hello, try this >
Code:
for (int i = 0; i < dataGrid1.Items.Count; i++)
{
    DataGridRow row = (DataGridRow)dataGrid1.ItemContainerGenerator.ContainerFromIndex(i);

    if (row.IsSelected == true)
    {
        row.IsSelected = false;
    }
    else
    {
        row.IsSelected = true;
    }
}