Repopulating selectlist for dropdownlist on postback when default selected ASP.NET MVC - asp.net-mvc

Question

New to ASP.NET MVC and I've come across this before, however I feel I've gone about fixing it in an incorrect way. I was hoping someone could point me in the direction of the correct way of doing it.
I have a page which the user selects a dropdown value and clicks next. Now, if they don't select an item and select the default value ("Select..."), there is a validation error. The controller seems to lose information about the dropdownlist even though the model is returned to the view on a "postback." So, say I don't basically repeat code in my [HTTPPost] from my [HTTPGet]. I get the error:
There is no ViewData item of type 'IEnumerable<SelectListItem>' that has the key 'SheetIndex'.
on
#Html.DropDownListFor(model => model.SheetIndex, Model.SheetsDropdown, "Select...", new { #class = "form-control" })
So unless I repeat code, I get that error when no selection is made. What am I doing wrong?
ViewModel:
public class SelectSheetViewModel
{
public int? Id { get; set; }
public string Name { get; set; }
[Required]
public string SheetIndex { get; set; }
public string SheetName { get; set; }
public int? ChainId { get; set; }
public int? SheetId { get; set; }
public int? FileId { get; set; }
public IEnumerable<SelectListItem> SheetsDropdown { get; set; }
public HeaderViewModel Header { get; set; }
}
Controller:
[HttpGet]
public ActionResult SelectSheet(int? chainId, int? sheetId, int? fileId)
{
if (sheetId == null || fileId == null)
{
return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
}
var fileDetails = db.FileDetails.Find(fileId);
SelectSheetViewModel selectSheet = new SelectSheetViewModel()
{
Id = fileDetails.FileId,
Name = fileDetails.Name,
ChainId = chainId,
SheetId = sheetId,
FileId = fileId
};
string fileName = fileDetails.UniqueName + fileDetails.Extension;
string relativeFileLocation = "~/uploads/" + fileName;
string absoluteFileLocation = HttpContext.Server.MapPath(relativeFileLocation);
if (System.IO.File.Exists(absoluteFileLocation))
{
DSDBuilder builder = new DSDBuilder();
selectSheet.SheetsDropdown = builder.GetSheets(absoluteFileLocation); // Where I get my selectlist
}
else
{
ModelState.AddModelError("SheetDropdown", "Excel workbook does not exist.");
}
selectSheet.Header = BuildHeaderViewModel(chainId, sheetId);
return View(selectSheet);
}
[HttpPost]
public ActionResult SelectSheet(SelectSheetViewModel selectSheet)
{
if (ModelState.IsValid)
{
FileDetail fileDetails = db.FileDetails.Find(selectSheet.FileId);
string[] sheetIndexAndName = selectSheet.SheetIndex.Split(':');
fileDetails.SheetIndex = Convert.ToInt32(sheetIndexAndName[0]);
fileDetails.SheetName = sheetIndexAndName[1];
db.SaveChanges();
return RedirectToAction("Build", "Sheets", new
{
ChainId = selectSheet.ChainId,
SheetId = selectSheet.SheetId,
FileId = selectSheet.FileId
});
}
// Probably not a good method vvv
var fileDetailsPostBack = db.FileDetails.Find(selectSheet.FileId);
string fileName = fileDetailsPostBack.UniqueName + fileDetailsPostBack.Extension;
string relativeFileLocation = "~/uploads/" + fileName;
string absoluteFileLocation = HttpContext.Server.MapPath(relativeFileLocation);
if (System.IO.File.Exists(absoluteFileLocation))
{
DSDBuilder builder = new DSDBuilder();
selectSheet.SheetsDropdown = builder.GetSheets(absoluteFileLocation);
}
else
{
ModelState.AddModelError("SheetDropdown", "Excel workbook does not exist.");
}
// Probably not a good method ^^^
return View(selectSheet);
}
View:
#using (Html.BeginForm())
{
#Html.AntiForgeryToken()
#Html.ValidationSummary(true, "", new { #class = "text-danger" })
#Html.HiddenFor(model => model.ChainId)
#Html.HiddenFor(model => model.SheetId)
#Html.HiddenFor(model => model.FileId)
#Html.HiddenFor(model => model.Header.ChainName)
#Html.HiddenFor(model => model.Header.SheetName)
#Html.HiddenFor(model => model.Header.SheetDescription)
<div class="form-horizontal">
<div class="form-group">
#Html.LabelFor(model => model.SheetIndex, htmlAttributes: new { #class = "control-label col-md-2" })
<div class="col-md-10">
#Html.DropDownListFor(model => model.SheetIndex, Model.SheetsDropdown, "Select...", new { #class = "form-control" })
#Html.ValidationMessageFor(model => model.SheetIndex, "", new { #class = "text-danger" })
</div>
</div>
<div class="form-group">
<div class="col-md-offset-2 col-md-10">
<input type="submit" value="Next" class="btn btn-default" />
</div>
</div>
</div>
}
I also would not like to repeat that code because it goes to a function which uses Excel.Interop and is kind of resource heavy. Let me know if anyone has a better solution. I'm always trying to improve my code and do things the "correct" way.