[英]Blazor WASM : error while ReadFromJsonAsync <List<Object>> Post crud
[英]Object reference not set error in Blazor WASM app
我很難追蹤以下錯誤代碼的原因。
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:\MusicManager\MusicManager\Client\Pages\Pieces.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)
以下是引用的 Pieces.razor 文件中的代碼。
@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;
}
}
如果需要,我將包含更多代碼,但我認為錯誤存在於此代碼中。 該頁面呈現正常,但該錯誤除外。 表單甚至與刪除、顯示和編輯按鈕一起正確提交。
錯誤中引用的第 20 行指向此行。
@if (PieceService.Pieces.Count == 0)
謝謝你能給我的任何幫助。
我會說 Pieces 是 null
@if (PieceService.Pieces?.Count == 0)
應該有幫助,或者 go 矯枉過正,並檢查 null 的 PieceService
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