Is it possible to “await yield return DoSomethingAsync()”

Question

Are regular iterator blocks (i.e. \"yield return\") incompatible with \"async\" and \"await\"?

This gives a good idea of what I\'m trying to do:

asyn         


        

Answer 1

According to the new features at C# 8.0 (link#1 and link#2) we'll have IAsyncEnumerable interface support that will allow to implement your second attempt. It will look like this:

async Task DoSomethingAsync(string url)
{
    ...
}       
// producing IAsyncEnumerable
async IAsyncEnumerable DownLoadAllURL(string[] strs)
{
    foreach (string url in strs)
    {
        yield return await DoSomethingAsync(url);
    }
}
...
// using
await foreach (Foo foo in DownLoadAllURL(new string[] { "url1", "url2" }))
{
    Use(foo);
}

We can achieve the same behavior at C# 5 but with a different semantics:

async Task DoSomethingAsync(string url)
{
    ...
}
IEnumerable> DownLoadAllURL(string[] strs)
{
    foreach (string url in strs)
    {
        yield return DoSomethingAsync(url);
    }
}

// using
foreach (Task task in DownLoadAllURL(new string[] { "url1", "url2" }))
{
    Foo foo = await task;
    Use(foo);
}

Brian Gideon's answer implies that the calling code will get asynchronously a collection of results that were obtained in parallel. The code above implies that the calling code will get results like from a stream one by one in asynchronous manner.