异步使用另一对象更新目标对象触发空引用异常问题排查

问题根因 #

该NullReferenceException的触发点有两个高概率场景：

1. OnInitializedAsync 执行时，[Parameter] 标记的 CustomerId 还未完成父组件传值，仍为默认值0，调用dataService.Get1056StatusById(CustomerId) 返回null，后续访问Statuses.CustomerId时触发空引用
2. UpdateValues方法中调用查询接口返回null，直接访问PreviousStatuses的属性赋值时触发空引用

修复方案 #

1. 调整初始化时机+增加空值校验，避免访问未初始化的对象

修改后的代码如下：

[Parameter]
public int CustomerId { get; set; }

[CascadingParameter]
public Task<AuthenticationState> AuthState { get; set; }

public Status Statuses;
public Status PreviousStatuses;

public Dspd1056Status StatusToAdd = new Dspd1056Status();

public Customer customer;

public int AccountStatusId = 0;

// 初始化逻辑挪到OnParametersSetAsync，保证Parameter已完成赋值
protected override async Task OnParametersSetAsync()
{
    // 先判断CustomerId有效再调用接口
    if (CustomerId <= 0) return;
    
    Statuses = await dataService.Get1056StatusById(CustomerId);
    // 校验Statuses不为空再执行后续逻辑
    if (Statuses == null || string.IsNullOrEmpty(Statuses.CustomerId))
    {
        // 可自行添加数据不存在的提示逻辑
        return;
    }
    Console.WriteLine(Statuses.CustomerId);
    await UpdateValues(Statuses.CustomerId);
}

async Task UpdateValues(string id)
{
    if (!int.TryParse(id, out int customerId))
    {
        // 可自行添加ID格式错误的处理逻辑
        return;
    }
    PreviousStatuses = await dataService.Get1056StatusById(customerId);
    // 校验PreviousStatuses不为空再赋值
    if (PreviousStatuses == null)
    {
        // 可自行添加无对应状态数据的处理逻辑
        return;
    }
    StatusToAdd.AccountCurrent = PreviousStatuses.AccountCurrent;
    StatusToAdd.StartDate = PreviousStatuses.StartDate;
    StatusToAdd.EndDate = PreviousStatuses.EndDate;
    StatusToAdd.Units = PreviousStatuses.Units;
    StatusToAdd.Ppc = PreviousStatuses.Ppc;
    StatusToAdd.EndStatus = PreviousStatuses.EndStatus;
    StatusToAdd.ContinuallyFunded = PreviousStatuses.ContinuallyFunded;
    StatusToAdd.AnnualUnits = PreviousStatuses.AnnualUnits;
    StatusToAdd.Elg = PreviousStatuses.Elg;
    StatusToAdd.ReceiptDate = PreviousStatuses.ReceiptDate;
    StatusToAdd.RahTripsFunded = PreviousStatuses.RahTripsFunded;
    StatusToAdd.Rate = PreviousStatuses.Rate;
    StatusToAdd.AccountTotal = PreviousStatuses.AccountTotal;
}

protected override async Task OnAfterRenderAsync(bool firstRender)
{
    if (firstRender)
    {
        if (CustomerId != 0)
        {
            customer = await dataService.GetCustomerById(CustomerId);
            StateHasChanged();
        }
    }
}

补充说明 #

Blazor组件的参数绑定在OnInitializedAsync执行完成后才会结束，部分场景下该阶段CustomerId还未收到父组件传值，默认值0会导致查询返回空数据。所有接口返回对象都增加空值校验，能从根源避免空引用异常。

内容的提问来源于stack exchange，提问作者Brsn