What is runtime polymorphism in C#, and how do interfaces and virtual methods provide it?

Runtime polymorphism with interfaces and virtual dispatch

Explains how callers use one contract while runtime dispatch selects implementation, then connects that mechanism to extensible behavior.

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Runtime polymorphism means the actual object's implementation is selected through a shared contract while the program runs. If a variable has type IShape and refers to a Circle, calling Area invokes Circle's implementation. With a base class, the base member must be virtual or abstract and the derived member must override it for runtime dispatch. This lets a collection of shapes be processed without a type switch, as long as each implementation preserves the contract.

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Static types, runtime types, interface dispatch, virtual methods, overrides, substitutability, and polymorphic collections.

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Runtime polymorphism lets code call one contract while the runtime selects behavior from the actual object. In C#, interface calls and overridden virtual methods are common forms of dynamic dispatch.
Runtime polymorphism means the actual object's implementation is selected through a shared contract while the program runs. If a variable has type IShape and refers to a Circle, calling Area invokes Circle's implementation. With a base class, the base member must be virtual or abstract and the derived member must override it for runtime dispatch. This lets a collection of shapes be processed without a type switch, as long as each implementation preserves the contract.
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Polymorphism is most useful when behavior varies behind a stable abstraction. Overloading is different because the compiler selects a method from the compile-time argument types. The new keyword hides a member rather than overriding it, so behavior can change depending on the reference's static type. I prefer explicit override and small interfaces so dispatch remains unsurprising.

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Calling method overloading runtime polymorphism.
Using new when override was intended.
Switching on concrete types after accepting an interface.
Ignoring behavior promised by the base contract.

What is runtime polymorphism in C#, and how do interfaces and virtual methods provide it?

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