What is the difference between shadow and override ?

When you define a class that inherits from a base class, you sometimes want to redefine one or more of the base class elements in the derived class. Shadowing and overriding are both available for this purpose.

Comparison

It is easy to confuse shadowing with overriding. Both are used when a derived class inherits from a base class, and both redefine one declared element with another. But there are significant differences between the two.The following table compares shadowing with overriding.

Point of comparison Shadowing OverridingPurposeShadowing

Protecting against a subsequent base class modification that introduces a member you have already defined in your derived classAchieving polymorphism by defining a different implementation of a procedure or property with the same calling sequence1Redefined elementShadowing

Any declared element typeOnly a procedure (Function, Sub, or Operator) or propertyRedefining elementShadowing

Any declared element typeOnly a procedure or property with the identical calling sequence1Access level of redefining elementShadowing

Any access levelCannot change access level of overridden elementReadability and writability of redefining elementShadowing

Any combinationCannot change readability or writability of overridden propertyControl over redefiningShadowing

Base class element cannot enforce or prohibit shadowingBase class element can specify MustOverride, NotOverridable, or OverridableKeyword usageShadowing

Shadows recommended in derived class; Shadows assumed if neither Shadows nor Overrides specified2Overridable or MustOverride required in base class; Overrides required in derived classInheritance of redefining element by classes deriving from your derived classShadowing

Shadowing element inherited by further derived classes; shadowed element still hidden3Overriding element inherited by further derived classes; overridden element still overridden

1 The calling sequence consists of the element type (Function, Sub, Operator, or Property), name, parameter list, and return type. You cannot override a procedure with a property, or the other way around. You cannot override one kind of procedure (Function, Sub, or Operator) with another kind.

2 If you do not specify either Shadows or Overrides, the compiler issues a warning message to help you be sure which kind of redefinition you want to use. If you ignore the warning, the shadowing mechanism is used.

3 If the shadowing element is inaccessible in a further derived class, shadowing is not inherited. For example, if you declare the shadowing element as Private, a class deriving from your derived class inherits the original element instead of the shadowing element.

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Overriding redefines only the implementation while shadowing redefines the whole element.

In Overriding derived classes can refer the parent class element by using "ME" keyword, but in shadowing you can access it by "MYBASE".