setting enctype = "multipart/form-data" causes all ViewBags value be lost

Question

this is my View containing an upload file control. so it needs "multipart/form-data" content type.

@using (Html.BeginForm("Create", "RoutinTest", FormMethod.Post,
new { enctype = "multipart/form-data" }))
{
    @Html.AntiForgeryToken()

<div>
            @Html.LabelFor(model => model.ScannedFile)
            <div>
                <input type="file" name="ScannedFile"  multiple="multiple" />
                @Html.ValidationMessageFor(model => model.ScannedFile)
            </div>
</div>
<div>
            <div>
                <input type="submit" value="Save"  />
            </div>
 </div>   

this is my controller and it's Create action methods:

[HttpGet]
 public ActionResult Create(string patientId, string fullName)
        {
            ViewBag.PatientId = patientId;
            ViewBag.FullName = fullName;
            return View();
        }

[HttpPost]
public ActionResult Create() RoutinTest routintest, string patientId, string fullName)
{

}

The problem is that On second Create action method (post method) both patientId and fullName are lost. (patientId = 0 and fullName = null) but I set both of them on Get method.

Amazingly when I change the form content type to it's default (by removing multipart/form-data), I am able to have those two parameters values. I know I can solve the problem without changing the form content type, by setting those two ViewBags value to hidden fields but I just want to know why this happens? What affect the "multiPart/form-data" form content type has on those ViewBags values?

thanks

Answer 1

Remember that your patientId and fullName is found in the URL. Reassigning them to the ViewBag has nothing to do with it (remove the ViewBag and see what happens)

// Looks familliar? This is your first request.
Create?patientId=0&fullName=

The parameters is passed to the second Create action method by default because the URL parameters is maintained.

// By default, parameters are not cleared during the second request.
Create?patientId=0&fullName=

When you set enctype = "multipart/form-data", the URL parameters gets cleared, resulting in something like this.

Create

Which means that it is the parameters that are lost and not the ViewBag data.

Related Problem: Form Post with enctype = "multipart/form-data" causing parameters to not get passed