在简单的 Blazor 应用程序中出现 "Object reference not set to an instance of an object." 错误。我究竟做错了什么？

Question

我收到以下错误。来自我的 API.

的简单所有记录调用

crit: 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 MusicManager.Client.Pages.Themes.BuildRenderTree(RenderTreeBuilder __builder) in
E:\Projects\marklalich\MusicManager\MusicManager\Client\Pages\Themes.razor:line 7
  at Microsoft.AspNetCore.Components.ComponentBase.<.ctor>b__6_0(RenderTreeBuilder builder)
  at Microsoft.AspNetCore.Components.Rendering.ComponentState.RenderIntoBatch(RenderBatchBuilder
  batchBuilder, RenderFragment renderFragment, Exception& renderFragmentException)

这是代码，我做错了什么？

@page "/themes"
@inject IThemeService ThemeService

<div class="container mt-3">
    <h2>Themes</h2>
    <p>asfgasfgasdfgasdfg</p>
    @if (ThemeService.Themes.Count == 0) <== this is the line the error is referencing
    {
        <span>Loading Themes...</span>
    }
    else
    {
        <table class="table table-striped">
            <thead>
                <tr>
                    <th>Title</th>
                </tr>
            </thead>
            <tbody>
                @foreach (var theme in ThemeService.Themes)
                {
                    <tr>
                        <td>@theme.Title</td>
                    </tr>
                }
            </tbody>
        </table>
    }
</div>

@code {
    protected override async Task OnInitializedAsync()
    {
        await ThemeService.GetThemes();
    }
}

Answer 1

组件首次渲染时

await ThemeService.GetThemes();

产量。

此时我猜测 ThemeService.Themes 为空，因此没有主题可计算：

@if (ThemeService.Themes.Count == 0)

所以你需要做一个空检查：

@if (ThemeService.Themes is null)

查看 FetchData 页面。

在简单的 Blazor 应用程序中出现 "Object reference not set to an instance of an object." 错误。我究竟做错了什么？

Getting an "Object reference not set to an instance of an object." error in a simple Blazor app. What am I doing wrong?

blazor

blazor-webassembly