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