Getting a Cannot await void, on a method that I have want to await on

Question

I'm on a team writing a WPF app. We have to make it so that when a user hides/shows different columns that it will reflect that in a ReportViewer control on one of the views. In testing we've found that it takes a long time to add the data to the ReportViewer's data sources; sometimes on the order of several seconds to possibly a minute. Too long I think for the users. So I'm trying to use C#'s async and await. However, when I applied await to the line that is the process hog and then compile it, I get an error from the C# compiler, "Cannot await 'void'". In this case, I cannot change what the .NET framework returns, its void. So how do I handle this situation? Here's the code:

private async Task GenerateReportAsync()
{
    DataSet ds = new DataSet();
    DataTable dt = new DataTable();
    dt.Clear();
    int iCols = 0;
    //Get the column names
    if (Columns.Count == 0)     //Make sure it isn't populated twice
    {
        foreach (DataGridColumn col in dataGrid.Columns)
        {
            if (col.Visibility == Visibility.Visible)
            {
                Columns.Add(col.Header.ToString());     //Get the column heading
                iCols++;
            }
        }
    }
    //Create a DataTable from the rows
    var itemsSource = dataGrid.ItemsSource as IEnumerable<Grievance>;
    if (this.rptViewer != null)
    {
        rptViewer.Reset();
    }
    rptViewer.LocalReport.DataSources.Clear();
    if (m_rdl != null)
    {
        m_rdl.Dispose();
    }
    Columns = GetFieldOrder();
    m_rdl = CoreUtils.GenerateRdl(Columns, Columns);
    rptViewer.LocalReport.LoadReportDefinition(m_rdl);
    //the next line is what takes a long time
    await rptViewer.LocalReport.DataSources.Add(new ReportDataSource("MyData", CoreUtils.ToDataTable(itemsSource)));
    rptViewer.RefreshReport();
}

You must have some false belief about what `await` does if you think that awaiting something that takes a long time is a solution to your problem. Can you describe what your beliefs are about `await`? It only makes sense to await something that is *already* asynchronous; do you believe that await *makes* something that is not asynchronous into an asynchronous operation? It does not. — Eric Lippert, Feb 14 '17 at 18:44
It is my understanding that using async/await makes it possible to return control to the UI thread, rather than hang it up, waiting for some long working process to finish. But thank you for asking. — Rod, Feb 14 '17 at 20:40
My point is that if the function is void returning *and* already asynchronous then *it should not be taking much time*. If it is void returning, synchronous, and high latency then there is nothing to `await`. Await manages an existing asynchronous operation. If you have a high latency operation that you'd like to be asynchronous, `await` isn't going to help you. You're going to have to figure out how to make it asynchronous via some other mechanism. — Eric Lippert, Feb 14 '17 at 21:12

BradleyDotNET · Accepted Answer · 2017-02-14T19:45:41.093

22

For methods that are inherently synchronous, you need to wrap them in your own Task so you can await it. In your case, I would just use Task.Run:

await Task.Run(() => 
{
   rptViewer.LocalReport.DataSources.Add(new ReportDataSource("MyData", CoreUtils.ToDataTable(itemsSource)));
});

There are other ways to generate a task, but this one is probably the easiest. If this updates a UI component, it will likely throw an exception because those operations must occur on the UI thread. In this case, try to get the UI related piece away from the long-running part and only wrap the long part in the Task.

edited Feb 14 '17 at 19:45

answered Feb 14 '17 at 18:43

BradleyDotNET

60,462
10
96
117

3

It is possible that this will throw a cross thread exception because you are accessing UI components like the report viewer from a background thread. – Scott Chamberlain Feb 14 '17 at 18:46
@ScottChamberlain Absolutely, I have noted this. – BradleyDotNET Feb 14 '17 at 19:46
@BradleyDotNET I've taken your suggestion and came up with the following: DataTable dtTmp = null; await Task.Run(() => dtTmp = CoreUtils.ToDataTable(itemsSource)); rptViewer.LocalReport.DataSources.Add(new ReportDataSource("MyData", dtTmp)); I didn't know that using await on a UI component would cause an exception. Thanks for the head's up! The above gives control back to the UI thread quickly. Thanks, again.. – Rod Feb 14 '17 at 20:44
2

@Rod it is not the await that causes the exception it is the Tas.Run that does. – Scott Chamberlain Feb 14 '17 at 21:02

score 5 · Answer 2 · answered Feb 14 '17 at 18:45

I can almost be certain that the entire line is not the slow line, it is much more likely that CoreUtils.ToDataTable(itemsSource) is the slow piece and that is the part that needs to be improved.

You did not include the source of ToDataTable so I can't say for sure what the best way would be, the first and best option would be write a new version of the function that leverages async calls internally to create a function that allows you to await it.

var data = await CoreUtils.ToDataTableAsync(itemsSource)
rptViewer.LocalReport.DataSources.Add(new ReportDataSource("MyData", data));

The second, less performant option is do the slow part on a background thread using Task.Run then back on the UI thread set the data source as needed.

var data = await Task.Run(() => CoreUtils.ToDataTable(itemsSource));
rptViewer.LocalReport.DataSources.Add(new ReportDataSource("MyData", data));

Getting a Cannot await void, on a method that I have want to await on

2 Answers2

Linked