interface IStateMessageService
{
    string StateMessages { get; }
    event Action OnChange;
    void ClearStateMessages();
    void SetStateMessages(string statusMessages);
}

public class StateMessageService : IStateMessageService
{
    public string StateMessages { get; private set; }
    public event Action OnChange;

    public void SetStateMessages(IStatusMessages statusMessages)
    {
        StateMessages = statusMessages;
        NotifyStateChanged();
    }

    public void ClearStateMessages()
    {
        StateMessages = null;
        NotifyStateChanged();
    }

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

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        // 略
        services.AddScoped<IStateMessageService, StateMessageService>(); // 追記
        // 略
    }
}

@inject IStateMessageService StateMessage
@implements IDisposable

<div class="sidebar">
    <NavMenu />
</div>

<div class="main">
    <AlertMessage Message="StateMessage.StateMessages" />
    <div class="content px-4 vh-100">
        @Body
    </div>
</div>

@code
{
    protected override void OnInitialized()
    {
        StateMessage.OnChange += StateHasChanged;
    }

    public void Dispose()
    {
        StateMessage.OnChange -= StateHasChanged;
    }
}

@inject IStateMessageService StateMessage
@if(!string.IsNullOrEmpty(Message))
{
    <div class="alert alert-success alert-dismissible @(IsVisible ? "show " : "")text-left shadow" role="alert">
        <button type="button" class="close" data-dismiss="alert" aria-label="Close" @onclick="OnClickCloseButton">
            <span aria-hidden="true">&times;</span>
        </button>
        <p class="m-0">
            @Message
        </p>
    </div>
}

@code {
    [Parameter]
    public string Message { get; set; } = string.Empty;
    public bool IsVisible { get; set; } = true;

    private void OnClickCloseButton()
    {
        IsVisible = false;
        Task.Run(() => StateMessage.ClearStateMessages());
    }
}

@inject IStateMessageService StateMessage
<button type="button" @onclick="OnClickButton">Show</button>

@code {
    public void OnClickButton()
    {
        StateMessage.SetStateMessages("Hello world!");
    }
}