Windows Live Writer 有点问题，着色代码看起来不清晰，所以贴的图片，完整代码在最后。

1：MVC实现

大致思路就是实现一个JsonpResult，在ExecuteResult内实现支持Jsonp，如图

MVC及WebAPI添加Jsonp支持

实际上这样就可以在Action内返回JsonpResult对象就可以了。但为了方便起见，也可以添加个拓展。

MVC及WebAPI添加Jsonp支持

这样一来我们就可以这样写

MVC及WebAPI添加Jsonp支持

前台用ajax调用如下

MVC及WebAPI添加Jsonp支持

2：WebApi实现

这里有两种用法，一种是自定义实现一个 JsonpMediaTypeFormatter，另外一种即为Filter

首先是JsonpMediaTypeFormatter，如下

public class JsonpMediaTypeFormatter : JsonMediaTypeFormatter
{
        public string Callback { get; private set; }

        public JsonpMediaTypeFormatter(string callback = null)
        {
            this.Callback = callback;
        }
        public override Task WriteToStreamAsync(Type type, object value, Stream writeStream, HttpContent content, TransportContext transportContext)
        {
            if (string.IsNullOrEmpty(this.Callback))
            {
                return base.WriteToStreamAsync(type, value, writeStream, content, transportContext);
            }
            try
            {
                this.WriteToStream(type, value, writeStream, content);
                return Task.FromResult<AsyncVoid>(new AsyncVoid());
            }
            catch (Exception exception)
            {
                TaskCompletionSource<AsyncVoid> source = new TaskCompletionSource<AsyncVoid>();
                source.SetException(exception);
                return source.Task;
            }
        }
        private void WriteToStream(Type type, object value, Stream writeStream, HttpContent content)
        {
            JsonSerializer serializer = JsonSerializer.Create(this.SerializerSettings);
            using (StreamWriter streamWriter = new StreamWriter(writeStream, this.SupportedEncodings.First()))
            using (JsonTextWriter jsonTextWriter = new JsonTextWriter(streamWriter) { CloseOutput = false })
            {
                jsonTextWriter.WriteRaw(this.Callback + "(");
                serializer.Serialize(jsonTextWriter, value);
                jsonTextWriter.WriteRaw(")");
            }
        }
        public override MediaTypeFormatter GetPerRequestFormatterInstance(Type type, HttpRequestMessage request, MediaTypeHeaderValue mediaType)
        {
            if (request.Method != HttpMethod.Get)
            {
                return this;
            }
            string callback;
            if (request.GetQueryNameValuePairs().ToDictionary(pair => pair.Key,
                 pair => pair.Value).TryGetValue("callback", out callback))
            {
                return new JsonpMediaTypeFormatter(callback);
            }
            return this;
        }
        [StructLayout(LayoutKind.Sequential, Size = )]
        private struct AsyncVoid
        { }
}

在Global.asax内注册

MVC及WebAPI添加Jsonp支持