Using EPPlus

UsingEPPlus.cs

using (var excelPackage = new ExcelPackage(newFileInfo))
{
  var ws = excelPackage.Workbook.Worksheets["Work Sheet Name"];
  
  // do the stuff, here are some examples
  var cellRange = ws.Cells[string.Format("{0}{1}:{0}{2}", "A", 2, 4)];
  // merging cells
  cellRange.Merge = true;
  // setting value
  cellRange.Value = "Any object that serves as cell value";
  // alignment
  cellRange.Style.HorizontalAlignment = ExcelHorizontalAlignment.Center;
  cellRange.Style.VerticalAlignment = ExcelVerticalAlignment.Center;
  // setting color
  cellRange.Style.Font.Color.SetColor(System.Drawing.Color.DarkRed);
  // height for a row
  ws.Row(5).Height = 50;
  
  excelPackage.Save();
}

UsingEPPlusInAspNetWebApiController.cs

[HttpGet]
[Auth(Roles = "SomeRole")]
public HttpResponseMessage ExelFileDownload(Guid documentId)
{
  using (var stream = new MemoryStream())
  {
    // get template stream
    using (var templateStream = Assembly.GetExecutingAssembly()
      .GetManifestResourceStream("TemplateResource.xlsx"))
    {
      using (var excelPackage = new ExcelPackage(stream, templateStream))
      {
        // do stuff

        excelPackage.Save();
      }
    }

    // content as array of bytes
    var content = stream.ToArray();

    // create response
    var response = new HttpResponseMessage(HttpStatusCode.OK);
    response.Content = new ByteArrayContent(content);
    response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/msexcel");
    response.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
    {
      FileName = "SomeNameOfYourFile.xlsx"
    };
    return response;
  }
}