加载 HTML 时出现 MVC 错误。 具有键“MY KEY”的 ViewData 项的类型为“System.String”，但必须为“IEnumerable”类型<selectlistitem> '</selectlistitem>

Question

I am doing an MVC APP. For this example..

In my definition of my DropDownListFor I defined something like this.

  @Html.DropDownListFor(model => model.SelectedSystem, Model.Systems, new { @class = "form-control listbox",id="Origin" })

My Model is loaded in the Controller, where it loads Model.System under certain circumstances. Model.System is of type List<SelectListItem>

The option selected is in model.SelectedSystem that is a string type . That works fine...

The problem i am facing is when Model.System is null.

My Controller looks like this

public ActionResult Index()
        {
            var apiEntitySyncViewModel = new ApiEntitySyncViewModel
            {
                Systems = _entitySyncService.GetEnvironments(),
            };
            return View(apiEntitySyncViewModel);
        }

On running appears the message The ViewData item that has the key SelectedSystemOrigin is of type System.String but must be of type IEnumerable<SelectListItem>

How can I draw an empty DropDownListFor without having that mistake

Answer 1

try this

public ActionResult Index()
{
   var apiEntitySyncViewModel = new ApiEntitySyncViewModel
   {
       Systems = _entitySyncService.GetEnvironments(),
  };
if(apiEntitySyncViewModel.Systems==null) apiEntitySyncViewModel.Systems = new List <SelectListItem>();

//or you can try
new List <SelectListItem>{ new SelectListItem{ Value="0", Text ="Empty" } 


  return View(apiEntitySyncViewModel);
}