Blazor Server 使用 EF Core 噴 Invalid attempt to call ReadAsync when reader is closed. Or A second operation started on this context before a previous operation completed.

前言

在 ASP.NET Core 中，EF Core 的 DI 通常是設定成 Scoped。
但 Blazor Server 的 Scoped 是以 SignalR Connection 來看的，所以並不是之前 ASP.NET Core 的每次 Request。
所以如果直接在 Blazor Server 中使用 EF Core 的 DbContext 時，有時會噴以下的錯誤，

Invalid attempt to call ReadAsync when reader is closed.

A second operation started on this context before a previous operation completed. This is usually caused by different threads using the same instance of DbContext. For more information on how to avoid threading issues with DbContext

解決方式

解法1 : 讓頁面繼承 OwningComponentBase

讓 Service 的生命週期變成是以 Page 為 Base ，也就是使用者切換頁面時，那個注入的服務才會 Release 掉。

@inherits OwningComponentBase<SyncfusionHelpDeskService>

@code{
    public async Task HandleValidSubmit(EditContext context)
    {
        try
        {
            var tickets = new HelpDeskTickets();
            //使用時，直接取用 Service 就是 SyncfusionHelpDeskService 物件
            var result = Service.CreateTicketAsync(tickets);
        }
        catch (Exception ex)
        {
            // error handle
        }
    }
}

但有這在同一個頁面中的 Method ，如果同時操作 async & sync 的 DbContext Method ，就有可能出現 Invalid attempt to call ReadAsync when reader is closed.

解法2 : 改用 Transient

解法3 : 要使用時，再透過 IServiceScopeFactory.CreateScope 建立

@inject IServiceScopeFactory serviceScopeFactory
@* other razor page code ... *@
@code{
    [CascadingParameter]
    private Task<AuthenticationState> authenticationStateTask { get; set; }

    public IQueryable<HelpDeskTickets> QryHelpDeskTickets { get; set; }
    private HelpDeskTickets SelectedTicket = new HelpDeskTickets();

    HelpDeskTicket objHelpDeskTicket = new HelpDeskTicket { TicketDate = DateTime.Now };

    public bool DeleteRecordConfirmVisibility { get; set; } = false;
    public bool EditDialogVisiblity { get; set; } = false;

    // page command event
    public void ConfirmDeleteYes()
    {
        using (var scope = serviceScopeFactory.CreateScope())
        {
            var serviceProvider = scope.ServiceProvider;
            var service = serviceProvider.GetRequiredService<SyncfusionHelpDeskService>();
            // call service delete data
            var result = service.DeleteHelpDeskTickets(SelectedTicket);
            if (result)
            {
                this.DeleteRecordConfirmVisibility = false;
                // call service get query data 
                QryHelpDeskTickets = service.GetHelpDeskTickets().ToList().AsQueryable();
            }
        }
    }
}

但如果有把 DbContext 直接回傳 IQueryable Bind 到 Blazor 控制項的話，也會噴 A second operation started on this context before a previous operation completed 的錯哦!

參考資料

Using Entity Framework in a Blazor Server Application
An Introduction to OwningComponentBase