GridInnerControls.cs

var box00 = new BoxView
{
    BackgroundColor = Color.FromHex("#ffaaaa")
};
var box01 = new BoxView
{
    BackgroundColor = Color.FromHex("#aaffaa")
};
var box10 = new BoxView
{
    BackgroundColor = Color.FromHex("#aaaaff")
};
var box20 = new BoxView
{
    BackgroundColor = Color.FromHex("#aaaaaa")
};
grid.Children.Add(box00, 0, 0);
grid.Children.Add(box01, 1, 0);
Grid.SetRowSpan(box01, 2);
grid.Children.Add(box10, 0, 1);
grid.Children.Add(box20, 0, 2);
Grid.SetColumnSpan(box20, 2);