How to check all selected check boxes when you check one in C# datagridview

Question

I'm pretty new to C# development, but here goes. I'm working on a plugin for Autodesk revit, which creates a windows form with a datagridview on it which gets populated with several checkbox columns when the form loads. What I would like to do is select multiple checkbox cells, and check one of the selected boxes in order to check all selected boxes.

I have tried using a SelectionChanged event handler to store the selected sells in another variable. Then I'm trying to use CellValueChanged in order to set all those cells to the new value

    DataGridViewSelectedCellCollection selCells = null;
    private void revDataGridView_SelectionChanged(object sender, EventArgs e)
    {
        selCells = revDataGridView.SelectedCells;
    }

    private void revDataGridView_CellValueChanged(object sender, DataGridViewCellEventArgs e)
    {
        foreach(DataGridViewCell cell in selCells)
        {
            cell.Value = revDataGridView[e.ColumnIndex, e.RowIndex].Value;
        }
    }

My problem is that as soon as I click on the one cell, it resets the DataGridView.SelectedCells to that one cell and I lose the previous selection. Any help would be greatly appreciated!

EDIT: I solved this by storing the selection in another variable (selCells) and using a combination of the CellMouseUp, CellMouseDown, and CellValueChanged event handlers:

private void revDataGridView_CellMouseUp(object sender, DataGridViewCellMouseEventArgs e)
    {
        if (selCells != null && selCells.Count>1)
        {
            revDataGridView.EndEdit();
            selCells = revDataGridView.SelectedCells; 
        }
        else if(selCells !=null && selCells.Count == 1)
        {
            selCells = revDataGridView.SelectedCells;
            revDataGridView.EndEdit();
        }
    }
    //when a value is changed, apply change to all selected cells
    private void revDataGridView_CellValueChanged(object sender, DataGridViewCellEventArgs e)
    {
        foreach(DataGridViewCell cell in selCells)
        {
            if (cell.ReadOnly == false)
            {
                cell.Value = revDataGridView[e.ColumnIndex, e.RowIndex].Value;
                cell.Selected = true; 
            }
        }
    }
    //clear selection upon mouse button down
    private void revDataGridView_CellMouseDown(object sender, DataGridViewCellMouseEventArgs e)
    {
        revDataGridView.ClearSelection();
    }