Blazor - how do I update a value in parent from child setting state

So I have created a state container:

public class StateContainer
{
    public List<Jewellery> basket = new List<Jewellery>();

    public void Add(Jewellery item)
    {
        basket.Add(item);
        NotifyStateChanged();
    }

    public void Remove(Jewellery item)
    {
        var removeVal = basket.FindIndex(m=>m.Id == item.Id);
        basket.RemoveAt(removeVal);
        NotifyStateChanged();
    }

    public event Action? OnChange;

    private void NotifyStateChanged() => OnChange?.Invoke();
}

On a Dialog page I have a button to add and another to remove the values From my StateContainer object and this works fine.

In my Program.cs file I have registered the service as Singleton:

builder.Services.AddSingleton<StateContainer>();

I have a header component for now with:

<div>@StateContainer.basket.Count</div>

After I adding an item into the basket, after I close the dialog I want to be able to see the count in my header. How should I bind the StateContainer in my header component so that it acts on the change immediately?

Your component containing the header should listen the OnChange event.
To do this, you can override the OnInitialized method and suscribe to the event. Then when the event is triggered, just call StateHasChanged() which will re-render the component.

protected override void OnInitialized()
{
    stateContainer.OnChange += StateContainedChanged;
	base.OnInitialized();
}

void StateContainedChanged() 
{ 
     InvokeAsync(StateHasChanged); 
}

PS: I call InvokeAsync() here because the event could be triggered by a non main thread. So I make sure the StateHasChangedmethod is called by the main thread.

When you subcribe to an event, in order to prevent memory leaks, it is important to unsubscribe.
You can do it when your component is disposing :

@implements IDisposable

public void Dispose()
{ 
    stateContainer.OnChange -= StateContainedChanged;
}

You could also use an ObservableCollection<Jewellery>instead of a List<Jewellery>.
The ObservableCollection already implements an OnCollectionChanged event and is able to tell what change is made