Never coming back from Task.WaitAll

Question

Still coming up to speed on Xamarin and C#.

I have some code like:

        List<Task<int>> taskList = new List<Task<int>>();
        ConfigEntry siteId = new ConfigEntry
        {
            ConfigKey = KEY_SITE_ID,
            ConfigValue = siteInfo.siteId
        };
        taskList.Add(ConfigDatabase.SaveConfigAsync(siteId));

        ConfigEntry productId = new ConfigEntry
        {
            ConfigKey = KEY_PRODUCT_ID1,
            ConfigValue = siteInfo.products[0].productId.ToString()
        };
        taskList.Add(ConfigDatabase.SaveConfigAsync(productId));

There's a total of nine of these getting added to taskList. Each of these inserts stuff into SQLITE. Here is the code being run:

    public async Task<int> SaveConfigAsync(ConfigEntry entry)
    {
        if (entry.ConfigKey == null)
        {
            throw new Square1Exception("Config entry key not defined:" + entry);
        }
        else
        {
            try
            {
                ConfigEntry existing = await GetConfigAsync(entry.ConfigKey);
                if (existing == null)
                {
                    return await _database.InsertAsync(entry);
                }
                else
                {
                    existing.UpdateFrom(entry);
                    return await _database.UpdateAsync(entry);
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine("Error while saving value:" + entry.ConfigKey);
                throw ex;
            }
        }
    }

So at the end of the building of this tasklist, I have the following line:

        Task.WaitAll(taskList.ToArray());

Which I had hoped would wait until all of the adds completed before exiting. Instead it is never coming back from this. It just hangs my whole app. Not seeing anything in the log either. Does it (potentially) start the task when created or wait until something like WaitAll?

If I replace each of the adds with an await and single thread them it works fine. Maybe blocking on the database or disk?

Ideas?

score 2 · Accepted Answer · answered Feb 09 '21 at 19:11

2

You shouldn't block on asynchronous code.

The best fix is to change Task.WaitAll(taskList.ToArray()); to await Task.WhenAll(taskList);.

If you must block, then you can use Task.Run to push the work to background threads, as such:

taskList.Add(Task.Run(() => ConfigDatabase.SaveConfigAsync(siteId)));
...
taskList.Add(Task.Run(() => ConfigDatabase.SaveConfigAsync(productId)));

But then you would be blocking your UI thread at the Task.WaitAll, so I don't recommend that approach. Using await Task.WhenAll is better.

answered Feb 09 '21 at 19:11

Stephen Cleary

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Can you explain the difference between these two? – Thom Feb 09 '21 at 19:22
`Task.WhenAll` is asynchronous; `Task.WaitAll` is synchronous (blocking). – Stephen Cleary Feb 09 '21 at 20:06

score -1 · Answer 2 · answered Feb 09 '21 at 19:20

You are missing await when doing the Task.WhenAll(Tasks). Try the following solution:

ConfigEntry siteId = new ConfigEntry
{
    ConfigKey = KEY_SITE_ID,
    ConfigValue = siteInfo.siteId
};
ConfigEntry productId = new ConfigEntry
{
    ConfigKey = KEY_PRODUCT_ID1,
    ConfigValue = siteInfo.products[0].productId.ToString()
};

var insertResultFirstTask = ConfigDatabase.SaveConfigAsync(siteId)
var insertResultSecondTask = ConfigDatabase.SaveConfigAsync(productId)

IEnumerable<Task> tasks = new List<Task>() {
    insertResultFirstTask,
    insertResultSecondTask
};

await Task.WhenAll(tasks);

var insertResultFirst = insertResultFirstTask.Result;
var insertResultSecond = insertResultSecondTask.Result;

And it is clear from your question. I just tried to help with a different approach. If this code is not suitable for your needs then I can accept your negative response, but you should consider it. — eugen, Feb 09 '21 at 19:30

Never coming back from Task.WaitAll

2 Answers2