Programmatically setting the width of a grid column with * in WPF

Grid grid = new Grid();
ColumnDefinition c1 = new ColumnDefinition();
c1.Width = new GridLength(20, GridUnitType.Star);
ColumnDefinition c2 = new ColumnDefinition();
c2.Width = new GridLength(80, GridUnitType.Star);
grid.ColumnDefinitions.Add(c1);
grid.ColumnDefinitions.Add(c2);