Blazor WASM应用程序中的对象引用未设置错误

Question

我很难找到以下错误代码的原因。

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.Pieces.BuildRenderTree(RenderTreeBuilder __builder) in D:MusicManagerMusicManagerClientPagesPieces.razor:line 20
at Microsoft.AspNetCore.Components.ComponentBase.<.ctor>b__6_0(RenderTreeBuilder builder)
at Microsoft.AspNetCore.Components.Rendering.ComponentState.RenderIntoBatch(RenderBatchBuilder batchBuilder, RenderFragment renderFragment, Exception& renderFragmentException)

这是引用的Pieces.razor文件中的代码。

@page "/pieces"
@inject IPieceService PieceService
@inject NavigationManager NavigationManager
@implements IDisposable 
<PageTitle>Pieces</PageTitle>
<h3>Pieces</h3>
<table class="table table-hover">
<thead>
<tr>
<th scope="col" style="width: 100px">#</th>
<th scope="col">Title</th>
<th scope="col">Number of Copies</th>
<th scope="col">Score?</th>
<th scope="col">Last Performed</th>
<th scope="col"></th>
</tr>
</thead>
<tbody>
@if (PieceService.Pieces.Count == 0)
{
<tr><td colspan="6"><span>Loading Pieces...</span></td></tr>
}
else
{
@foreach (var piece in PieceService.Pieces)
{
<tr>
<td>@piece.Id</td>
<td>@piece.Title</td>
<td>@piece.NumOfCopies</td>
<td>@piece.ScoreYesNo</td>
<td>@piece.LastPerformed</td>
<td align="right">
<button class="btn btn-primary" @onclick="(() =>     Show(piece.Id))">Show</button>
<button class="btn btn-primary" @onclick="(() => Edit(piece.Id))">Edit</button>
<button class="btn btn-primary" @onclick="(() => Delete(piece.Id))">Delete</button>
</td>
</tr>
}
}
</tbody>
</table>
<FormPiece piece="pieceToEdit"></FormPiece>
@code {
Piece pieceToEdit = new Piece();
protected override async Task OnInitializedAsync()
{
await PieceService.GetAll();
PieceService.OnChange += StateHasChanged;
}
void Show(int id)
{
NavigationManager.NavigateTo($"pieces/{id}");
}
void Edit(int id)
{
pieceToEdit = PieceService.Pieces.Find(x => x.Id == id);
}
void Delete(int id)
{
PieceService.Delete(id);
}
public void Dispose()
{
PieceService.OnChange -= StateHasChanged;
}
}

如果请求，我将包含更多的代码，但我认为错误存在于此代码中。该页面呈现良好，但该错误除外。表单甚至与删除、显示和编辑按钮一起正确提交。

错误中引用的第20行指向这一行。

@if (PieceService.Pieces.Count == 0)

谢谢你能给我的任何帮助。

Answer 1

我认为Pieces为空

@if (PieceService.Pieces?.Count == 0)

应该有帮助，或者过分夸张地检查PieceService是否为空以及