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Index

@page "/"

@code {
    private bool isNavCollapsed = true;

    private void ToggleNavMenu()
    {
        isNavCollapsed = !isNavCollapsed;
    }
}
<link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.2/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-T3c6CoIi6uLrA9TneNEoa7RxnatzjcDSCmG1MXxSR1GAsXEV/Dwwykc2MPK8M2HN" crossorigin="anonymous">
<nav class="navbar navbar-expand-lg navbar-light bg-light">
    <a class="navbar-brand" href="#">Navbar</a>
    <button class="navbar-toggler" type="button" @onclick="ToggleNavMenu">
        <span class="navbar-toggler-icon"></span>
    </button>

    <div class="@($"collapse navbar-collapse {(isNavCollapsed ? "" : "show")}")" id="navbarNav">
        <ul class="navbar-nav">
            <li class="nav-item active">
                <NavLink class="nav-link" href="" Match="NavLinkMatch.All">
                    Home <span class="sr-only">(current)</span>
                </NavLink>
            </li>
            <li class="nav-item">
                <NavLink class="nav-link" href="counter">
                    Counter
                </NavLink>
            </li>
            <li class="nav-item">
                <NavLink class="nav-link" href="fetchdata">
                    Fetch data
                </NavLink>
            </li>
            <li class="nav-item dropdown">
                <a class="nav-link dropdown-toggle" id="navbarDropdown" role="button" data-toggle="collapse" data-target="#submenu1" aria-expanded="false">
                    Submenu
                </a>
                <ul class="collapse" id="submenu1">
                    <li class="nav-item"><a class="nav-link" href="#">Submenu Item 1</a></li>
                    <li class="nav-item"><a class="nav-link" href="#">Submenu Item 2</a></li>
                    <!-- More submenu items -->
                </ul>
            </li>
        </ul>
    </div>
</nav>

namespace BlazorFiddleProject
{
    using Microsoft.Extensions.DependencyInjection;


    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
        }

        // Warning: IComponentsApplicationBuilder - not the original .NET 6.0 interface. This was done to make old saves work. 
        public void Configure(IComponentsApplicationBuilder app)
        {
            app.AddComponent<App>("app");
        }
    }
}

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8"/>
    <meta name="viewport" content="width=device-width">
    <title>BlazorFiddleProject</title>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous">

    <link href="_content/MatBlazor/dist/matBlazor.css" rel="stylesheet" />
    <script src="_content/MatBlazor/dist/matBlazor.js"></script>    

    <style>
        app { }
    </style>

    <script type="text/javascript">
    </script>
</head>
<body>
<app>Loading...</app>

<script src="_framework/blazor.webassembly.js"></script>
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js" integrity="sha384-UO2eT0CpHqdSJQ6hJty5KVphtPhzWj9WO1clHTMGa3JDZwrnQq4sF86dIHNDz0W1" crossorigin="anonymous"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js" integrity="sha384-JjSmVgyd0p3pXB1rRibZUAYoIIy6OrQ6VrjIEaFf/nJGzIxFDsf4x0xIM+B07jRM" crossorigin="anonymous"></script>
</body>
</html>

BlazorFiddle was updated from Blazor 0.7 to .NET 6.0. Your old source code could not work. You need to upgrade to latest.