c# - Blazor 服务器端,剃刀页面:对象引用未设置为对象的实例
问题描述
我正在使用 .NET 5 版本5.0.100-rc.1.20452.10
,ASP.NET Core Blazor 服务器端。
我的错误
System.NullReferenceException
HResult=0x80004003
Message=Object reference not set to an instance of an object.
Source=acc133blazor
StackTrace:
at acc133blazor.Pages.DeclaredData.Account.AccountsPage.<OnInitializedAsync>d__10.MoveNext() in D:\acc133blazor\acc133blazor\Pages\DeclaredData\Account\AccountsPage.razor:line 144
@foreach (Tuple<string, string> urlInfo in ExportUrlInfo)
{
<a href="@urlInfo.Item1" download target="_blank">@urlInfo.Item2</a>
}
<br />
<br />
}
@code {
private readonly ObservableCollection<Tuple<string, string>> exportUrlInfo = new ObservableCollection<Tuple<string, string>>();
protected Task<LoadResult> LoadOrderData(DataSourceLoadOptionsBase options, CancellationToken cancellationToken) {
string baseUri = NavigationManager.BaseUri.ToString();
exportUrlInfo.Clear();
exportUrlInfo.Add(Tuple.Create(options.ConvertToGetRequestUri(baseUri + "exportPdf"), "Export PDF"));
exportUrlInfo.Add(Tuple.Create(options.ConvertToGetRequestUri(baseUri + "exportXlsx"), "Export XLSX"));
exportUrlInfo.Add(Tuple.Create(options.ConvertToGetRequestUri(baseUri + "exportDocx"), "Export DOCX"));
return Task.FromResult(DataSourceLoader.Load(AccountList, options));
}
public ObservableCollection<Tuple<string, string>> ExportUrlInfo { get; set; }
//...
protected override async Task OnInitializedAsync()
{
AccountList = await Controller.GetAccountAsync();
ListOfAccountTypes = new List<ComboBoxItem>()
{
new ComboBoxItem(0,"Dư Nợ"),
new ComboBoxItem(1,"Dư Có"),
new ComboBoxItem(2,"Lưỡng tính")
};
ExportUrlInfo.CollectionChanged += (s, e) =>
{
InvokeAsync(StateHasChanged);
};
}
如何修复错误?
解决方案
问题就在这里ExportUrlInfo.CollectionChanged
。ExportUrlInfo
尚未初始化。
引用类型需要获取一个初始值:
以下声明并不意味着它将具有值
public ObservableCollection<Tuple<string, string>> ExportUrlInfo { get; set; }
你有两个选择:
- 更改属性以使用支持变量:
public ObservableCollection<Tuple<string, string>> ExportUrlInfo { get { return exportUrlInfo; } set { exportUrlInfo = value; } }
- 更改对私有成员的引用
this.exportUrlInfo.CollectionChanged += (s, e) =>
{
InvokeAsync(StateHasChanged);
};