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

Index

@page "/"

<MatH4>Source</MatH4>
<MatList>
    @foreach (var item in items)
    {
        <MatListItem>
            <MatCheckbox TValue="bool" Value="@item.IsSelected" ValueChanged="@(value=>OnItemSelected(item, value))" Label="@item.Name"></MatCheckbox>
        </MatListItem>
    }
</MatList>


<MatH4>Result</MatH4>
@foreach (var item in items)
{
    <div>@item.Name - @item.IsSelected</div>
}

@code
{
    void OnItemSelected(ItemClass item, bool value)
    {
        item.IsSelected = value;
        // here you can dot something
    }
    

    ItemClass[] items = Enumerable.Range(1, 10)
        .Select(i => new ItemClass($"Item {i}", i % 2 == 0))
        .ToArray();

    class ItemClass
    {
        public string Name { get; set; }
        public bool IsSelected { get; set; }

        public ItemClass(string name, bool isSelected)
        {
            Name = name;
            IsSelected = isSelected;
        }
    }
}

namespace BlazorFiddleProject
{
    using Microsoft.AspNetCore.Components.Builder;
    using Microsoft.Extensions.DependencyInjection;


    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
        }

        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">
	
	<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.