Question

I’m catching the DataGridView control’s SelectionChanged event and my event handler takes about 1/2 a second to complete its task (setting the values of several controls, etc.). In UI terms, that’s an eternity.

The problem is that the DataGridView’s user interface doesn’t update the selection immediately. What I want is for the user to see the selection change in the DataGridView as soon as they click and then the lengthy work to be done. It will still take 1/2 a second to complete the whole task and the UI will not be responsive during that time, and that’s OK — at least the user will get immediate feedback.

You can see this behaviour by inserting the following code into a new Form1 class:

    private System.Windows.Forms.DataGridView dataGridView1;

    public Form1()
    {
        InitializeComponent();
        dataGridView1 = new DataGridView();
        dataGridView1.Dock = DockStyle.Fill;
        dataGridView1.Columns.Add("Column0", "Column 0");
        dataGridView1.Rows.Add("Row 0");
        dataGridView1.Rows.Add("Row 1");
        dataGridView1.Rows.Add("Row 2");
        dataGridView1.Rows.Add("Row 3");
        dataGridView1.SelectionChanged += 
            new EventHandler(dataGridView1_SelectionChanged);
        this.Controls.Add(dataGridView1);
    }
    private void dataGridView1_SelectionChanged(object sender, EventArgs e)
    {
        if (this.Handle != null)
        {
            this.DoSomethingForAWhile();
            // Even BeginInvoke doesn't help
            //this.BeginInvoke((MethodInvoker)this.DoSomethingForAWhile);
        }
    }
    private void DoSomethingForAWhile()
    {
        // Do anything that causes a noticable delay

        DateTime t0 = DateTime.Now;
        while ((DateTime.Now - t0).TotalSeconds < 2)
        {
            // Do nothing
        }
    }

I would normally use Control.BeginInvoke so that the paint messages could be processed before my 1/2 second task starts. This works with a ListView control but for some reason it doesn’t work with DataGridView.

I could do the processing on another thread and use Invoke to set the UI values, but that seems like a lot of complexity for something that should be a simple task.

Surely there’s a better way.

Answer 1

Whilst I think it’s appropriate to stick your long running task into a BackgroundWorker and callback to the UI to update it, you can get the behaviour you want with a call to DataGridView.Refresh(), and use of BeginInvoke;

private void dataGridView1_SelectionChanged(object sender, EventArgs e)
{
    dataGridView1.Refresh();
    if (this.Handle != null)
    {
        this.BeginInvoke((MethodInvoker)this.DoSomethingForAWhile);
    }
}

This approach sticks a Paint message on the UI thread before your long running task, which is blocking the UI thread. At that point, I don’t believe the DataGridView has yet been instructed to Paint itself, hence why you weren’t seeing it update.