There are a few differences between defining data bindings in MXML at compile time and in defining them at runtime in ActionScript:

* You cannot include ActionScript code in a data binding expression defined by the bindProperty() or bindSetter() method. Instead, use the bindSetter() method to specify a method to call when the binding occurs.

* You cannot include an E4X expression in a data binding expression defined in ActionScript.

* You cannot include functions or array elements in property chains in a data binding expression defined by the bindProperty() or bindSetter() method. For more information on property chains, see Working with bindable property chains.

* The MXML compiler has better warning and error detection support than runtime data bindings defined by the bindProperty() or bindSetter() method.