c# – 为什么我无法读取Http请求输入流两次？

这是在.NET 4中并使用WebApi,我试图在调试代码中打印出http请求的正文.为了做到这一点,我寻求输入流回读流.它第一次工作正常,但如果我再次读取,我会得到一个空字符串.

为什么我不能再次回头看看InputStream？在下面的示例中,body2始终为空.在第二组中,CanSeek仍然为真,第二次调用ReadToEnd()返回一个空字符串覆盖默认值.

using System.IO;
using System.Net;
using System.Net.Http;
using System.Web;
using System.Web.Http;

public class TestController : ApiController
{

    public class TestOutuput
    {
        public string firstRead;
        public string secondRead;
    }

    public HttpResponseMessage Post()
    {
        string body1 = "default for one";
        string body2 = "default for two";
        if (HttpContext.Current.Request.InputStream.CanSeek)
        {
            HttpContext.Current.Request.InputStream.Seek(0,System.IO.SeekOrigin.Begin);
        }
        using (var reader = new StreamReader(HttpContext.Current.Request.InputStream))
        {
            body1 = reader.ReadToEnd();
        }

        if (HttpContext.Current.Request.InputStream.CanSeek)
        {
            HttpContext.Current.Request.InputStream.Seek(0,System.IO.SeekOrigin.Begin);
        }
        using (var reader2 = new StreamReader(HttpContext.Current.Request.InputStream))
        {
            // this is always empty,even after seek back to origin
            body2 = reader2.ReadToEnd();
        }

        TestOutuput testOutput = new TestOutuput() { firstRead = body1,secondRead = body2 };
        HttpResponseMessage response = new HttpResponseMessage();
        return Request.CreateResponse<TestOutuput>(HttpStatusCode.OK,testOutput);
    }
}