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();
}

Answer 1

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.