C# Parallel.ForEach循环调用API时员工详情对象偶尔为空问题

问题根源分析 #

1. GetEmployeeDetails返回非null的空对象：该方法无论API调用或解析是否失败，都会返回一个新创建的ImportedEmployee实例，只是失败时属性未赋值（全为null）。你判断importedEmployee != null永远为true，导致这些空属性的对象被加入列表，看起来像是“员工对象为null”。
2. Parallel.ForEach与Task.Run的冗余嵌套：Parallel.ForEach本身已使用线程池并行执行，内部再调用Task.Run会额外创建任务，导致线程池资源过度消耗，增加异常概率。
3. 同步阻塞调用引发的潜在问题：ServiceRequest中使用.Result阻塞异步操作，在存在同步上下文的环境中容易引发死锁，也可能导致API请求超时或失败。

解决方案 #

1. 修复GetEmployeeDetails，失败时返回null

修改该方法，仅当API调用和解析完全成功时才返回员工对象，失败则返回null：

public ImportedEmployee GetEmployeeDetails(string empNum)
{
    try
    {
        var responseString = ServiceRequest($"Employees/{empNum}", null, HttpVerbs.Get);
        if (string.IsNullOrEmpty(responseString))
            return null;

        JObject result = JObject.Parse(responseString);
        return new ImportedEmployee
        {
            FirstName = result["FirstName"]?.ToString(),
            LastName = result["LastName"]?.ToString(),
            Username = result["LoginId"]?.ToString(),
            EmployeeNumber = empNum // 补充员工编号，避免丢失
        };
    }
    catch (Exception ex)
    {
        Log.Error($"Error retrieving employee details for {empNum}.", ex);
        return null;
    }
}

2. 改用异步并行任务替代Parallel.ForEach+Task.Run

直接使用Task.WhenAll并行执行所有API请求，避免线程池资源浪费，同时简化代码：

public async Task<List<ImportedEmployee>> GetEmployeesAsync(List<string> empNumbers)
{
    var employeeList = new List<ImportedEmployee>();

    try
    {
        // 生成所有异步任务
        var tasks = empNumbers.Select(empNum => Task.Run(() => GetEmployeeDetails(empNum)));
        // 等待所有任务完成
        var results = await Task.WhenAll(tasks);
        // 过滤掉null结果，添加到列表
        employeeList.AddRange(results.Where(e => e != null));
    }
    catch (Exception ex)
    {
        Log.Error("Error retrieving employees.", ex);
    }

    return employeeList;
}

3. 修复ServiceRequest的异步阻塞问题

将ServiceRequest改为异步方法，使用await替代.Result，避免死锁和阻塞：

public async Task<string> ServiceRequestAsync(string URI, string PostJSON, HttpVerbs Method)
{
    try
    {
        using (HttpClient client = new HttpClient() { Timeout = TimeSpan.FromSeconds(Configuration.ClientTimeout) })
        {
            HttpRequestMessage request = new HttpRequestMessage();
            switch (Method)
            {
                case HttpVerbs.Get:
                    request = new HttpRequestMessage(HttpMethod.Get, BaseURL + URI);
                    break;
                default:
                    request = new HttpRequestMessage(HttpMethod.Post, BaseURL + URI);
                    request.Content = new StringContent(PostJSON, null, "application/json");
                    break;
            }
            request.Headers.Add("Authorization", $"Bearer {AuthToken.Token}");
            var resp = await client.SendAsync(request);
            resp.EnsureSuccessStatusCode(); // 确保HTTP请求成功，否则抛出异常
            return await resp.Content.ReadAsStringAsync();
        }
    }
    catch (Exception ex)
    {
        Log.Error($"Error in service request for URI: {URI}", ex);
        return null;
    }
}

同时更新GetEmployeeDetails调用异步的ServiceRequestAsync：

public async Task<ImportedEmployee> GetEmployeeDetailsAsync(string empNum)
{
    try
    {
        var responseString = await ServiceRequestAsync($"Employees/{empNum}", null, HttpVerbs.Get);
        if (string.IsNullOrEmpty(responseString))
            return null;

        JObject result = JObject.Parse(responseString);
        return new ImportedEmployee
        {
            FirstName = result["FirstName"]?.ToString(),
            LastName = result["LastName"]?.ToString(),
            Username = result["LoginId"]?.ToString(),
            EmployeeNumber = empNum
        };
    }
    catch (Exception ex)
    {
        Log.Error($"Error retrieving employee details for {empNum}.", ex);
        return null;
    }
}

最终的GetEmployeesAsync也相应调整为全异步：

public async Task<List<ImportedEmployee>> GetEmployeesAsync(List<string> empNumbers)
{
    var employeeList = new List<ImportedEmployee>();

    try
    {
        var tasks = empNumbers.Select(GetEmployeeDetailsAsync);
        var results = await Task.WhenAll(tasks);
        employeeList.AddRange(results.Where(e => e != null));
    }
    catch (Exception ex)
    {
        Log.Error("Error retrieving employees.", ex);
    }

    return employeeList;
}

额外优化点 #

• HttpClient复用：当前ServiceRequest每次创建新的HttpClient，建议复用单例HttpClient以避免套接字耗尽问题。
• 添加员工编号：在返回的ImportedEmployee中设置EmployeeNumber，方便后续排查哪个员工数据获取失败。
• HTTP状态码检查：在ServiceRequestAsync中添加resp.EnsureSuccessStatusCode()，确保捕获HTTP错误（如404、500等）。

内容的提问来源于stack exchange，提问作者M.G