How to display the time elapsed in a label

Question

What should be straight forward is not here and I couldnt find a way yet in spite of reading a lot.

I have a button which executes a time consuming function. So on clicking the button should show time elapsed in milliseconds in a label with an interval of 500 ms. And when the desired result is achieved I want the timer to stop. I dont just need the final time (the total time consumed) in a label, but the label should dynamically show the time being elapsed. My code would be:

    private void btnHistory_Click(object sender, EventArgs e)
    {
        Class1 c = new Class1();
        c.StartClock(ref label12);

        Utility.PopulateHistory(dgvRecords_history, _util); //time consuming function

        c.StopClock();
    }

And in Class1 I write this:

    internal void StartClock(ref Label l)
    {
        Timer t = new Timer();
        t.Interval = 500;
        t.Enabled = true;
        t.Tag = l;
        t.Tick += new EventHandler(t_Tick);
        t.Start();
    }

    int i;
    bool stop;
    void t_Tick(object sender, EventArgs e)
    {
        if (stop)
        {
            ((Timer)sender).Stop();
            return;
        }

        ((Label)((Timer)sender).Tag).Text = (++i).ToString();
    }

    internal void StopClock()
    {
        i = 0;
        stop = true;
    }

What happens is, the t_Tick event is fired only after the complete code under button event is fired. That is the tick event is fired after it goes through the StopClock function! I got no idea why on earth it should be that!

2 questions basically:

1. How can my requirement be achieved in the right way to handle these? I know I should use other built in classes to evaluate performance, but this is just for display purpose. For this, what is the ideal approach?

2. Why is my code not working?

EDIT: I have used here System.Windows.Forms Timer here, but the result is not any different with System.Timers Timer

Answer 1

@Dainel Rose's answer worked for me perfectly, but only if invalid cross thread operation is handled. I could do so like:

private void btnHistory_Click(object sender, EventArgs e) 
{ 
    Class1 c = new Class1(ref label12); 
    c.StartClock(); 

    var backgroundWorker = new BackgroundWorker();
    backgroundWorker.DoWork += ((s, e) =>
        {
            // time consuming function
            Utility.PopulateHistory(dgvRecords_history, _util);
        });

    backgroundWorker.RunWorkerCompleted += ((s, e) =>
        {
            c.StopClock();
        });

    backgroundWorker.RunWorkerAsync();
} 

And in the Utility class where the time consuming function runs,

    internal static void PopulateHistory(DataGridView dgv, Utility util)
    {
        SetDataGridView_History(dgv, util);
    }

    delegate void UpdateDataGridView_History(DataGridView dgv, Utility util);
    static void SetDataGridView_History(DataGridView dgv, Utility util)
    {
        if (dgv.InvokeRequired)
        {
            UpdateDataGridView_History updaterDelegate = new UpdateDataGridView_History(SetDataGridView_History);
            ((Form)util._w).Invoke(updaterDelegate, new object[] { dgv, util });
        }
        else
            //code that utilizes UI thread (long running process in my case)
    }

Thanks all who helped. I'm marking Daniel's answer..

Answer 2

The problem is that your long-running task is also running on the UI thread. So the timer can't fire and update the UI, since the thread is busy handling the long-running task.

Instead, you should use a BackgroundWorker to handle the long-running task.

In code:

private void btnHistory_Click(object sender, EventArgs e) 
{ 
    Class1 c = new Class1(ref label12); 
    c.StartClock(); 

    var backgroundWorker = new BackgroundWorker();
    backgroundWorker.DoWork += (s, e) =>
        {
            // time consuming function
            Utility.PopulateHistory(dgvRecords_history, _util);
        };

    backgroundWorker.RunWorkerCompleted += (s, e) =>
        {
            c.StopClock();
        };

    backgroundWorker.RunWorkerAsync();
} 

As ChrisWue noted, since you now have the long-running task in a separate thread, it needs to invoke any access to the UI controls on the UI thread.

If your long-running task just needs some data from the UI to start, you can pass that data as parameter of RunWorkerAsync(). If you need to output some result data to the UI, you can do that in the handler of the RunWorkerCompleted event. If you occasionally need to update the UI as progress is being made, you can do that in the handler of the ProgressChanged event, calling ReportProgress() in your DoWork handler.

If none of the above are needed, you could use the ThreadPool, as in StaWho's answer.

Answer 3

Add the timer to the Components collection of the form. Or store the timer in a field in the class.

The timer is garbage collected because it is not longer reachable when your method returns.

I don't know about your long running code, but out should new run on a separate thread, or make calls to Application.DoEvents

(And remove the ref in your code, it is not used).

Answer 4

Your time consuming function is blocking the main thread. You can use BackgroundWorker or below trick:

    public Form1()
    {
        InitializeComponent();
        t.Tick +=new EventHandler(t_Tick);
        t.Interval = 500;
    }

    int timeElapsed = 0;
    System.Windows.Forms.Timer t = new System.Windows.Forms.Timer();
    private void button1_Click(object sender, EventArgs e)
    {
        t.Start();
        ThreadPool.QueueUserWorkItem((x) =>
        {
            TimeConsumingFunction();
        });

    }

    void TimeConsumingFunction()
    {
        Thread.Sleep(10000);
        t.Stop();
    }

    void t_Tick(object sender, EventArgs e)
    {
        timeElapsed += t.Interval;
        label1.Text = timeElapsed.ToString();
    }