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;
}
除了其他答案，还有await (c#参考)
更具体地说，在包含的例子中，它解释了您的情况
下面的Windows窗体示例说明了await在
异步方法，WaitAsynchronouslyAsync。对比一下它的行为
方法使用waitsynchrontically的行为。没有等待
应用到任务的操作符，waitsynchronize同步运行
尽管在定义中使用了async修饰符，并且调用了
线程。睡在它的身体里。
private async void button1_Click(object sender, EventArgs e)
{
// Call the method that runs asynchronously.
string result = await WaitAsynchronouslyAsync();
// Call the method that runs synchronously.
//string result = await WaitSynchronously ();
// Display the result.
textBox1.Text += result;
}
// The following method runs asynchronously. The UI thread is not
// blocked during the delay. You can move or resize the Form1 window
// while Task.Delay is running.
public async Task<string> WaitAsynchronouslyAsync()
{
await Task.Delay(10000);
return "Finished";
}
// The following method runs synchronously, despite the use of async.
// You cannot move or resize the Form1 window while Thread.Sleep
// is running because the UI thread is blocked.
public async Task<string> WaitSynchronously()
{
// Add a using directive for System.Threading.
Thread.Sleep(10000);
return "Finished";
}
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。
在更高的层次上:
1) Async关键字启用等待，这就是它所做的一切。Async关键字不会在单独的线程中运行该方法。beginf async方法同步运行，直到它命中一个耗时任务的await。
2)你可以等待一个返回Task或t类型Task的方法。你不能等待async void方法。
3)主线程遇到等待耗时任务或实际工作开始时，主线程返回到当前方法的调用方。
4)如果主线程看到一个仍在执行的任务在等待，它不会等待它，而是返回到当前方法的调用者。通过这种方式，应用程序保持响应性。
5)等待处理任务，现在将在线程池的独立线程上执行。
6)当这个await任务完成时，它下面的所有代码将由单独的线程执行
下面是示例代码。执行它并检查线程id
using System;
using System.Threading;
using System.Threading.Tasks;
namespace AsyncAwaitDemo
{
class Program
{
public static async void AsynchronousOperation()
{
Console.WriteLine("Inside AsynchronousOperation Before AsyncMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
//Task<int> _task = AsyncMethod();
int count = await AsyncMethod();
Console.WriteLine("Inside AsynchronousOperation After AsyncMethod Before Await, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
//int count = await _task;
Console.WriteLine("Inside AsynchronousOperation After AsyncMethod After Await Before DependentMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
DependentMethod(count);
Console.WriteLine("Inside AsynchronousOperation After AsyncMethod After Await After DependentMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
}
public static async Task<int> AsyncMethod()
{
Console.WriteLine("Inside AsyncMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
int count = 0;
await Task.Run(() =>
{
Console.WriteLine("Executing a long running task which takes 10 seconds to complete, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
Thread.Sleep(20000);
count = 10;
});
Console.WriteLine("Completed AsyncMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
return count;
}
public static void DependentMethod(int count)
{
Console.WriteLine("Inside DependentMethod, Thread Id: " + Thread.CurrentThread.ManagedThreadId + ". Total count is " + count);
}
static void Main(string[] args)
{
Console.WriteLine("Started Main method, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
AsynchronousOperation();
Console.WriteLine("Completed Main method, Thread Id: " + Thread.CurrentThread.ManagedThreadId);
Console.ReadKey();
}
}
}
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。
它们是为了让异步代码易于编写和阅读。
这和生成后台线程来执行长时间逻辑是一样的吗?
一点也不。
//我不明白为什么这个方法必须被标记为'async'。
async关键字启用await关键字。所以任何使用await的方法都必须被标记为async。
// DoSomethingAsync()方法在5秒休眠后到达该行。难道不应该立即到达吗?
不会，因为异步方法默认情况下不会在另一个线程上运行。
//是否在后台线程上执行?
No.
你可能会发现我的async/await介绍很有用。官方MSDN文档也非常好(尤其是TAP部分)，异步团队还发布了一个很好的FAQ。
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
{
}
}
