asp.net-core Tag Helpers Select Tag Helper

Example

Assuming your view is strongly typed to a view model like this

public class CreateProduct
{       
   public IEnumerable<SelectListItem> Categories { set; get; }
   public int SelectedCategory { set; get; }
}

And in your GET action method, you are creating an object of this view model, setting the Categories property and sending to the view

public IActionResult Create()
{
    var vm = new CreateProduct();
    vm.Categories = new List<SelectListItem>
    {
        new SelectListItem {Text = "Books", Value = "1"},
        new SelectListItem {Text = "Furniture", Value = "2"}
    };
    return View(vm);
}

and in your view

@model CreateProduct

<form asp-action="create" asp-controller="Home">
    <select asp-for="SelectedCategory" asp-items="@Model.Categories">
        <option>Select one</option>
    </select>
    <input type="submit"/>
</form>

This will render the below markup(included only relevant parts of form/fields)

<form action="/Home/create" method="post">  
    <select data-val="true" id="SelectedCategory" name="SelectedCategory">
        <option>Select one</option>
        <option value="1">Shyju</option>
        <option value="2">Sean</option>
    </select>
    <input type="submit"/>
</form>

Getting the selected dropdown value in form submission

You can use the same view model as your HttpPost action method parameter

[HttpPost]
public ActionResult Create(CreateProduct model)
{
  //check model.SelectedCategory value
  / /to do : return something
}

Set an option as the selected one

If you want to set an option as the selected option, you may simply set the SelectedCategory property value.

public IActionResult Create()
{
    var vm = new CreateProduct();
    vm.Categories = new List<SelectListItem>
    {
        new SelectListItem {Text = "Books", Value = "1"},
        new SelectListItem {Text = "Furniture", Value = "2"},
        new SelectListItem {Text = "Music", Value = "3"}
    };
    vm.SelectedCategory = 2;
    return View(vm);
}

Rendering a Multi select dropdown/ListBox

If you want to render a multi select dropdown, you can simply change your view model property which you use for asp-for attribute in your view to an array type.

public class CreateProduct
{       
   public IEnumerable<SelectListItem> Categories { set; get; }
   public int[] SelectedCategories { set; get; }
}

In the view

@model CreateProduct

<form asp-action="create" asp-controller="Home" >
    <select asp-for="SelectedCategories" asp-items="@Model.Categories">
        <option>Select one</option>
    </select>
    <input type="submit"/>
</form>

This will generate the SELECT element with multiple attribute

<form action="/Home/create" method="post">
     <select id="SelectedCategories" multiple="multiple" name="SelectedCategories">
        <option>Select one</option>
        <option value="1">Shyju</option>
       <option value="2">Sean</option>
     </select>
    <input type="submit"/>
</form>