如何从第一个任务中获取结果然后在循环中使用 ContinueWith 将其传递给第二个任务？

Question

So here's what I'm doing:

var resultObj = GetFirstResultAsync()
     .ContinueWith(t => GetSecondResultAsync(resultObj))
     .Wait();

Essentially telling me that I can't use the variable before it's even declared, which I get hence the question I posted. How do I do this?

Here's my goal, I have a list resultObj then on that list I will loop through the Id's to get an array of my another list that I want GetSecondResultAsync kinda drilling down on the list, I wanted to use async since it will be using HttpClient to get the data.

Answer 1

You should be able to pass the result by changing the ContinueWith statement to t.Result. Something like:

var resultObj = GetFirstResultAsync()
 .ContinueWith(t => GetSecondResultAsync(t.Result));

I haven't tested the code.

The first run thread will return a "Task". Which you will have to access by Task.Result. This is what you will have to use.

Edit: As @fahadash stated, "wait()" should not be used. Use "await" instead.

Answer 2

Creating a continuation Task that has a result of different type than the first task, with ContinueWith , is not trivial. You have to use the Unwrap method in order to flatten the resulting Task<Task<MySecondResult>> , handle carefully the failure and cancellation cases, and specify explicitly the TaskScheduler that will execute the continuation:

Task<MySecondResult> task = GetFirstResultAsync().ContinueWith(t =>
{
    if (t.IsFaulted)
    {
        var tcs = new TaskCompletionSource<MySecondResult>();
        tcs.SetException(t.Exception.InnerExceptions);
        return tcs.Task;
    }
    if (t.IsCanceled)
    {
        TaskCompletionSource<MySecondResult> tcs = new();
        tcs.SetCanceled(new TaskCanceledException(t).CancellationToken);
        return tcs.Task;
    }
    return GetSecondResultAsync(t.Result);
}, TaskScheduler.Default).Unwrap();

This is quite cumbersome. You don't want to have code like this appear frequently in your application code. You can either encapsulate this functionality in an extension method, like the Then / ContinueWithResult extension method requested in this GitHub proposal, or use async/await instead of the ContinueWith . Async/await is composable, so you can write a third async method that combines the GetFirstResultAsync and the GetSecondResultAsync to one:

async Task<MySecondResult> GetSecondAfterFirstAsync()
{
    MyFirstResult result = await GetFirstResultAsync();
    return await GetSecondResultAsync(result);
}