Redirect back to Previous Page without losing original data

I have trouble preserving the original data by redirecting the same page when my custom error handling is executed in the controller.
Assume that I have a web page call Create.cshtml.
In that create webpage, I have a few form control which require the user to enter class code but the class code cannot be duplicated.
Assume that the user entered a class code that is existed in the system, my system should redirect back to Create.cshtml and pass error message (E.g. ViewBag.error = “Class Code duplicated”) and simulatenously .
But my current implementation does not revert back the original content/data after redirect.

ClassController:

  [HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> Create([Bind("ID,ClassCode,ClassName,DateCreation,DegreeID,CourseChapterID")] Class @class)
        {
            if (ModelState.IsValid)
            {
                Class cls = await _context.Class.SingleOrDefaultAsync(c => c.ClassCode == @class.ClassCode);
                if (cls != null)
                {
                    TempData["error"] = "This class code has been existed in the system";
                ModelState.AddModelError("error", "This class code has been existed in the system");
                return RedirectToAction(nameof(Create),@class);
                }
                _context.Add(@class);
                await _context.SaveChangesAsync();
                return RedirectToAction(nameof(Index));
            }
            return View(@class);
        }

Create.cshtml

<form asp-action="Create">
    <div asp-validation-summary="ModelOnly" class="text-danger"></div>
    <div class="form-group">
        <label asp-for="ClassCode" class="control-label"></label>
        <input asp-for="ClassCode" class="form-control" />
        <span asp-validation-for="ClassCode" class="text-danger"></span>
    </div>
    <div class="form-group">
        <label asp-for="ClassName" class="control-label"></label>
        <input asp-for="ClassName" class="form-control" />
        <span asp-validation-for="ClassName" class="text-danger"></span>
    </div>
    @if (@TempData["error"] != null)
    {
        <div class="form-group">
            <label class="control-label">@TempData["error"]</label>
        </div>
    }
    <div class="form-group">
        <input type="submit" value="Create" class="btn btn-primary" />
    </div>
</form>

System environment: .NET Core Entity Framework

Answers:

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Method 1

When you use redirection, you should use TempData instead of ViewBag.

TempData["error"] = "This class code has been existed in the system";

return RedirectToAction(nameof(Create));

You are redirect to the create get method, if it doesn’t return the model, the data will lose, I think you can directly return View() here.

if (cls != null)
{
    TempData["error"] = "This class code has been existed in the system";
    ModelState.AddModelError("error", "This class code has been existed in the system");
    return View(nameof(Create), @class);
}

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