为什么 Blazor WebAssembly @foreach{} 无法呈现列表？

Question

我不明白为什么这个列表渲染不再工作并抛出一个奇怪的异常：

Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100] Unhandled exception rendering component: Nullable object must have a value. System.InvalidOperationException: Nullable object must have a value. at System.Nullable`1[T].get_Value () <0x28f3be8 + 0x0000a> in :0 at .Client.Components.Threads.BuildRenderTree (Microsoft.AspNetCore.Components.Rendering.RenderTreeBuilder __builder) [0x010c7] in C:\Users\vOId\Desktop\projects<redacted><redacted>\Client\Components\Threads.razor:106 at Microsoft.AspNetCore.Components.ComponentBase.<.ctor>b__6_0 (Microsoft.AspNetCore.Components.Rendering.RenderTreeBuilder builder) <0x2d39550 + 0x0001a> in :0 at Microsoft.AspNetCore.Components.Rendering.ComponentState.RenderIntoBatch (Microsoft.AspNetCore.Components.Rendering.RenderBatchBuilder batchBuilder, Microsoft.AspNetCore.Components.RenderFragment renderFragment) <0x2d38ea8 + 0x00062> in :0 at Microsoft.AspNetCore.Components.RenderTree.Renderer.RenderInExistingBatch (Microsoft.AspNetCore.Components.Rendering.RenderQueueEntry renderQueueEntry) <0x2d388a8 + 0x0004c> in :0 at Microsoft.AspNetCore.Components.RenderTree.Renderer.ProcessRenderQueue () <0x2d35ce8 + 0x00098> in :0

列表的初始化是正常的，从服务器获取数据e将其添加到列表中进行渲染(ViewThreads):

protected async override Task OnInitializedAsync()
{
    var result = await ThreadService.GetThreads();
    if (result.IsValid)
        ViewThreads.AddRange((List<ViewThread>)result.Data);
    //...
}

同时在@foreach的第一条指令的渲染点，第106行异常，这是渲染

//...
@foreach (var thread in ViewThreads)
{
    <p class="button is-light is-rounded neoFile petite-caps mb-3 @(CurrentThread != null && CurrentThread.Id == thread.Id ? "isSelected" : null)"
            @onclick="() => SelectThread(thread)" @key="thread">
        @thread.Title
    </p>
}
//...

这部分代码没有改变，只是突然渲染该列表抛出异常并且不再渲染元素。

它可能是什么？有人遇到过类似的问题吗？

Answer 1

初始化ViewThreads对象：

public List<ViewThread> ViewThreads { get; set; } = new List<ViewThread>();

或将 foreach 环绕在 if (ViewThreads != null) 周围：

if (ViewThreads != null)
{
    @foreach (var thread in ViewThreads)
    {
        <p class="button is-light is-rounded neoFile petite-caps mb-3 @(CurrentThread != null && CurrentThread.Id == thread.Id ? "isSelected" : null)"
            @onclick="() => SelectThread(thread)" @key="thread">
            @thread.Title
        </p>
    }
}

Answer 2

Nullable object must have a value.

您在此处发布的代码不会产生该错误。正如您自己所说，“这部分代码没有改变”

所以你用 ... 替换了某些部分。寻找可为 null 的属性.

而“突然渲染那个列表抛出”很可能是由数据引起的。某些专栏在您意想不到的地方偷偷加入了空值。

为什么 Blazor WebAssembly @foreach{} 无法呈现列表？

Why a Blazor WebAssembly @foreach{} can't render a list?

c#

razor

blazor

blazor-webassembly