c# - 任务 ContinueWith 行为中的异步？

Question

你如何解释以下行为:

await Task.Run(() => { }).ContinueWith(async prev =>
{
    Console.WriteLine("Continue with 1 start");
    await Task.Delay(1000);
    Console.WriteLine("Continue with 1 end");
}).ContinueWith(prev =>
{
    Console.WriteLine("Continue with 2 start");
});

为什么我们会在“Continue with 1 end”之前得到“Continue with 2 start”？

Answer 1

下面的代码等同于您的示例，其中显式声明了变量，因此更容易看出发生了什么:

Task task = Task.Run(() => { });

Task<Task> continuation1 = task.ContinueWith(async prev =>
{
    Console.WriteLine("Continue with 1 start");
    await Task.Delay(1000);
    Console.WriteLine("Continue with 1 end");
});

Task continuation2 = continuation1.ContinueWith(prev =>
{
    Console.WriteLine("Continue with 2 start");
});

await continuation2;
Console.WriteLine($"task.IsCompleted: {task.IsCompleted}");
Console.WriteLine($"continuation1.IsCompleted: {continuation1.IsCompleted}");
Console.WriteLine($"continuation2.IsCompleted: {continuation2.IsCompleted}");

Console.WriteLine($"continuation1.Unwrap().IsCompleted:" +
    $" {continuation1.Unwrap().IsCompleted}");

await await continuation1;

输出:

Continue with 1 start
Continue with 2 start
task.IsCompleted: True
continuation1.IsCompleted: True
continuation2.IsCompleted: True
continuation1.Unwrap().IsCompleted: False
Continue with 1 end

棘手的部分是变量 continuation1 ，即 Task<Task> 类型. ContinueWith 方法不会自动解包 Task<Task>返回值如 Task.Run 确实如此，所以你最终会得到这些嵌套的任务。外层Task的工作只是创建内部 Task .当内Task已创建(未完成!)，然后是外部 Task已经完成。这就是为什么 continuation2在内Task之前完成的 continuation1 .

有一个内置的扩展方法 Unwrap 这使得打开 Task<Task> 变得容易.一个展开的 Task当外部和内部任务都完成时，完成。打开 Task<Task> 的另一种方法是使用await运算符两次:await await .