C# 知识点集锦（三）并发例子集锦

await 作用于 Task或者返回Task的方法，无论其有没有async修饰。

注意：如果没有使用await仅仅调用标识了async的方法，内部不会执行，只是返回一个Task

如果该方法为void，内部会执行。

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〇。简单使用实例。

        private async void button1_Click(object sender, EventArgs e)
        {

            try
            {
                this.label1.Text= await JustDO2();//或者await JustDO();
            }
            catch (Exception ex)
            {
                
                this.label1.Text=ex.Message;
            }
        }

        private  async Task<string> JustDO()
        {
            await Task.Delay(2000);
            //throw new Exception("ooops");
            return "OK";
        }


        private Task<string> JustDO2()
        {
            return Task.Run(() =>
            {
                Task.Delay(TimeSpan.FromSeconds(4)).Wait();
                throw new Exception("oooops");
                return Task.FromResult("OK");
            });

        }

一。使用并发的泛型方法

        static async Task<T> DelayResult<T>(T result,TimeSpan delay)
        {
            await Task.Delay(delay);
            return result;
        }

如何调用，点击按钮，更新文本。

        private async void button1_Click(object sender, EventArgs e)
        {
            this.label1.Text = await DelayResult<string>("OK", TimeSpan.FromSeconds(5));
             

        }

二。指数退避使用HttpClient访问

        static async Task<string> DownloadStringWithRetries(string uri)
        {
            using (var client =new HttpClient())
            {
                var nextDelay = TimeSpan.FromSeconds(1);
                for(int i=0;i!=3;++i)
                {
                    try
                    {
                        return await client.GetStringAsync(uri);
                    }
                    catch (Exception)
                    {
                       
                    }
                    await Task.Delay(nextDelay);
                    nextDelay = nextDelay +nextDelay;
                }

                return await client.GetStringAsync(uri);
            }
        }

另外一种超时机制使用Task.WhenAny

        private async Task<string> DownloadStringWithTimeout(string uri)
        {
            using(var client=new HttpClient())
            {
                var downloadTask = client.GetStringAsync(uri);
                var timeoutTask = Task.Delay(2000);
                var completedTask = await Task.WhenAny(downloadTask, timeoutTask);
                if (completedTask == timeoutTask)
                    return null;
                return await downloadTask;
            }
        }

三。取消

//Form上2个按钮，点击start启动线程，点击cancel取消线程

        private async void start_Click(object sender, EventArgs e)
        {
            start.Enabled = false;
            cancel.Enabled = true;
            try
            {
                cts = new CancellationTokenSource();
                var token = cts.Token;
                await Task.Run(() =>
                {
                    for (int i = 1; i <= 100; i++)
                    {
                        dispatcher.Invoke(() =>
                        {
                            label1.Text = i + "%";
                        });
                        Task.Delay(100).Wait();
                        token.ThrowIfCancellationRequested();
                    }
                }
                    , token
                );
                MessageBox.Show(" task Done.");
            }
            catch(OperationCanceledException )
            {
                MessageBox.Show(" Task Canceled.");
            }
            catch (Exception ex)
            {
                
                MessageBox.Show("task DONE with errors");
            }finally{
                start.Enabled=true;
                cancel.Enabled=false;
            }
        }

        private void cancel_Click(object sender, EventArgs e)
        {
            cts.Cancel();
        }

超时取消

4秒取消
方式1：
cts.CancelAfter(4000);

方式2：

cts = new CancellationTokenSource(4000);