How to set default value to select list in MVC during run time

I have view in which it loop through the model and display the details in editable mode. One of the model value is from a select list like below

@if (Model != null)
    {
    for (int i = 0; i < Model.provider_service_dtls.Count; i++)
     {
    <tr>
    <td> @Html.DropDownListFor(m => m.provider_service_dtls[i].activity_code_type,
 (SelectList)@ViewBag.activity_code_type, "--- Select Activity Code Type ---", new { @class = "m-wrap" })</td>
    <td>@Html.TextBoxFor(m => m.provider_service_dtls[i].activity_name)</td>    
    </tr>

    }
    } 

Here the ViewBag.activity_code_type contain the values Internal & Standard when submitting if user selected Internal its value 1 will pass to controller and if Standard it will be 2 and here the default value will be "--- Select Activity Code Type ---"

Now when i open the same request in edit mode if the model value for provider_service_dtls[i].activity_code_type is 1 the select list should be default select as Internal and Standard if it is 2.

I coded like this

    @Html.DropDownListFor(m => m.provider_service_dtls[i].activity_code_type,
 (SelectList)@ViewBag.activity_code_type, Model.provider_service_dtls[i].activity_code_type)

But it is not working as expected it is giving the result as below picture

Here it should default selected Internal. What is the change to do achieve the same?

Edited

Model

public partial class provider_service_dtls
    {
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public long service_id { get; set; }

        public long preapproval_id { get; set; }

        public string activity_code_type { get; set; }
        public string activity_type { get; set; }

        public string activity_type_name { get; set; }


        public string activity_code { get; set; }
        public string activity_name { get; set; }
        public string internal_activity_code { get; set; }
        public string internal_activity_name { get; set; }


        [ForeignKey("preapproval_id"), InverseProperty("provider_service_dtls")]
        public virtual provider_preapproval preapproval { get; set; }


    }

Editor template

@model Provider.Models.provider_preapproval
@Html.DropDownListFor(m => m.activity_code_type, (SelectList)ViewData["options"])

View

Inside the for loop i coded like this

  @Html.EditorFor(m => m.provider_service_dtls,
new { options = (SelectList)@ViewBag.activity_code_type })

I am getting an error

'System.Web.Mvc.HtmlHelper' does not contain a definition for 'EditorFor' and the best extension method overload 'System.Web.Mvc.Html.EditorExtensions.EditorFor(System.Web.Mvc.HtmlHelper, System.Linq.Expressions.Expression>, string, object)' has some invalid arguments

Unfortunately @Html.DropDownListFor() behaves a little differently than other helpers when rendering controls in a loop. This has been previously reported as an issue on CodePlex (not sure if its a bug or just a limitation)

Create a custom EditorTemplate for the type in the collection.

In /Views/Shared/EditorTemplates/provider_service_dtls.cshtml (note the name must match the name of the type)

@model yourAssembly.provider_service_dtls

@Html.HiddenFor(m => m.service_id)
@Html.DropDownListFor(m => m.activity_code_type, (SelectList)ViewData["options"], "--- Select Activity Code Type ---")
.... // html helpers for other properties of provider_service_dtls

and then in the main view, pass the SelectList to the EditorTemplate as additionalViewData

@model yourAssembly.provider_preapproval 

@using (Html.BeginForm())
{
  .... // html helpers for other properties of provider_preapproval

  @Html.EditorFor(m => m.provider_service_dtls, new { options = (SelectList)@ViewBag.activity_code_type })
  ...

The EditorFor() methods accepts IEnumerable<T> and will generate the controls for each item in the collection

Edit

An alternative is to create a new SelectList in each iteration of the for loop, where you need to set the Selected property of SelectList. This means that your ViewBag property must be IEnumerable<T>, not a SelectList, for example in the controller

ViewBag.ActivityCodeTypeList = new[]
{
  new { ID = 1, Name = "Internal" },
  new { ID = 2, Name = "Standard" }
}

and in the view

for (int i = 0; i < Model.provider_service_dtls.Count; i++)
{
  @Html.DropDownListFor(m => m => m.provider_service_dtls[i].activity_code_type,
    new SelectList(ViewBag.ActivityCodeTypeList, "ID", "Name", Model.provider_service_dtls[i].activity_code_type),
    "--- Select Activity Code Type ---")
}