AK3800 AK3800 - 1 year ago 294
Vb.net Question

HTTP 415 unsupported media type error when calling Web API 2 endpoint

I have an existing Web API 2 service and need to modify one of the methods to take a custom object as another parameter, currently the method has one parameter which is a simple string coming from the URL. After adding the custom object as a parameter I am now getting a 415 unsupported media type error when calling the service from a .NET windows app. Interestingly, I can successfully call this method using javascript and the jquery ajax method.

The Web API 2 service method looks like this:

Public Function GetResultsWithView(view As String, pPaging As Paging) As HttpResponseMessage
Dim resp As New HttpResponseMessage
Dim lstrFetchXml As String = String.Empty
Dim lstrResults As String = String.Empty

'... do some work here to generate xml string for the response
'// write xml results to response
resp.Content = New StringContent(lstrResults)
resp.Content.Headers.ContentType.MediaType = "text/xml"
resp.Headers.Add("Status-Message", "Query executed successfully")
resp.StatusCode = HttpStatusCode.OK
Catch ex As Exception
resp.StatusCode = HttpStatusCode.InternalServerError
resp.Headers.Add("Status-Message", String.Format("Error while retrieving results from view {0}: {1}", view, ex.Message))
End Try
Return resp
End Function

The method allows both
because the
object is optional. If I call this method with a
request it works.

And the simple .NET client code calling the service looks like this:

Dim uri As String = BASE_URI + "fetch/someview"
Dim resp As HttpWebResponse
Dim sr As StreamReader
Dim lstrResponse As String
Dim reqStream As Stream
Dim bytData As Byte()
Dim req As HttpWebRequest = WebRequest.Create(uri)
Dim lstrPagingJSON As String
Dim lPaging As New Paging
lPaging.Page = 1
lPaging.Count = 100
lPaging.PagingCookie = ""
req.Method = "POST"
lstrPagingJSON = JsonSerializer(Of Paging)(lPaging)
bytData = Encoding.UTF8.GetBytes(lstrPagingJSON)
req.ContentLength = bytData.Length
reqStream = req.GetRequestStream()
reqStream.Write(bytData, 0, bytData.Length)
req.ContentType = "application/json"

resp = req.GetResponse()

sr = New StreamReader(resp.GetResponseStream, Encoding.UTF8)
lstrResponse = sr.ReadToEnd
'// do something with the response here
Catch exweb As WebException
txtOutput.AppendText("Error during request: " + exweb.Message)
Catch ex As Exception
txtOutput.AppendText(String.Format("General error during request to {0}: {1}", uri, ex.Message))
End Try

The .NET client is running on the 4.5 framework and the service is on 4.5.2 framework. The error is thrown at the
resp = req.GetResponse()
line. Some things I tried already:

  • on the client, set the
    value to "application/xml" or

  • in the service method, removed the line
    `resp.Content.Headers.ContentType.MediaType = "text/xml"

  • replace the XML response content with some static JSON, tried to rule out any problems with sending in JSON on the request and getting XML back on the response

So far I keep getting the same 415 error response no matter what I try.

I mentioned this works when called from javascript, here's my ajax call that is working:

headers: {},
url: "api/fetch/someview",
type: "POST",
data: "{Count:100,Page:1,PagingCookie:\"\"}",
contentType: "application/json; charset=utf-8",
dataType: "xml",
success: function (data) {
alert("call succeeded");
failure: function (response) {
alert("call failed");

On the service side, there is nothing fancy going on with the route config or anything else, it's pretty much all out-of-the-box Web API 2. I know the routing is working, calls are being correctly routed to the method, they're not going somewhere else unexpectedly, so what am I missing in the .NET client? Any help is much appreciated!

--- UPDATE ---

I tried to create a completely new Web API service to rule out any possible issues with the existing service, I created a controller with a single method that takes a custom object as the parameter. I then tried calling that from the .NET client and got the same error. I also tried using WebClient instead of HttpWebRequest, but still get the same error. This is also something that previously worked for me with Web API (prior to Web API 2).

--- UPDATE ---

I also tried creating a new web app using Web API 1, when I call that with a POST my complex object parameter is now coming in null. I have another web service running Web API 1 and verified that I can still call that successfully with complex objects. Whatever my problem is, it appears to be something with the JSON passing between the client and server. I have checked the JSON I'm sending it and its valid, the object definition is also an exact match between the client and server so the JSON should be able to be parsed by the server.

Answer Source

After banging my head on the wall for a couple days with this issue, it was looking like the problem had something to do with the content type negotiation between the client and server. I dug deeper into that using Fiddler to check the request details coming from the client app, here's a screenshot of the raw request as captured by fiddler:

Fiddler capture of http request from client app

What's obviously missing there is the Content-Type header, even though I was setting it as seen in the code sample in my original post. I thought it was strange that the Content-Type never came through even though I was setting it, so I had another look at my other (working) code calling a different Web API service, the only difference was that I happened to be setting the req.ContentType property prior to writing to the request body in that case. I made that change to this new code and that did it, the Content-Type was now showing up and I got the expected success response from the web service. The new code from my .NET client now looks like this:

req.Method = "POST"
req.ContentType = "application/json"
lstrPagingJSON = JsonSerializer(Of Paging)(lPaging)
bytData = Encoding.UTF8.GetBytes(lstrPagingJSON)
req.ContentLength = bytData.Length
reqStream = req.GetRequestStream()
reqStream.Write(bytData, 0, bytData.Length)
'// Content-Type was being set here, causing the problem
'req.ContentType = "application/json"

That's all it was, the ContentType property just needed to be set prior to writing to the request body

I believe this behavior is because once content is written to the body it is streamed to the service endpoint being called, any other attributes pertaining to the request need to be set prior to that. Please correct me if I'm wrong or if this needs more detail.

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