C＃等待的问题

Question

documents is a IDictionary<string, string> where the parameters are <filename, fileUrl>

DocumentHandler.Download() returns a Task<Memorystram>

This code works:

foreach (var x in documents.Keys)
    {
        var result = await DocumentHandler.Download(new Uri(documents[x]));
        // other code
    }

however it rund synchronously.

In order to run it all async i wrote this code:

var keys =
documents.Keys.Select(async x =>
    {
        return Tuple.Create(x, await DocumentHandler.Download(new Uri(documents[x])));
    });
await Task.WhenAll(keys);
foreach (var item in keys)
{
    var tpl = item.Result;
    // other code
}

It doesn't work, it crashes without showing an exception on the last line var tpl = item.Result; Why?

Answer 1

Your keys variable will create a new set of tasks every time you evaluate it... so after waiting for the first set of tasks to complete, you're iterating over a new set of unfinished tasks. The simple fix for this is to add a call to ToList() :

var keys = documents
    .Keys
    .Select(async x => Tuple.Create(x, await DocumentHandler.Download(new Uri(documents[x]))))
    .ToList();
await Task.WhenAll(keys);
foreach (var item in keys)
{
    var tpl = item.Result;
    // other code
}