WebApi 中开启跨域请求方式

public class JsonpFormatter : JsonMediaTypeFormatter
    {
　　　　　//当请求过来是带有text/javascript时处理JSONP请求
        public JsonpFormatter()
        {
            SupportedMediaTypes.Add(new MediaTypeHeaderValue("application/json"));
            SupportedMediaTypes.Add(new MediaTypeHeaderValue("text/javascript"));

            JsonpParameterName = "callback";
        }

        //查找函数名  
        public string JsonpParameterName { get; set; }

        private string JsonpCallbackFunction;

        public override bool CanWriteType(Type type)
        {
            return true;
        }

        //重写此方法来捕获请求对象
        public override MediaTypeFormatter GetPerRequestFormatterInstance(Type type, System.Net.Http.HttpRequestMessage request, MediaTypeHeaderValue mediaType)
        {
            var formatter = new JsonpFormatter()
            {
                JsonpCallbackFunction = GetJsonCallbackFunction(request)
            };
            //运用JSON.NET来序列化自定义         
            formatter.SerializerSettings.Converters.Add(new StringEnumConverter());
            formatter.SerializerSettings.Formatting = Newtonsoft.Json.Formatting.Indented;

            return formatter;
        }

       //重写此方法写入到流并返回
        public override Task WriteToStreamAsync(Type type, object value,
                                        Stream stream,
                                        HttpContent content,
                                        TransportContext transportContext)
        {
            if (string.IsNullOrEmpty(JsonpCallbackFunction))
                return base.WriteToStreamAsync(type, value, stream, content, transportContext);

            StreamWriter writer = null;

            try
            {
                writer = new StreamWriter(stream);
                writer.Write(JsonpCallbackFunction + "(");
                writer.Flush();
            }
            catch (Exception ex)
            {
                try
                {
                    if (writer != null)
                        writer.Dispose();
                }
                catch { }

                var tcs = new TaskCompletionSource<object>();
                tcs.SetException(ex);
                return tcs.Task;
            }

            return base.WriteToStreamAsync(type, value, stream, content, transportContext)
                       .ContinueWith(innerTask =>
                       {
                           if (innerTask.Status == TaskStatus.RanToCompletion)
                           {
                               writer.Write(")");
                               writer.Flush();
                           }

                       }, TaskContinuationOptions.ExecuteSynchronously)
                        .ContinueWith(innerTask =>
                        {
                            writer.Dispose();
                            return innerTask;

                        }, TaskContinuationOptions.ExecuteSynchronously)
                        .Unwrap();
        }

        //从查询字符串中获得JSONP Callback回调函数
        private string GetJsonCallbackFunction(HttpRequestMessage request)
        {
            if (request.Method != HttpMethod.Get)
                return null;

            var query = HttpUtility.ParseQueryString(request.RequestUri.Query);
            var queryVal = query[this.JsonpParameterName];

            if (string.IsNullOrEmpty(queryVal))
                return null;

            return queryVal;
        }
    }

  

//加入自定义配置
GlobalConfiguration
                .Configuration
                .Formatters
                .Insert(0, new JsonpFormatter());

///首先加入全局配置：

 var cors = new EnableCorsAttribute("*", "*", "*");
            config.EnableCors(cors);


 eg:在需要使用的接口上加入特性标记即可

[EnableCors(origins: "*", headers: "*", methods: "*")]
    public class HomeController : Controller
    {
         //todo
    }