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Razor Html.DropDownListFor automatically set the selected value

I am new to Razor, I pass dropdown list as a viewbag and another values, when drop down list show in web it has selected values but i haven't set the any selected value, I think I use same name in viewbag value and dropdownlist value,it can be happen?
This is the viewbag,here i set the values and drop downlist:
ViewBag.GlobalInstance = globalInstance;
ViewBag.LegislativeCode = legislativeCode.Trim();
ViewBag.Legislatives = new SelectList(legislativeEntities, "LegislativeCode", "LegislativeName");
This is frontend code:
#if (ViewBag.GlobalInstance == true)
{
<div class="col-xs-5 hcm-form_controller mandatory gap" >
<label for="LegislativeCode" class="w-100" mi-resourcekey="atr-LegislativeCode" >Legislative Entity</label>
#Html.DropDownListFor(m => m.LegislativeCode, ViewBag.Legislatives as SelectList, "--- Select ---", new { #class = "calc-w-100" })
</div>
}

It seems that the page model has a value for LegislativeCode,
m => m.LegislativeCode
so when there is a same LegislativeCode in the ViewBag.Legislatives, this option will be selected.

How to set default value for ASP.NET MVC DropDownList from ViewBag

In the controller Index I have the following:
ViewBag.Assignees = (await GetAllUsers()).Select(a =>
new SelectListItem
{
Text = a.DisplayName,
Value = a.Username,
Selected = a.DisplayName == "John Smith"
}).OrderBy(x => x.Text).ToList();
In the View, I have the following:
#Html.DropDownListFor(model => model.Assignee,
ViewBag.Assignees as List<SelectListItem>,
"Select Assignee",
new { id = "ddlAssignee", #class = "form-control"})
The dropdownlist populates as expected, however, the default (selected = true) value, which does exist, does not get set. Can someone advise what is wrong in the above?
UPDATE:
By Changing the SelectListItem.Value to a.DisplayName (same as SelectedListItem.Text) I achieved it. Still not sure what prevents the dropdownlist from displaying the item with Selected = true

If the model.Assignee comes with value, and if it is an int it will be defaulted to 0, it will override the SelectListItem selected value.
I suggest to set up the model.Assignee.

Here two ways that i use.
WAY 1
#Html.DropDownListFor(model => model.Assignee,
ViewBag.Assignees as List<SelectListItem>,
"Value", // property to be set as Value of dropdown item
"Text", // property to be used as text of dropdown item
"1"), // value that should be set selected of dropdown
new { id = "ddlAssignee", #class = "form-control"})
WAY 2
<select name="SelectName" value="1" class="w-100">
#foreach (var item in ViewBag.Collection) {
<option value="#item.Id">#item.Name</option>
}
</select>
I hope it work for you
#Html.DropDownListFor how to set default value

In your view set:
#Html.DropDownListFor(model => model.Assignee,
ViewBag.Assignees as List<SelectListItem>,
"Select Assignee",
new { id = "ddlAssignee", #class = "form-control", #value = "Default value"})

When you have list already defined.
Use This
#Html.DropDownList("CoverageDropDown", new SelectList(Model.youList, "Code", "Description",item.seletecItem), "Select")

How to get disabled DropDownList control value to controller

Work on asp.net mvc5.My project DropDownList work perfectly problem arise when i disabled DropDownList control then can not get value in controller.
My razor syntax is bellow:
<div class="form-group">
#Html.LabelFor(m => m.ProductName, new {#class = "col-md-3 control-label"})
<div class="col-md-3">
#Html.DropDownList("ProductID", null, "Select product", new {#placeholder = "Select product", #class = "form-control", #disabled = "disabled" })
</div>
</div>
my controller syntax to fill the above DropDownList
private void LoadProductsInViewData(object selectedValue = null)
{
var datas = productRepository.GetAll().OrderBy(p => p.Name);
ViewBag.ProductID = new SelectList(datas, "ProductID", "Name", selectedValue);
}
Why In controller create/edit event can not get the DropDownList value?
Note:I know it's disabled control default behavior,i try to use hidden column like this.Problem is how hidden column used with my approach.

You can use Jquery to retrive the value of Currently Selected item of DropList which is disabled in your case, using
var dropDownValue=("#Id_of_u_Dropdownlist").val()
and you can send the DropDownValue variable which contains value of Disable Dropdown list to Controller.

This is a normal behavior. Disabled fields cannot be posted, then it's a common pratice to add a Html.HiddenFor() to post the disabled value.
As you said, there is no "ProductId" property in your model. However, if you create a Html.Hidden("ProductId", "YourValue") and add a parameter named 'productId' in your Controller Create/Edit action, the value should be subimitted.

Create an Httpost Action and then use FormCollection Object like this
[HttpPost]
public ActionResult GetDropDownValue(FormCollection Form)
{
string dropdownValue=form["ID_of_dropdownlist"].toString();
}

how to show the selected value in html.dropdownlistfor box when open the edit view

in my action method I used
ViewBag.type = new SelectList(
db.UserDetails.AsEnumerable().GroupBy(x => x.type).Select(x => x.First()),
"type",
"type",
user.type);
in my view I used
#Html.DropDownListFor(
m => m.type,
(IEnumerable<SelectListItem>)ViewBag.type,
new { #class = "form-control" })
don't know whats wrong why not showing the selected item on load

DropDownListFor Not Selecting Value
the correct answer I found on this link
The reason is you are using ViewBag's name the same as the model's property.

Default value for Html dropdown list

I am using MVC4 and c#. I have a DropDownList which I set using ViewBag.
#Html.DropDownList("category_Parent", new SelectList(ViewBag.Category, "Id", "Name"), new { #class = "form-control" })
By default it always selects the first value but I want it to select a null value (a default value). How can I do this?