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

Question

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    lt HttpPost gt   lt HttpGet gt   lt Route   view    gt  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     Try            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 POST and GET because the Paging object is optional   If I call this method with a GET 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 Try    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     reqStream Close      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 req Accept value to  application xml  or  text xml  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     ajax      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

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I was failing to send a body on a DELETE that required one and was getting this message as a result

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I also experienced this error   I add into header Content-Type  application json  Following the change  my submissions succeed

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SOLVED 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     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  reqStream Close       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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I experienced this issue when calling my web api endpoint and solved it   In my case it was an issue in the way the client was encoding the body content  I was not specifying the encoding or media type  Specifying them solved it   Not specifying encoding type  caused 415 error   var content   new StringContent postData   httpClient PostAsync uri  content     Specifying the encoding and media type  success   var content   new StringContent postData  Encoding UTF8   application json    httpClient PostAsync uri  content

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In my case it is Asp Net Core 3 1 API  I changed the HTTP GET method from public ActionResult GetValidationRulesForField  GetValidationRulesForFieldDto getValidationRulesForFieldDto  to public ActionResult GetValidationRulesForField  FromQuery  GetValidationRulesForFieldDto getValidationRulesForFieldDto  and its working

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I was trying to write a code that would work on both Mac and Windows  The code was working fine on Windows  but was giving the response as  Unsupported Media Type  on Mac  Here is the code I used and the following line made the code work on Mac as well    Request AddHeader  Content-Type    application json    Here is the snippet of my code   Dim Client As New WebClient Dim Request As New WebRequest Dim Response As WebResponse Dim Distance As String  Client BaseUrl    http   1 1 1 1 8080 config  Request AddHeader  Content-Type    application json      The line that made the code work on mac  Set Response   Client Execute Request

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

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