在C#4.5的Windows窗体应用程序编程中，使用Task和async关键字创建异步操作“精确地测量长时间运行任务的持续时间并实时显示已执行时间”的完整示例代码。

在C# 4.5及以上版本的Windows窗体应用程序中，你可以使用`Task`、`async`以及`DateTime`来精确测量长时间运行任务的持续时间和实时更新进度。以下是一个简单的示例，展示了如何在一个`BackgroundWorker`的基础上使用`async`和`await`来更新UI：

首先，在Windows Forms项目中添加一个计时器和一些文本框用于显示执行时间和暂停/继续按钮：

<Label Text="执行时间:" AutoSize=False/>
<TextBox Name="timeElapsedTextBox" />
<Button Text="开始" Click="buttonStart_Click" />
<Button Text="暂停" Enabled=False Click="buttonPause_Click" />
<Button Text="继续" Enabled=False Click="buttonContinue_Click" />
<Timer Interval="1000" Tick="timer_Tick" />

然后编写C#代码：

using System;
using System.Threading.Tasks;
using System.Windows.Forms;

public partial class Form1 : Form
{
    private BackgroundWorker backgroundWorker;
    private Timer timer;
    private DateTime startTime;
    private bool isPaused;

    public Form1()
    {
        InitializeComponent();

        backgroundWorker = new BackgroundWorker();
        backgroundWorker.WorkerReportsProgress = true;
        backgroundWorker.DoWork += BackgroundWorker_DoWork;
        backgroundWorker.ProgressChanged += BackgroundWorker_ProgressChanged;
        backgroundWorker.RunWorkerCompleted += BackgroundWorker_RunWorkerCompleted;

        timer = new Timer();
        timer.Tick += Timer_Tick;
    }

    private async void buttonStart_Click(object sender, EventArgs e)
    {
        isPaused = false;
        startTime = DateTime.Now;
        timer.Start();
        backgroundWorker.RunWorkerAsync();
    }

    private void buttonPause_Click(object sender, EventArgs e)
    {
        isPaused = !isPaused;
    }

    private async void buttonContinue_Click(object sender, EventArgs e)
    {
        if (isPaused)
        {
            isPaused = false;
            await backgroundWorker;
        }
    }

    private void BackgroundWorker_DoWork(object sender, DoWorkEventArgs e)
    {
        for (int i = 0; i < 1000000; i++)
        {
            if (!isPaused)
                e.ReportProgress(i);
        }
    }

    private void BackgroundWorker_ProgressChanged(object sender, ProgressChangedEventArgs e)
    {
        timeElapsedTextBox.Text = $"{DateTime.Now - startTime.ToString("HH:mm:ss.FFF")} 已执行: {e.ProgressPercentage}%";
    }

    private void BackgroundWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        timer.Stop();
    }

    private void Timer_Tick(object sender, EventArgs e)
    {
        if (!isPaused && backgroundWorker.IsBusy)
        {
            TimeSpan elapsedTime = DateTime.Now - startTime;
            double progress = Math.Min((double)elapsedTime.TotalMilliseconds / (double)(backgroundWorker.WorkerTotalLoops * 10), 100);
            backgroundWorker.ReportProgress(Convert.ToInt32(progress));
        }
    }
}

这个示例中，当用户点击"开始"按钮时，一个计数器开始运行，每秒报告一次进度。如果用户点击"暂停"，则计时器暂停，直到再次点击"继续"。注意`ProgressChanged`事件会在工作线程上调用，所以我们需要通过`await`传递进度给UI线程。