Rectangle rect = GetCellDisplayRectangle(union.ColumnIndex, union.RowIndex, true);
string text = Rows[union.RowIndex].Cells[union.ColumnIndex].Value.ToString();
for (int i = 1; i < union.UnionCells; i++)
{
        Rectangle r = GetCellDisplayRectangle(union.ColumnIndex + i, union.RowIndex, true);
        rect.Width += r.Width;
}
rect.Height -= 1;
rect.Width -= 2;
Graphics gr = e.Graphics;
gr.FillRectangle(Brushes.LightGray, rect);
StringFormat sf = new StringFormat();
sf.Alignment = StringAlignment.Center;
sf.LineAlignment = StringAlignment.Center; 
e.Graphics.DrawString(text, Font, Brushes.Black, rect,sf);