Execute two queries as tasks

Question

I need help understanding how Task work in c#. I want to call ExecuteClause once for every clause in clauses. I've figured out that I should create a list of tasks and then

await Task.WhenAll(taskList);

Finally I can get the result of each task. This is what I have so far:

public async Task Execute(string[] clauses)
{
    var taskList = new List<Task<in>>();

    // Here I want to call ExecuteClause for each string in clauses
    // and wait for the result.
}

private async Task<int> ExecuteClause(string clause)
{
    var connectionString = ConfigurationManager.ConnectionStrings["default"].ConnectionString;
    var connection = new SqlConnection(connectionString);
    await connection.OpenAsync();

    var r = await new SqlCommand(clause, connection).ExecuteReaderAsync();

    while (await r.ReadAsync())
    {
        ...
    }

    r.Close();

    connection.Close();

    // Just an example returning an int
    return 1;
}

`var taskList = clauses.Select(c => ExecuteClause(c));` or jus tuse a simple `for` loop. — DavidG, Feb 09 '18 at 15:19
@DavidG Brilliant! I think I finally understand how it works now. Much easier than I thought! — Asken, Feb 09 '18 at 15:21
@DavidG - you can also use ExecuteClause as a method group for the call to Select — Josh E, Feb 09 '18 at 15:26
@JoshE Yes and that's how I would probably write it in code, but for SO purposes it often confuses people. — DavidG, Feb 09 '18 at 15:26

score 0 · Accepted Answer · answered Feb 09 '18 at 15:21

I think the answer is simpler than you suspected.

You'll loop over the clauses array and add a new Task to the taskList in each iteration. Afterwards, you'll use WhenAll to aggregate all of the separate tasks into a single one that returns when all constituent tasks have finished:

public async Task Execute(string[] clauses)
{
    var taskList = clauses.Select(ExecuteClause);
    return Task.WhenAll(taskList);
}

You'll want to fix your ExecuteClause method to ensure proper disposal, since the way it's currently written may not handle the connections in all situations:

private async Task<int> ExecuteClause(string clause)
{
    var connectionString = ConfigurationManager
                 .ConnectionStrings["default"].ConnectionString;
    using (var connection = new SqlConnection(connectionString)) {

        await connection.OpenAsync();

        var r = await new SqlCommand(clause, connection).ExecuteReaderAsync();

        while (await r.ReadAsync())
        {
            ...
        }

        r.Close();

        connection.Close();
    }
    // Just an example returning an int
    return 1;
}

Thanks. Good point about the `using`. What about using method group as you commented above? — Asken, Feb 10 '18 at 06:40

Execute two queries as tasks

1 Answers1