BlazorFiddle - Blazor .Net Developer Playground and Code Editor in the Browser.

Index

@page "/"

<h1>Hello, world!</h1>

Welcome to your new app.

<div>
    <label>Async void (@VoidCount)</label>
    <select @onchange="@IncrementVoid">
        <option>a</option>
        <option>b</option>
        <option>c</option>
    </select>
</div>
<div>
    <label>Async task (@TaskCount)</label>
    <select @onchange="@IncrementTask">
        <option>a</option>
        <option>b</option>
        <option>c</option>
    </select>
</div>
@foreach (string message in Messages)
{
<div>
    @message
</div>
}

@code {
    private List<string> Messages {get;} = new List<string>();
    private int TaskCount {get; set;}
    private int VoidCount {get; set;}

    private async Task IncrementTask()
    {   
        AddMessage("Running IncrementTask");
        await RunFakeTask();
        AddMessage("Incrementing TaskCount");
        TaskCount++;
    }

    private async void IncrementVoid()
    {   
        AddMessage("Running IncrementVoid");
        await RunFakeTask();
        AddMessage("Incrementing VoidCount");
        VoidCount++;
    }

    private void AddMessage(string message)
    {
        Messages.Add(message);
        StateHasChanged();
    }

    private async Task RunFakeTask()
    {
        await Task.Delay(TimeSpan.FromSeconds(1));
    }
}

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.