Blazor使用InputSelect时触发NullReferenceException空引用异常

问题表现 #

• Razor组件中使用InputSelect时，控制台持续抛出System.NullReferenceException（未将对象引用设置到对象的实例）
• 异常堆栈指向Add.razor文件第18行的InputSelect渲染逻辑，无法直接定位错误点

异常堆栈 #

System.NullReferenceException: Object reference not set to an instance of an object.
at VidaConfortoApplication.Client.Pages.ServiceTypes.Add.b__0_7(RenderTreeBuilder __builder3) in C:\repos\sources\VidaConfortoTese\src\VidaConfortoApplication\VidaConfortoApplication\Client\Pages\ServiceTypes\Add.razor:line 18
at Microsoft.AspNetCore.Components.Rendering.RenderTreeBuilder.AddContent(Int32 sequence, RenderFragment fragment)
at Microsoft.AspNetCore.Components.Forms.InputSelect`1[[System.Int64, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].BuildRenderTree(RenderTreeBuilder builder)
at Microsoft.AspNetCore.Components.ComponentBase.<.ctor>b__6_0(RenderTreeBuilder builder)
at Microsoft.AspNetCore.Components.Rendering.ComponentState.RenderIntoBatch(RenderBatchBuilder batchBuilder, RenderFragment renderFragment, Exception& renderFragmentException)

相关代码 #

Add.razor 组件 #

<EditForm Model="@_model"  OnValidSubmit="OnValidSubmit">
    <DataAnnotationsValidator />
    <div class="form-row">
        <div class="form-group col">
            <label>Serviço pai</label>
            <InputSelect class="form-control" @bind-Value="@_model.ParentId">
                <option value="">--</option>
                @foreach (var service in _services)
                {
                    <option value="@service.Id">@service.Name</option>
                }
            </InputSelect>
            <label>@_serviceHierarchy</label>
            <ValidationMessage For="@(() => _model.ParentId)"/>
        </div>
    </div>
    <br />
    <div class="form-group">
        <button class="btn btn-primary">
            Save
        </button>
    </div>
</EditForm>

@code {
    private readonly AddServiceTypeViewModel _model = new();
    private IEnumerable<ServiceType> _services;

    protected override async Task OnInitializedAsync()
    {
        _services = await ServiceTypeService.GetAll();
    }
}

AddServiceTypeViewModel 类 #

public class AddServiceTypeViewModel
{
    [Required]
    public string Name { get; set; }
    public long? ParentId { get; set; }
    public string? Description { get; set; }
}

ServiceType 类 #

public class ServiceType
{
    public string Name { get; set; }
    public long? ParentId { get; set; }
    public ServiceType? Parent { get; set; }
    public string? PathToService { get; set; }
    public string? Description { get; set; }
    public DateTime? AddedWhen { get; set; }
    public DateTime? UpdatedWhen { get; set; }
    public long Id { get; set; }
}

触发原因 #

异常的直接触发点是首次渲染时_services为null：
Blazor组件在执行OnInitializedAsync时，遇到第一次await会先返回并触发一次组件渲染，此时ServiceTypeService.GetAll()的异步请求还未完成，_services字段没有被赋值，是null值。@foreach循环直接遍历null的IEnumerable<ServiceType>，就会抛出空引用异常，堆栈指向的第18行正好是foreach循环渲染option的位置，和现象完全匹配。
另外代码还存在一个潜在问题：InputSelect没有显式声明TValue为long?，默认识别到的绑定类型是long（非空值类型），后续选中空选项时，空字符串无法转换为long类型，会触发转换异常。

修复方法 #

1. 初始化_services为空集合，避免首次渲染遍历null：
   将字段声明修改为

   private IEnumerable<ServiceType> _services = Enumerable.Empty<ServiceType>();
   

2. 显式指定InputSelect的泛型类型为可空long，匹配绑定值的类型，解决空选项转换问题：
   将InputSelect的声明修改为

   <InputSelect TValue="long?" class="form-control" @bind-Value="@_model.ParentId">
   

内容的提问来源于stack exchange，提问作者user2382627