following:

<handlers>
  <remove name="ExtensionlessUrlHandler-Integrated-4.0" />
  <remove name="OPTIONSVerbHandler" />
  <remove name="TRACEVerbHandler" />
  <add name="ExtensionlessUrlHandler-Integrated-4.0" path="*." verb="*" type="System.Web.Handlers.TransferRequestHandler" preCondition="integratedMode,runtimeVersionv4.0" />
</handlers>

If you already have that exact content in place, try removing it, running the server, stopping the server, re-adding the content and then running the server again. I know that sounds odd but I think it's what ended up fixing it for me. Good luck.

UPDATE #1

Now that we have something happening on the server side of things, try removing the Content-Type header and JSON.stringify stuff in your Angular code. i.e.:

postEmployeeForm(employee: Employee): Observable<any>{
    console.log('posting employee ' , employee);

    return this.http.post("http://localhost:54429/api/employees/", employee)
        .map(this.extractData)
        .catch(this.handleError)
}

UPDATE #2

I think your IIS Express configuration may be overriding some of the changes we've made. Close Visual Studio, delete the .vs/config folder at the root of the solution in Windows Explorer and then try re-running the project. This should reset your IIS Express settings. If you're not completely comfortable with deleting the folder, just rename it and follow the same process.

UPDATE #3

I've managed to get your example project running. It demonstrates a problem with CORS, but this was fixed by using my initial suggestion of adding config.EnableCors:

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        // Web API configuration and services
        config.EnableCors();

        // Web API routes
        config.MapHttpAttributeRoutes();

        config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "api/{controller}/{id}",
            defaults: new { id = RouteParameter.Optional }
        );
    }
}

Answer 2

The problem seems to be on the server side.

Before any non-simple CORS request, the browser sends an OPTIONS preflight request to make sure that this server allows non-simple CORS requests. (POST requests with application/json are not considered simple)

In your case, as the error implies, only if the response to the preflight OPTIONS request contains the access-control-allow-origin header with the value 'http://localhost:4200' your original POST request will be sent. (The response in your screenshot doesn't contain this header)

You didn't write much about your server implementation, but if you're using asp.net-web-api here is a simple (a bit hacky) solution on how to make it respond to these preflight OPTIONS requests - link (Also see the comment there by Marcus Cunningham)