ASP.Net Core Web App - cshtml file can't invoke Controller method

Form

An HTML <form> needs a submit button (and usually some controls) before it will call the controller action. It looks like the form in your example is empty.

You haven't shown the controller, but let's assume you want to pass a string to the controller action, maybe to search or filter:

public ActionResult Index(string searchTerm)
{
    // do something with parameters then return view
    return View();
}

Instead of this in your view:

Html.BeginForm("Index", "Default", FormMethod.Get);    // empty form

It should be something like this:

@Html.BeginForm("Index", "Default", FormMethod.Get) 
{
    // add controls here, what parameters are you passing? 
    @Html.TextBox("SearchTerm")
    <input type="submit" value="Search" />
}

Tag Helpers

Since you're using ASP.Net-Core you can take advantage of tag helpers which allow you to write code in a more HTML like manner. I encourage you to read Tag Helpers in forms in ASP.NET Core. One way the above could be written using tag helpers is:

<form asp-action="Index" asp-controller="Default" method="get">
    <input type="text" name="SearchTerm" />
    <button>Search</button>
</form>

ActionLink

Perhaps you want to create a hyperlink to Default/Index? In that case, use the @Html.ActionLink helper:

@Html.ActionLink("go to this link", "Index", "Default")

Which will create a regular anchor <a>:

<a href="/Default">go to this link</a>

Tag Helper version

<a asp-action="Index" asp-controller="Default" >Click this link</a>

When you load index.cshtml,it would get into index action by default.You need to return model in your index action.

Here is a simple demo like below:

1.Model:

public class Test
{
    public int ID { get; set; }
    public string Title { get; set; }
}

2.Index.cshtml:

@model IEnumerable<TestModel>
<h1>Index</h1>
<table class="table">
    <thead>
        <tr>
            <th>
                @Html.DisplayNameFor(model => model.Name)
            </th>
        </tr>
    </thead>
    <tbody>
@foreach (var item in Model) {
        <tr>
            <td>
                @Html.DisplayFor(modelItem => item.Name)
            </td>            
        </tr>
}
    </tbody>
</table>

3.Index action:

public class TestModelsController : Controller
{
    private readonly MyDbContext _context;

    public TestModelsController(MyDbContext context)
    {
        _context = context;
    }

    // GET: TestModels
    public async Task<IActionResult> Index()
    {
        var model = await _context.TestModel.ToListAsync();
        return View(model);
    }
}

Reference: Passing data to views