Blazor从数据库绑定InputSelect下拉列表遇空引用异常问题

问题根源 #

你代码里的context字段仅做了声明但未初始化，而StarRatings属性直接依赖这个未赋值的context，页面渲染时遍历StarRatings就会触发对象引用未设置为对象实例异常。另外还有一处ValidationMessage的绑定错误需要修正。

解决步骤 #

1. 初始化DbContext并异步加载下拉选项数据 #

在组件的生命周期方法OnInitializedAsync中，通过注入的DbFactory创建DbContext实例，并加载MovieStarRating数据（转为List而非直接用IQueryable，避免后续重复查询）。

2. 修正ValidationMessage的绑定目标 #

原来的ValidationMessage指向Movie.MovieStarRating，但你绑定的是Movie.MovieStarRatingId，需要修改匹配的表达式。

修改后的完整代码 #

@page "/movies/create"
@rendermode InteractiveServer
@using Microsoft.EntityFrameworkCore
@using BlazorWebAppMovies.Models
@using BlazorWebAppMovies.Data
@inject IDbContextFactory<BlazorWebAppMovies.Data.BlazorWebAppMoviesContext> DbFactory
@inject NavigationManager NavigationManager

<PageTitle>Create</PageTitle>

<h1>Create</h1>

<h2>Movie</h2>
<hr />
<div class="row">
    <div class="col-md-4">
        <EditForm method="post" Model="Movie" OnValidSubmit="AddMovie" FormName="create" Enhance>
            <DataAnnotationsValidator />
            <ValidationSummary class="text-danger" role="alert"/>
            <div class="mb-3">
                <label for="title" class="form-label">Title:</label> 
                <InputText id="title" @bind-Value="Movie.Title" class="form-control" /> 
                <ValidationMessage For="() => Movie.Title" class="text-danger" /> 
            </div>        
            <div class="mb-3">
                <label for="releasedate" class="form-label">Release Date:</label> 
                <InputDate id="releasedate" @bind-Value="Movie.ReleaseDate" class="form-control" /> 
                <ValidationMessage For="() => Movie.ReleaseDate" class="text-danger" /> 
            </div>        
            <div class="mb-3">
                <label for="genre" class="form-label">Genre:</label> 
                <InputText id="genre" @bind-Value="Movie.Genre" class="form-control" /> 
                <ValidationMessage For="() => Movie.Genre" class="text-danger" /> 
            </div>        
            <div class="mb-3">
                <label for="price" class="form-label">Price:</label> 
                <InputNumber id="price" @bind-Value="Movie.Price" class="form-control" /> 
                <ValidationMessage For="() => Movie.Price" class="text-danger" /> 
            </div>
            <div class="mb-3">
                <label for="rating" class="form-label">Rating:</label>
                <InputText id="rating" @bind-Value="Movie.Rating" class="form-control" />
                <ValidationMessage For="() => Movie.Rating" class="text-danger" />
            </div>
            <div class="mb-3">
                <label for="moviestarrating" class="form-label">Movie Star Rating:</label>
                <InputSelect id="moviestarrating" @bind-Value="Movie.MovieStarRatingId" class="form-control">
                    <option value="">-- Select --</option>
                    @foreach (var movieStarRating in StarRatings)
                    {
                        <option value="@movieStarRating.Id">@movieStarRating.Descr</option>
                    }
                </InputSelect>
                <ValidationMessage For="() => Movie.MovieStarRatingId" class="text-danger"></ValidationMessage>
            </div>
            <button type="submit" class="btn btn-primary">Create</button>
        </EditForm>
    </div>
</div>

<div>
    <a href="/movies">Back to List</a>
</div>

@code {
    private List<MovieStarRating> StarRatings = new();

    [SupplyParameterFromForm]
    private Movie Movie { get; set; } = new();

    protected override async Task OnInitializedAsync()
    {
        using var context = DbFactory.CreateDbContext();
        StarRatings = await context.MovieStarRating.ToListAsync();
    }

    private async Task AddMovie()
    {
        using var context = DbFactory.CreateDbContext();
        context.Movie.Add(Movie);
        await context.SaveChangesAsync();
        NavigationManager.NavigateTo("/movies");
    }
}

关键修改点说明 #

• 移除了未初始化的context字段，改为在OnInitializedAsync中通过DbFactory创建临时context加载数据
• 将StarRatings从IQueryable改为List<MovieStarRating>，避免页面渲染时重复触发数据库查询
• 修正了ValidationMessage的绑定表达式，与InputSelect绑定的MovieStarRatingId保持一致

内容的提问来源于stack exchange，提问作者Rusty0918-B