Why return 500 error while calling another controller using WebClient? - asp.net-mvc-4

Question

I'm testing example on this link: http://msdn.microsoft.com/en-us/vs11trainingcourse_aspnetmvc4_topic5#_Toc319061802 but I have 500 error calling another controller using WebClient.
When I access to "http://localhost:2323/photo/gallery directly is running, but I'm trying from action using WebClient it return 500 error? Why?"
public ActionResult Index()
{
WebClient client = new WebClient();
var response = client.DownloadString(Url.Action("gallery", "photo", null, Request.Url.Scheme));
var jss = new JavaScriptSerializer();
var result = jss.Deserialize<List<Photo>>(response);
return View(result);
}
500 error created by below exception:
[ArgumentNullException: Value cannot be null.
Parameter name: input]
System.Text.RegularExpressions.Regex.Match(String input) +6411438
Microsoft.VisualStudio.Web.Runtime.Tracing.UserAgentUtilities.GetEurekaVersion(String userAgent) +79
Microsoft.VisualStudio.Web.Runtime.Tracing.UserAgentUtilities.IsRequestFromEureka(String userAgent) +36
Microsoft.VisualStudio.Web.Runtime.Tracing.SelectionMappingExecutionListenerModule.OnBeginRequest(Object sender, EventArgs e) +181
System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +136
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +69

Answer 1

It's hard to tell. Maybe the controller action that you are calling requires authorization? Or uses session? When you send your WebClient request it doesn't delegate any of the client cookies that were sent by the client to the Index action.
Here's how you can debug your code and see the exact response returned by the server:
WebClient client = new WebClient();
try
{
var response = client.DownloadString(Url.Action("gallery", "photo", null, Request.Url.Scheme));
}
catch (WebException ex)
{
using (var reader = new StreamReader(ex.Response.GetResponseStream()))
{
string responseText = reader.ReadToEnd(); // <-- Look here to get more details about the error
}
}
And if it turns out that the problem is related to the ASP.NET Session that your target controller action depends upon, here's how you could delegate the client cookies with the request:
WebClient client = new WebClient();
client.Headers[HttpRequestHeader.Cookie] = Request.Headers["Cookie"];

Answer 2

error occurred because of the User-Agent Header
Resolution is:
public ActionResult Index()
{
WebClient client = new WebClient();
client.Headers[HttpRequestHeader.UserAgent] = Request.Headers["User-Agent"];
var response = client.DownloadString(Url.Action("gallery", "photo", null, Request.Url.Scheme));
var jss = new JavaScriptSerializer();
var result = jss.Deserialize<List<Photo>>(response);
return View(result);
}