c# - Blazor Server托管，null引用异常

Question

我正在我们的一个项目上测试 blazor 服务器 (.net 6.05)，我有以下代码:

@page "/"
@using Microsoft.EntityFrameworkCore

@using Microsoft.JSInterop
@inject IDbContextFactory<MyDbContext> DbContextFactory;


@code {
    [Inject]
    private IHttpContextAccessor HttpContextAccessor { get; set; } = default!;

    MyDbContextContext { get; set; }
    public Dish? RandomDish { get; set; }
    public Region? Region { get; set; }


    async Task getRandomDish()
    {
        var dish = await Context.Dishes.OrderBy(m => Guid.NewGuid()).FirstOrDefaultAsync();
        RandomDish = dish;
    }

    async Task SaveDishName()
    {
        await Context.SaveChangesAsync();
    }

    protected override async Task OnInitializedAsync()
    {

        var a = HttpContextAccessor.HttpContext.User.Identity;
        Context = await DbContextFactory.CreateDbContextAsync();
        Region = await Context.Regions.SingleOrDefaultAsync(m => m.Name == "Italy");
        await getRandomDish();

    }
     protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (!firstRender) return;

    }
}

<PageTitle>Index</PageTitle>

@if (RandomDish != null)
{
    <h1>@RandomDish?.Name</h1>
    <h2>@RandomDish?.DishId</h2>
    <p>@Region?.Borders?.AsText()</p>
}<-- it errors here
<button @onclick="@(async () => await getRandomDish())">GET NEW DISH</button>

<div>
    Change dish name
    <input @bind="RandomDish.Name" />
    <button @onclick="@(async () => await SaveDishName())">Save</button>
</div>

当我运行代码时，它抛出以下异常

System.NullReferenceException: 'Object reference not set to an instance of an object.'

****.Cms.Web.Pages.Index.RandomDish.get returned null.

我也尝试过“is not null”和referenceEquals(RandomDish, null)，但总是同样的问题。

如果 getter 会抛出空值，我应该如何进行空值检查:/

如果我用下面的行初始化盘子，问题就会消失，但这并不是我猜想的问题的解决方案。

public Dish? RandomDish { get; set; } = new Dish();

Answer 1

@bind 中不能有 null。

<input @bind="RandomDish.Name" />

使用 @if (RandomDish != null) 或类似内容包围 Save block 。