The problem is that Margin is a property, and its type (Thickness) is a value type. That means when you access the property you’re getting a copy of the value back.
Even though you can change the value of the Thickness.Left property for a particular value (grr… mutable value types shouldn’t exist), it wouldn’t change the margin.
Instead, you’ll need to set the Margin property to a new value. For instance (coincidentally the same code as Marc wrote):
Thickness margin = MyControl.Margin;
margin.Left = 10;
MyControl.Margin = margin;
As a note for library design, I would have vastly preferred it if Thickness were immutable, but with methods that returned a new value which was a copy of the original, but with one part replaced. Then you could write:
MyControl.Margin = MyControl.Margin.WithLeft(10);
No worrying about odd behaviour of mutable value types, nice and readable, all one expression…