Task.WaitAll() vs. Task.WhenAll(): When Should I Use Each in Async Programming?

Deeply explore the difference between Task.WaitAll() and Task.WhenAll() in .NET asynchronous programming

In .NET asynchronous programming, it is crucial to understand the difference between Task.WaitAll() and Task.WhenAll() in Async CTP. These two methods serve different purposes, and their characteristics also affect the performance and flow of your code.

Task.WaitAll()

Task.WaitAll() is a blocking operation that blocks the current thread until all tasks in the specified array are completed. This means that the method will only return after each task has been completed, no matter how long it takes.

Task.WhenAll()

In contrast, Task.WhenAll() returns a Task instance that represents the completion of all tasks in the specified array. This means that the method does not block the current thread but continues executing the next line of code. The returned Task will be completed when all input tasks have been completed.

When to use which?

The choice between Task.WaitAll() or Task.WhenAll() depends on the specific needs of your code. Here are some real-world use cases:

• When to use Task.WaitAll():

  • You need to wait until all tasks are completed before continuing.
  • You want to ensure that there are no concurrent accesses to resources or data.

• When to use Task.WhenAll():

  • You want to avoid blocking the current thread while waiting for the task to complete.
  • You want to delegate task completion to another part of the code (for example, using the await keyword).

Code Example

Consider the following code snippets to illustrate different use cases:

// 使用 Task.WaitAll() 等待任务
var tasks = new[] { Task.Delay(1000), Task.Delay(2000), Task.Delay(3000) };
Task.WaitAll(tasks); // 阻塞当前线程，直到所有任务完成

// 使用 Task.WhenAll() 创建延续任务
var tasks = new[] { Task.Delay(1000), Task.Delay(2000), Task.Delay(3000) };
var continuationTask = Task.WhenAll(tasks); // 返回一个任务，当所有输入任务完成后该任务完成

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