In this article I will explain with an example, how to use DropDownList SelectIndexChanged event in ASP.Net Core (.Net Core 8) MVC..
Note: For beginners in ASP.Net Core (.Net Core 8) MVC, please refer my article ASP.Net Core 8: Hello World Tutorial with Sample Program example.
 
 

Model

The Model class consists of the following property.
public class PersonModel
{
    /// <summary>
    /// Gets or sets Gender.
    /// </summary>
    [Display(Name = "Gender:")]
    public string Gender { get; set; }
 
}
 
 

Controller

The Controller consists of following Action methods.

Action method for handling GET operation

Inside this Action method, simply the View is returned.
 

Action method for handling POST operation

Inside the Action Method, the values posted from the Form inside the View are received through this parameter.
public class HomeController : Controller
{
    // GET: Home
    public IActionResult Index()
    {
        return View();
    }
 
    [HttpPost]
    public IActionResult Index(PersonModel person)
    {
        string gender = person.Gender;
        return View();
    }
}
 
 

View

HTML Markup

Inside the View, in the very first line the PersonModel class is declared as model for the View and ASP.Net TagHelpers is inherited.
The View consists of an HTML Form which has been created using the following TagHelpers attributes.
asp-action – Name of the Action. In this case the name is Index.
asp-controller – Name of the Controller. In this case the name is Home.
method – It specifies the Form Method i.e. GET or POST. In this case it will be set to POST.
 
The Form also consists of a HTML Table, which consists Label.
Label - Created using Html.LabelFor for displaying text.
Then Form consists of an HTML Table which consists DropDownList created using HTML SELECT (DropDownList).
Finally, HTML SELECT (DropDownList) has been assigned with an onchange event handler, and when the option value is click or unclick, a JavaScript function OnGenderChanged submits the Form and in turn calls the Controller’s Action method.
@model DropDownList_OnChange_MVC_Core.Models.PersonModel
@addTagHelper*,Microsoft.AspNetCore.Mvc.TagHelpers
@{
    Layout = null;
}
 
<!DOCTYPE html>
 
<html>
<head>
    <meta name="viewport" content="width=device-width" />
    <title>Index</title>
</head>
<body>
    <form asp-action="Index" asp-controller="Home" method="post" id="Form1">
        <table>
            <tr>
                <td><label asp-for="Gender"></label></td>
                <td>
                   <select asp-for="Gender" onchange="OnGenderChanged()">
                       <option value="">Please select</option>
                       <option value="M">Male</option>
                       <option value="F">Female</option>
                   </select>
                </td>
            </tr>
        </table>
    </form>
 
    <script>
        function OnGenderChanged() {
            document.getElementById("Form1").submit();
        };
    </script>
</body>
</html>
 
 

Screenshots

The Form

DropDownList SelectedIndexChanged event in ASP.Net Core
 

Values captured in Controller when Form is submitted

DropDownList SelectedIndexChanged event in ASP.Net Core
 
 

Downloads

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