c# - 遵循异步进度和取消的 MSDN 博客示例时出现 Lamba 错误

Question

我正尝试通过阅读博客文章 here 来了解异步编程.我遇到了一个编译器错误，指出我必须将 lambda 表达式与 await 一起使用。提供了答案here , 和 here ，但我没有掌握如何将解决方案合并到我尝试使用的 MSDN 示例中。

此行抛出以下错误:

int processed = await UploadAndProcessAsync(image);

'await' 运算符只能在异步 lambda 表达式中使用。考虑使用“async”修饰符标记此 lambda 表达式。

来自这段代码:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows;
using System.Threading;
using System.Threading.Tasks;
using System.IO;
using System.Windows.Forms;

namespace AsyncAttempt
{
    public partial class Form1 : Form
    {

        public Form1()
        {
            InitializeComponent();
        }

        async Task<int> UploadPicturesAsync(List<Image> imageList, IProgress<int> progress)
        {
            int totalCount = imageList.Count;
            int processCount = await Task.Run<int>(() =>
            {
                int tempCount = 0;
                foreach (var image in imageList)
                {
                    //await the processing and uploading logic here
                    int processed = await UploadAndProcessAsync(image);
                    if (progress != null)
                    {
                        progress.Report((tempCount * 100 / totalCount));
                    }
                    tempCount++;
                }

                return tempCount;
            });
            return processCount;
        }

        private Task<int> UploadAndProcessAsync(Image image)
        {
            throw new NotImplementedException();
        }

        private async void Start_Button_Click(object sender, RoutedEventArgs e)
        {
            //construct Progress<T>, passing ReportProgress as the Action<T> 
            var progressIndicator = new Progress<int>(ReportProgress);
            //call async method
            int uploads = await UploadPicturesAsync(GenerateTestImages(), progressIndicator);
        }

        private List<Image> GenerateTestImages()
        {
            throw new NotImplementedException();
        }

        void ReportProgress(int value)
        {
            //Update the UI to reflect the progress value that is passed back.
        }

    }
}

Answer 1

这个:

int processCount = await Task.Run<int>(() => /* process image */)

将使用 lambda 表达式创建委托(delegate)。为了让您能够在其中await，需要将其标记为async:

int processCount = await Task.Run<int>(async () => /* process image */)