Question

How can you show the tooltip for datagridview when cell is selected, not from mouseover but from using the arrow keys?

Answer 1

As you’ve noticed, you won’t be able to use the DataGridView’s built in tooltip. In fact, you will need to disable it so set your DataGridView’s ShowCellToolTips property to false (it’s true by default).

You can use the DataGridView’s CellEnter event with a regular Winform ToolTip control to display tool tips as the focus changes from cell to cell regardless of whether this was done with the mouse or the arrow keys.

private void dataGridView1_CellEnter(object sender, DataGridViewCellEventArgs e) {
    var cell = dataGridView1.CurrentCell;
    var cellDisplayRect = dataGridView1.GetCellDisplayRectangle(e.ColumnIndex, e.RowIndex, false);
    toolTip1.Show(string.Format("this is cell {0},{1}", e.ColumnIndex, e.RowIndex), 
                  dataGridView1,
                  cellDisplayRect.X + cell.Size.Width / 2,
                  cellDisplayRect.Y + cell.Size.Height / 2,
                  2000);
    dataGridView1.ShowCellToolTips = false;
}

Note that I added an offset to the location of the ToolTip based on the cell’s height and width. I did this so the ToolTip doesn’t appear directy over the cell; you might want to tweak this setting.