DataGridViewRowPrePaintEventArgs.State Property

// Determine whether the cell should be painted 
// with the custom selection background. 
if ((e.State & DataGridViewElementStates.Selected) ==
            DataGridViewElementStates.Selected)
{
    // Calculate the bounds of the row.
    Rectangle rowBounds = new Rectangle(
        this.dataGridView1.RowHeadersWidth, e.RowBounds.Top,
        this.dataGridView1.Columns.GetColumnsWidth(
            DataGridViewElementStates.Visible) -
        this.dataGridView1.HorizontalScrollingOffset + 1,
        e.RowBounds.Height);

    // Paint the custom selection background. 
    using (Brush backbrush =
        new System.Drawing.Drawing2D.LinearGradientBrush(rowBounds,
            this.dataGridView1.DefaultCellStyle.SelectionBackColor,
            e.InheritedRowStyle.ForeColor,
            System.Drawing.Drawing2D.LinearGradientMode.Horizontal))
    {
        e.Graphics.FillRectangle(backbrush, rowBounds);
    }
}