根据我的理解，async和await所做的主要事情之一是使代码易于编写和阅读-但使用它们是否等于生成后台线程来执行长时间的逻辑?
我目前正在尝试最基本的例子。我内联添加了一些注释。你能给我解释一下吗?
// I don't understand why this method must be marked as `async`.
private async void button1_Click(object sender, EventArgs e)
{
Task<int> access = DoSomethingAsync();
// task independent stuff here
// this line is reached after the 5 seconds sleep from
// DoSomethingAsync() method. Shouldn't it be reached immediately?
int a = 1;
// from my understanding the waiting should be done here.
int x = await access;
}
async Task<int> DoSomethingAsync()
{
// is this executed on a background thread?
System.Threading.Thread.Sleep(5000);
return 1;
}
这里的答案可以作为await/async的一般指导。它们还包含一些关于await/async如何连接的细节。我想和大家分享一些在使用这个设计模式之前应该知道的实践经验。
术语“await”是字面意义上的，所以无论您在哪个线程上调用它，都将在继续之前等待该方法的结果。在前台线程上，这是一个灾难。前台线程承担了构建应用程序的负担，包括视图、视图模型、初始动画，以及其他任何与这些元素捆绑在一起的东西。所以当你等待前台线程时，你会停止应用程序。当什么都没有发生时，用户会一直等待。这提供了一种消极的用户体验。
你当然可以使用各种方法来等待后台线程:
Device.BeginInvokeOnMainThread(async () => { await AnyAwaitableMethod(); });
// Notice that we do not await the following call,
// as that would tie it to the foreground thread.
try
{
Task.Run(async () => { await AnyAwaitableMethod(); });
}
catch
{}
这些注释的完整代码在https://github.com/marcusts/xamarin-forms-annoyances。参见名为AwaitAsyncAntipattern.sln的解决方案。
GitHub网站还提供了关于此主题的更详细讨论的链接。
回答你的第二个问题-何时使用async -这里有一个相当简单的方法，我们使用:
长时间运行的I/O绑定任务，运行时间超过50ms -使用异步。
长时间运行的cpu绑定任务——使用并行执行、线程等。
解释:当你在做I/O工作时——发送网络请求，从磁盘读取数据等——实际的工作是由“外部”硅(网卡，磁盘控制器等)完成的。一旦工作完成，I/O设备驱动程序将“ping”回操作系统，操作系统将执行你的延续代码，回调等。在此之前，CPU可以自由地做自己的工作(作为奖励，你还可以释放一个线程池线程，这对web应用程序的可伸缩性来说是一个非常好的奖励)
附注:50ms阈值是MS的推荐值。否则，异步所增加的开销(创建状态机、执行上下文等)会消耗掉所有的好处。现在找不到MS的原始文章，但这里也提到了https://www.red-gate.com/simple-talk/dotnet/net-framework/the-overhead-of-asyncawait-in-net-4-5/
下面是通过打开对话框读取excel文件的代码，然后使用async和等待运行异步代码，从excel逐行读取并绑定到网格
namespace EmailBillingRates
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
lblProcessing.Text = "";
}
private async void btnReadExcel_Click(object sender, EventArgs e)
{
string filename = OpenFileDialog();
Microsoft.Office.Interop.Excel.Application xlApp = new Microsoft.Office.Interop.Excel.Application();
Microsoft.Office.Interop.Excel.Workbook xlWorkbook = xlApp.Workbooks.Open(filename);
Microsoft.Office.Interop.Excel._Worksheet xlWorksheet = xlWorkbook.Sheets[1];
Microsoft.Office.Interop.Excel.Range xlRange = xlWorksheet.UsedRange;
try
{
Task<int> longRunningTask = BindGrid(xlRange);
int result = await longRunningTask;
}
catch (Exception ex)
{
MessageBox.Show(ex.Message.ToString());
}
finally
{
//cleanup
// GC.Collect();
//GC.WaitForPendingFinalizers();
//rule of thumb for releasing com objects:
// never use two dots, all COM objects must be referenced and released individually
// ex: [somthing].[something].[something] is bad
//release com objects to fully kill excel process from running in the background
Marshal.ReleaseComObject(xlRange);
Marshal.ReleaseComObject(xlWorksheet);
//close and release
xlWorkbook.Close();
Marshal.ReleaseComObject(xlWorkbook);
//quit and release
xlApp.Quit();
Marshal.ReleaseComObject(xlApp);
}
}
private void btnSendEmail_Click(object sender, EventArgs e)
{
}
private string OpenFileDialog()
{
string filename = "";
OpenFileDialog fdlg = new OpenFileDialog();
fdlg.Title = "Excel File Dialog";
fdlg.InitialDirectory = @"c:\";
fdlg.Filter = "All files (*.*)|*.*|All files (*.*)|*.*";
fdlg.FilterIndex = 2;
fdlg.RestoreDirectory = true;
if (fdlg.ShowDialog() == DialogResult.OK)
{
filename = fdlg.FileName;
}
return filename;
}
private async Task<int> BindGrid(Microsoft.Office.Interop.Excel.Range xlRange)
{
lblProcessing.Text = "Processing File.. Please wait";
int rowCount = xlRange.Rows.Count;
int colCount = xlRange.Columns.Count;
// dt.Column = colCount;
dataGridView1.ColumnCount = colCount;
dataGridView1.RowCount = rowCount;
for (int i = 1; i <= rowCount; i++)
{
for (int j = 1; j <= colCount; j++)
{
//write the value to the Grid
if (xlRange.Cells[i, j] != null && xlRange.Cells[i, j].Value2 != null)
{
await Task.Delay(1);
dataGridView1.Rows[i - 1].Cells[j - 1].Value = xlRange.Cells[i, j].Value2.ToString();
}
}
}
lblProcessing.Text = "";
return 0;
}
}
internal class async
{
}
}
