How to find out which DataGridView rows are currently onscreen?

    private bool RowIsVisible(DataGridViewRow row)
    {
        DataGridView dgv = row.DataGridView;
        int firstVisibleRowIndex = dgv.FirstDisplayedCell.RowIndex;
        int lastVisibleRowIndex = firstVisibleRowIndex + dgv.DisplayedRowCount(false) - 1;
        return row.Index >= firstVisibleRowIndex && row.Index <= lastVisibleRowIndex;
    }

IMHO Regards

I haven't tried this myself, but it seems to me that determining the rectangle of a row using DataGridView.GetRowDisplayRectangle and checking if it overlaps the current DataGridView.DisplayRectangle would be the way to go. Rectangle.IntersectsWith is useful in to do this.

As an optimization I would use DataGridView .DisplayedRowCount after finding the first visible row to determine what rows are visible.

public void GetVisibleCells(DataGridView dgv)
    {
        var visibleRowsCount = dgv.DisplayedRowCount(true);
        var firstDisplayedRowIndex = dgv.FirstDisplayedCell.RowIndex;
        var lastvisibleRowIndex = (firstDisplayedRowIndex + visibleRowsCount) - 1;
        for (int rowIndex = firstDisplayedRowIndex; rowIndex <= lastvisibleRowIndex; rowIndex++)
        {
            var cells = dgv.Rows[rowIndex].Cells;
            foreach (DataGridViewCell cell in cells)
            {
                if (cell.Displayed)
                {
                    // This cell is visible...
                    // Your code goes here...
                }
            }
        }
    }

Updated: It now finds visible cells.