Set RelativeLayout constraint expression from code

How do i change the RelativeLayout of a view from code? I have this XAML:

<StackLayout
                    x:Name="SemiLayout"
                    RelativeLayout.HeightConstraint = 
                        "{ConstraintExpression Type=RelativeToParent,
                                               Property=Height,
                                               Factor=1}"

                    RelativeLayout.WidthConstraint = 
                        "{ConstraintExpression Type=RelativeToParent,
                                               Property=Width,
                                               Factor=1}">
                </StackLayout>

How do i change the constraint expression from code? I didn't find anything related to this in the forum.