WebAPI接收JSON参数注意事项

运行环境：ASP.NET 4.5.2。

当我们向GlobalConfiguration.Configuration.MessageHandlers添加一个DelegatingHandler派生类后，很容易发生即使命中了Action，但方法参数值为null的问题。

在大多数情况下，我们会在DelegatingHandler派生类里，重写async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)方法。

在方法内，做一些事情，比如说访问日志记录，任意校验，添加Header，修改URL(重写)等工作。

其中最重要需要注意的一点在于request.Content，当我们在方法内访问了request.Content (get)之后，而不对request.Content进行赋值(set)的话，会发生什么呢？

这会导致我们的方法(action)无法获取到客户端Post上来的数据，导致方法参数值为null。

这是为什么呢，这个中原因，我没去深究。

现在附上解决代码：

 public class DefaultHandler : DelegatingHandler
 {
     protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
     {
         request.RequestUri = new Uri(request.RequestUri.ToString());

         MediaTypeHeaderValue contentType = request.Content.Headers.ContentType;

         if (contentType != null)
         {
             switch (contentType.MediaType)
             {
                 case "application/x-www-form-urlencoded":
                     {
                         NameValueCollection formData = await request.Content.ReadAsFormDataAsync(cancellationToken);
                         request.Content = new FormUrlEncodedContent(Correct(formData));
                         //TODO:在这里对formData进行业务处理
                     }
                     break;
                 case "multipart/form-data":
                     {
                         NameValueCollection formData = await request.Content.ReadAsFormDataAsync(cancellationToken);
                         request.Content = new FormUrlEncodedContent(Correct(formData));
                         //TODO:在这里对formData进行业务处理
                     }
                     break;
                 case "application/json":
                     {
                         HttpContentHeaders oldHeaders = request.Content.Headers;
                         string formData = await request.Content.ReadAsStringAsync();
                         request.Content = new StringContent(formData);
                         ReplaceHeaders(request.Content.Headers, oldHeaders);
                         //TODO:在这里对formData进行业务处理
                     }
                     break;
                 default:
                     throw new Exception("Implement It!");
             }
         }

         return await base.SendAsync(request, cancellationToken);
     }

     private static IEnumerable<KeyValuePair<string, string>> Correct(NameValueCollection formData)
     {
         return formData.Keys.Cast<string>().Select(key => new KeyValuePair<string, string>(key, formData[key])).ToList();
     }

     private static void ReplaceHeaders(HttpHeaders currentHeaders, HttpHeaders oldHeaders)
     {
         currentHeaders.Clear();
         foreach (var item in oldHeaders)
             currentHeaders.Add(item.Key, item.Value);
     }
 }

在Global.asax添加代码：

 protected void Application_Start()
 {
     GlobalConfiguration.Configure(WebApiConfig.Register);
     GlobalConfiguration.Configuration.MessageHandlers.Add(new DefaultHandler());
 }

模型类：

 public class TestModel
 {
     [JsonProperty(PropertyName ="I")]
     public long Id { get; set; }

     [JsonProperty(PropertyName = "N")]
     public string Name { get; set; }

     [JsonProperty(PropertyName = "M")]
     public decimal Money { get; set; }

     [JsonProperty(PropertyName = "IE")]
     public bool IsEnable { get; set; }

     [JsonProperty(PropertyName = "CD")]
     public DateTime CreateDate { get; set; }

     [JsonProperty(PropertyName = "UD")]
     public DateTime? UpdateDate { get; set; }
 }

ApiController：

 public class DefaultController : ApiController
 {
     [HttpPost]
     public TestModel Find(TestModel model)
     {
         return model;
     }
 }

Request Body：

{"I":10000,"N":"TestModel","M":21547855.0001,"IE":true,"CD":"2015-12-10 12:12:12","UD":"2016-01-01 01:01:01"}

Fiddler4测试：

测试结果：

解决办法来自：http://stackoverflow.com/questions/27333419/modify-request-content-in-webapi-delegatinghandler