Task.WaitAll() vs. Task.WhenAll(): When Should I Use Which?

In-depth understanding of the difference between Task.WaitAll() and Task.WhenAll()

In asynchronous programming, task operations are the cornerstone of Async CTP. Task.WaitAll() and Task.WhenAll() are two of the key methods, but they have different characteristics that affect their application scenarios.

Task.WaitAll()

Task.WaitAll() Adopt a more traditional way of synchronous task processing. What it does is block the current thread until all tasks it contains are completed. This approach is useful when you need immediate access to the results of all tasks. However, it is important to note that blocking threads may cause performance penalties in asynchronous scenarios.

Task.WhenAll()

In contrast, Task.WhenAll() takes an asynchronous approach. It does not block the current thread, but returns a task that represents the operation of waiting for all tasks to complete. This task can be awaited in an async method, allowing your method to continue execution after all tasks have completed.

Code Example

Consider the following code snippet:

async Task DoSomethingAsync()
{
    // 创建一个任务列表
    var tasks = new List<Task>();
    for (int i = 0; i < 5; i++)
    {
        tasks.Add(Task.Run(() => { /* 执行一些耗时操作 */ }));
    }

    // 使用 Task.WaitAll() 等待所有任务完成
    Task.WaitAll(tasks.ToArray());

    // 所有任务完成后执行此代码
    Console.WriteLine("所有任务已完成");
}

In this example, Task.WaitAll() is used to ensure that all tasks are completed before the method continues execution. This method is suitable if synchronous execution is required.

Alternatively, consider the following code:

async Task DoSomethingAsync()
{
    // 创建一个任务列表
    var tasks = new List<Task>();
    for (int i = 0; i < 5; i++)
    {
        tasks.Add(Task.Run(() => { /* 执行一些耗时操作 */ }));
    }

    // 使用 Task.WhenAll() 异步等待所有任务
    await Task.WhenAll(tasks);

    // 所有任务完成后执行此代码
    Console.WriteLine("所有任务已完成");
}

Here, Task.WhenAll() is used to await all tasks asynchronously. The method continues execution immediately, but the await operator pauses the current task until all other tasks are completed. This approach keeps execution asynchronous and prevents threads from blocking.

Through the above comparison, you can choose the appropriate method according to your actual needs. If you need to wait for all tasks to complete synchronously, use Task.WaitAll(); if you need to maintain asynchronous execution and avoid blocking threads, use Task.WhenAll().

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