C# WinForms DataGridView background color rendering too slow

Question

I'm painting my rows in a DataGridView like this:

private void AdjustColors()
    {            
        foreach (DataGridViewRow row in aufgabenDataGridView.Rows)
        {
            AufgabeStatus status = (AufgabeStatus)Enum.Parse(typeof(AufgabeStatus), (string)row.Cells["StatusColumn"].Value);

            switch (status)
            {
                case (AufgabeStatus.NotStarted):
                    row.DefaultCellStyle.BackColor = Color.LightCyan;
                    break;
                case (AufgabeStatus.InProgress):
                    row.DefaultCellStyle.BackColor = Color.LemonChiffon;
                    break;
                case (AufgabeStatus.Completed):
                    row.DefaultCellStyle.BackColor = Color.PaleGreen;
                    break;
                case (AufgabeStatus.Deferred):
                    row.DefaultCellStyle.BackColor = Color.LightPink;
                    break;
                default:
                    row.DefaultCellStyle.BackColor = Color.White;
                    break;
            }
        }        
    }

Then I call it in the OnLoad method:

protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);

            AdjustColors();           
        } 

I prefer OnLoad to OnPaint or something.. because OnPaint is called very often.

The question: Why does it take about 100 - 200 ms to change the background of every row? Early, I was doint CellPaint.. but I had problems when scrolling with refreshing..

Answer 1

Instead of changing the color of the whole DataGrid at once, you should let it manage the rendering by overriding the CellFormatting event. The rows will only be painted when they are actually displayed on the screen.

private void aufgabenDataGridView_CellFormatting(object sender, DataGridViewCellFormattingEventArgs e)
{
  DataGridViewRow row = aufgabenDataGridView.Rows[e.RowIndex];
  AufgabeStatus status = (AufgabeStatus) Enum.Parse(typeof(AufgabeStatus), (string) row.Cells["StatusColumn"].Value);

  switch (status)
  {
    case (AufgabeStatus.NotStarted):
      e.CellStyle.BackColor = Color.LightCyan;
      break;
    case (AufgabeStatus.InProgress):
      e.CellStyle.BackColor = Color.LemonChiffon;
      break;
    case (AufgabeStatus.Completed):
      e.CellStyle.BackColor = Color.PaleGreen;
      break;
    case (AufgabeStatus.Deferred):
      e.CellStyle.BackColor = Color.LightPink;
      break;
    default:
      e.CellStyle.BackColor = Color.White;
      break;
  }
}

If this is still too slow, try getting the real object the row is bound to:

...
DataGridViewRow row = aufgabenDataGridView.Rows[e.RowIndex];
var aufgabe = (Aufgabe) row.DataBoundItem;
AufgabeStatus status = aufgabe.Status;
...