How can I debug a 500 Internal Server Error when calling a WebApi from ajax?

Question

I am receiving a 500 Internal Server Error in a MVC 4.5 WebApi project. I can successfully call my webservice with a GET and a GET with an Id. But, when I POST a file I am getting the error. I can set a breakpoint in Application_BeginRequest() and confirm that I am receiving first an OPTIONS request and then the POST. The method in the controller is not getting called and I have added an Application_Error() method to Global.asax.cs that does not get hit either. The html page is doing a CORS but I have already handled that using ThinkTecture.IdentityModel. I am following the code here for the file upload.

Any ideas?

Here is the client code:

    <script type="text/javascript">
        var UploadDocument = function () {
            var url = sessionStorage.getItem("url");
            var auth = sessionStorage.getItem("auth");
            var data = new FormData();

            jQuery.each($('#fileToUpload')[0].files, function (i, file) {
                data.append('file-' + i, file);
            });

            jQuery.support.cors = true;
            $.ajax({
                type: "POST",
                url: url,
                data: data,
                cache: false,
                processData: false,
                contentType: false,
                //dataType: "json",
                headers: { Authorization: 'Basic ' + auth },
                crossDomain: true,
                success: function (data, textStatus, jqXHR) {
                    alert('Success');
                },
                error: function (jqXHR, textStatus, errorThrown) { alert('Error: ' + textStatus + ' ' + errorThrown.message); }
            });
        };

    </script>

My controller code looks like this:

    public int Post(HttpPostedFileBase FileToUpload)
    {
        // Do stuff with the file
    }

The request and response look like this:

Request URL:http://localhost:51234/api/TaxDocument
Request Method:POST
Status Code:500 Internal Server Error

Request Headers
Accept:*/*
Accept-Charset:ISO-8859-1,utf-8;q=0.7,*;q=0.3
Accept-Encoding:gzip,deflate,sdch
Accept-Language:en-US,en;q=0.8
Authorization:Basic YmNhbGxlbjpuZWxsYWM=
Connection:keep-alive
Content-Length:2054044
Content-Type:multipart/form-data; boundary=----WebKitFormBoundaryIGzPKhvRVwFXbupu
Host:localhost:51234
Origin:http://localhost:52386
Referer:http://localhost:52386/Upload.html
User-Agent:Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.4 (KHTML, like Gecko) Chrome/22.0.1229.94 Safari/537.4

Response Headers:
Access-Control-Allow-Credentials:true
Access-Control-Allow-Origin:http://localhost:52386
Cache-Control:no-cache
Content-Length:1133
Content-Type:application/json; charset=utf-8
Date:Tue, 06 Nov 2012 21:10:23 GMT
Expires:-1
Pragma:no-cache
Server:Microsoft-IIS/8.0
X-AspNet-Version:4.0.30319
X-Powered-By:ASP.NET
X-SourceFiles:=?UTF-8?B?QzpccHJvamVjdHNcU2F2ZU15VzJcU2F2ZU15VzIuV2Vic2VydmljZVxhcGlcVGF4RG9jdW1lbnQ=?=

Answer 1

To see all exceptions turn following setting on:

• VS 2013 (and below): Debug -> Exceptions -> CLR - check "when thrown".
• VS 2015: Debug -> Windows -> Exception Settings -> CLR

You may need to uncheck "Tools->Options->Debugging->My code only" option if exception is thrown really outside of your code.