List.Add沒有與Parallel和async完美添加

我有一個 Blazor WASM 應用程式，它需要在不阻塞 UI 的情況下每秒呼叫一個 API。此代碼演示了我如何嘗試這樣做：

List<int> testList = new();
testList.Add(1);
testList.Add(2);
testList.Add(3);
testList.Add(4);

List<int> emptyTestlist = new();

CancellationTokenSource cts;

Test();

void Test()
{
    Parallel.Invoke(async () =>
    {
        do
        {
            Console.WriteLine("Start");
            await Task.Delay(1000);
            await Test2();
            Console.WriteLine("END");
        } while (true);
    });
}
Console.ReadLine();

async ValueTask Test2()
{
    emptyTestlist.Clear();
    cts = new();
    await Parallel.ForEachAsync(testList, cts.Token, async (test, token) =>
    {
        await Test4(test);
    });
    foreach (var test in emptyTestlist)
    {
        await Test3(test);
    }
}

async Task Test4(int i)
{
    await Task.Delay(300);
    //Console.WriteLine("if I Add this console.WriteLine It's added perfectly");
    emptyTestlist.Add(i);
    Console.WriteLine($"from TEST4: {i}");
}

async Task Test3(int i)
{
    Console.WriteLine($"TEST3 {i}.");
    await Task.Delay(1000);
    Console.WriteLine($"TEST3 {i}, after 1sec");
}

如果我評論該行Console.WriteLine("if I Add this console.WriteLine It's added perfectly");，它并沒有完美添加。（emptyTestlist.Count并不總是 4）。但是如果我Console.WriteLine在emptyTestlist.Add(i)它正常作業之前添加（emptyTestlist.Count總是 4）。

我不知道如何解決。有什么問題？

uj5u.com熱心網友回復：

輪詢 API 的最簡單方法是使用計時器：

@code {
    private List<Customer> custs=new List<Customer>();
    
    private System.Threading.Timer timer;

    protected override async Task OnInitializedAsync()
    {
        await base.OnInitializedAsync();
        custs = await Http.GetFromJsonAsync<List<Customer>>(url);

        timer = new System.Threading.Timer(async _ =>
        {
            custs = await Http.GetFromJsonAsync<List<Customer>>("/api/customers");
            InvokeAsync(StateHasChanged); 
        }, null, 1000, 1000);
    }

在這種情況下InvokeAsync(StateHasChanged);是需要的，因為狀態是從計時器執行緒修改的，而 Blazor 不知道資料已更改。

但是，如果我們想將結果添加到串列中，我們必須使用鎖或執行緒安全集合，例如ConcurrentQueue.

@code {
    private ConcurrentQueue<Customer> custs=new ConcurrentQueue<Customer>();
    
    private System.Threading.Timer timer;

    protected override async Task OnInitializedAsync()
    {
        await base.OnInitializedAsync();
        custs = await Http.GetFromJsonAsync<List<Customer>>(url);

        timer = new System.Threading.Timer(async _ =>
        {
            var results = await Http.GetFromJsonAsync<List<Customer>>("/api/customers");
            foreach(var c in results)
            {
                custs.Enqueue(c);
            }
            InvokeAsync(StateHasChanged); 
        }, null, 1000, 1000);
    }

每秒輪詢一個 API 以防萬一有任何新資料雖然效率不高。最好讓 API 使用例如SignalR或Push Notifications通知客戶端任何新資料

借用檔案示例，這足以從服務器接收訊息：

@code {
    private HubConnection hubConnection;
    private List<string> messages = new List<string>();
    private string userInput;
    private string messageInput;

    protected override async Task OnInitializedAsync()
    {
        hubConnection = new HubConnectionBuilder()
            .WithUrl(NavigationManager.ToAbsoluteUri("/chathub"))
            .Build();

        hubConnection.On<string, string>("ReceiveMessage", (user, message) =>
        {
            var encodedMsg = $"{user}: {message}";
            messages.Add(encodedMsg);
            StateHasChanged();
        });

        await hubConnection.StartAsync();
    }

標籤：C＃ 异步等待 blazor-webassembly 并行.foreach .net-6.0

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