Compile-time and runtime casting c#

by
  • Upcasts can be checked at compile time – the type system guarantees that the cast succeeds.
  • Downcasts cannot (in general) be checked at compile time, so they are always checked at runtime.
  • Unrelated types cannot be cast to each other.

The compiler considers only the static types. The runtime checks the dynamic (runtime) type.
Looking at your examples:

Other x = new Other();
Derived d = (Derived)x;

The static type of x is Other. This is unrelated to Derived so the cast fails at compile time.

Base x = new Base();
Derived d = (Derived)x;

The static type of x is now Base. Something of type Base might have dynamic type Derived, so this is a downcast. In general the compiler can’t know from the static type of x if it the runtime type is Base, Derived, of some other subclass of Base. So the decision of whether the cast is allowed is left to the runtime.

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