c# - 任务不更新WPF中的UI

Question

我写了这段代码:

public double SumRootN(int root)
{
    double result = 0;
    for (int i = 1; i < 10000000; i++)
    {
        tokenSource.Token.ThrowIfCancellationRequested();
        result += Math.Exp(Math.Log(i) / root);
    }
    return result;
}

private void btnclick_Click(object sender, RoutedEventArgs e)
{
    tokenSource = new CancellationTokenSource();
    txttest.Text = "";

    var watch = Stopwatch.StartNew();
    List<Task> tasks = new List<Task>();
    var ui = TaskScheduler.FromCurrentSynchronizationContext();
    for (int i = 2; i < 20; i++)
    {
        int j = i;
        var compute = Task.Factory.StartNew(() =>
        {
            return SumRootN(j);
        }, tokenSource.Token);

        tasks.Add(compute);

        var displayResults = compute.ContinueWith(
                                resultTask =>
                                   txttest.Text
                                    += "root " + j.ToString() + " " +
                                        compute.Result.ToString() +
                                        Environment.NewLine,
                                CancellationToken.None,
                                TaskContinuationOptions.OnlyOnRanToCompletion,
                                ui);             
    }      
}

它可以在WPF中工作，但是当我以这种方式编写此代码时；

tokenSource = new CancellationTokenSource();
var watch = Stopwatch.StartNew();
List<Task> tasks = new List<Task>();
var ui = TaskScheduler.FromCurrentSynchronizationContext();

Report 
   += ((Microsoft.Office.Interop.Excel.Range)_sheet.Cells[row, "B"]).Value2;

var compute = Task.Factory.StartNew(() =>
             {
                return Report;                             
             }, tokenSource.Token);

            tasks.Add(compute);

            var displayResults
                = compute.ContinueWith(resultTask =>
                     txtReport.Text 
                        += compute.Result.ToString() +
                           Environment.NewLine,
                     CancellationToken.None,
                     TaskContinuationOptions.OnlyOnRanToCompletion,
                     ui);

它不起作用，但是 txtReport.Text具有正确的值，但是它不在操作中间显示，但是当操作结束时 txtReport显示它的值

为什么此代码不起作用？

Answer 1

ContinueWith创建一个继续执行，该继续执行在目标Task完成时异步执行。也就是说，只有在您的SumRootN操作完成后，它才会更新结果。