How to properly create and download a dynamically generated binary *.xlsx file on .net MVC4?

Question 1

So this is how i eventually got it :

You can't use ajax to return a binary file - you'll receive its data - but will have no way of manifesting the binary file

I've used EPPlus as the excel library and passed the data through a post method within generic HTML form hidden fields. The output was generated using a stream as the return value thus keeping the user on the page without refreshes/redirects

Server:

//the expected data description - automatic tool @ http://json2csharp.com/
public class ExpectedJSON
{
    public class SomeDataSet
    {
        public string dude{ get; set; }
        public string what { get; set; }
        public string foo { get; set; }
        public string bar { get; set; }
        public string baz { get; set; }
        public int asd { get; set; }
        public string someDate { get; set; }
        public string wat { get; set; }
        public string grrr { get; set; }
    }

    public class AnotherDataSet
    {
        public int type { get; set; }
        public string date { get; set; }
        public string dancing { get; set; }
        public double camels { get; set; }
        public int are { get; set; }
        public int crying { get; set; }
        public double _for { get; set; }
        public double some { get; set; }
        public double beer { get; set; }
    }

    public class MoreData
    {
        public int dark { get; set; }
        public double side { get; set; }
        public int of { get; set; }
        public int the { get; set; }
        public double moon { get; set; }
        public double iz { get; set; }
        public double da { get; set; }
        public string bomb { get; set; }
    }
}

public class ExportToController : Controller
{
    public ActionResult Excel([FromUri(Name= "someData")] string someDataSet,
                              [FromUri(Name = "anotherData")] string anotherDataSet,
                              [FromUri(Name = "moreData")] string moreData)
    {
        ExpectedJSON.SomeDataSet someDataSetJson = JsonConvert.DeserializeObject<ExpectedJSON.SomeDataSet>(someData);
        ExpectedJSON.AnotherDataSet[] anotherDataSetJson = JsonConvert.DeserializeObject<Campaign.OnlineComulativeDailyBuildChart[]>(anotherData);
        ExpectedJSON.MoreData[] moreDataJson = JsonConvert.DeserializeObject<Campaign.OnlineSitesGrid[]>(moreData);
        string fileName = "file.xlsx";

        ExcelPackage p = new ExcelPackage();
        p.Workbook.Worksheets.Add("Some Sheet");
        ExcelWorksheet ws = p.Workbook.Worksheets[1];

        //excel library logic follows...

        //...
        //..


       //stream logic

        MemoryStream stream = new MemoryStream(p.GetAsByteArray());


        FileStreamResult result = new FileStreamResult(stream, "application/vnd.ms-excel")
        {
            FileDownloadName = fileName
        };

        return result;
    }
}

HTML:

<form id="exportToExcel" action="../ExportTo/Excel/" target="_blank" method="post">
  <input type="hidden" id="someData" name="someData" />
  <input type="hidden" id="anotherData" name="anotherData" />
  <input type="hidden" id="moreData" name="moreData" />
  <button type="submit">export</button>
</form>

All my data components (which needed to be exported) sign their data objects into a generic object so that on submit - the value attribute is populated with the json representation of it using JSON.stringify:

function bindEvents() {
    var exportForm = $("#exportToExcel");
    var someData= exportForm.find("#someData");
    var anotherData= exportForm.find("#anotherData");
    var moreData = exportForm.find("#moreData");
    exportForm.on('submit', function (e) {
        someData.attr("value", JSON.stringify(exporter.getData().someData));
        anotherData.attr("value", JSON.stringify(exporter.getData().anotherData));
        moreData .attr("value", JSON.stringify(exporter.getData().moreData));
    });
}

Minutes with rails/sinatra/nodeJS - days/weeks with .net - but there you have it.

Question 2

The are tons of questions like this:

I could go on with this :)

Bottom line they all suggest using some kind of redirection through a <form>, an <iframe> or window.location.

This is probably the way to go, however if you're feeling adventurous enough I would also try some combination of HTML5 Blob API, File System and/or Typed Arrays.