Blazor双向绑定搜索栏报空引用异常，模型处理是否有误？

原字符串版本可正常运行的代码 #

<MudTextField @bind-Value="@searchTerm" Placeholder="Sök" />

@foreach (var item in filteredItems)
{
    <p>@item</p>
}

@code {
    private string searchTerm = "";
    private List<string> items = new List<string> {
        "1","2","3"
    };

    private IEnumerable<string> filteredItems =>
        items.Where(i => i.ToLower().Contains(searchTerm.ToLower()));
}

切换为Exercise模型后出现异常 #

控制台抛出错误：

Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]
Unhandled exception rendering component: Object reference not set to an instance of an object.
System.NullReferenceException: Object reference not set to an instance of an object.
at

修改后的Exercise模型代码 #

<MudTextField @bind-Value="@searchTerm" Placeholder="Sök" />

@foreach (var item in filteredItems)
{
    <p>@item.Title</p>
}

@code {
    private string searchTerm = "";
    private List<Exercise> items = new List<Exercise> {
        new Exercise
        {
            Id = 1,
            Title = "Push-ups",
            Instructions = new Dictionary<int, string>()
            {
                { 1, "Get into a plank position with your hands slightly wider than shoulder-width apart." },
                { 2, "Lower your body until your chest nearly touches the floor." },
                { 3, "Push your body back up to the starting position, keeping your elbows close to your body." }
            }
        },
        new Exercise
        {
            Id = 2,
            Title = "Squats",
            Instructions = new Dictionary<int, string>()
            {
                { 1, "Stand with your feet shoulder-width apart." },
                { 2, "Lower your body as if you are sitting back into a chair." },
                { 3, "Keep your knees over your ankles and your weight in your heels." },
                { 4, "Push back up to the starting position." }
            }
        },
        new Exercise
        {
            Id = 3,
        }
    };

    private IEnumerable<Exercise> filteredItems =>
        items.Where(i => i.Title.ToLower().Contains(searchTerm.ToLower()));
}

使用数据仓库时同样出错 #

@inject IExerciseDataStore<Exercise> exerciseDataStore
...

protected override async Task OnInitializedAsync()
{
    await base.OnInitializedAsync();

    items = await exerciseDataStore.GetAllExercisesAsync();
}

原因及解决办法 #

• 直接原因：

  1. 手动初始化的第三个Exercise对象未赋值Title属性，该属性为null，调用i.Title.ToLower()时触发空引用异常。
  2. 使用数据仓库时，返回的集合中可能存在Title为null的Exercise实例，同样会引发该异常。

• 解决方式：

  1. 初始化Exercise对象时确保Title不为空，比如给第三个对象设置Title = "未命名训练"。
  2. 在过滤逻辑中增加空值判断，避免对null调用方法：

     private IEnumerable<Exercise> filteredItems =>
         items.Where(i => !string.IsNullOrEmpty(i.Title) && i.Title.ToLower().Contains(searchTerm.ToLower()));
     

  3. 使用数据仓库时，要么确保返回的所有Exercise的Title都已正确赋值，要么同样在过滤条件中加入空值检查。

内容的提问来源于stack exchange，提问作者LordAhune