Values of a given type may or may not be explicitly or implicitly convertible to other types depending on predefined conversion rules, inheritance structure, and explicit cast definitions.
Predefined conversions
Many predefined value types have predefined conversions to other predefined value types. If the type conversion is guaranteed not to lose information, the conversion can be implicit (i.e. an explicit cast is not required).
Inheritance polymorphism
A value can be implicitly converted to any class from which it inherits or interface that it implements. To convert a base class to a class that inherits from it, the conversion must be explicit in order for the conversion statement to compile. Similarly, to convert an interface instance to a class that implements it, the conversion must be explicit in order for the conversion statement to compile. In either case, the runtime environment throws a conversion exception if the value to convert is not an instance of the target type or any of its derived types.
Predefined conversions
Many predefined value types have predefined conversions to other predefined value types. If the type conversion is guaranteed not to lose information, the conversion can be implicit (i.e. an explicit cast is not required).
Inheritance polymorphism
A value can be implicitly converted to any class from which it inherits or interface that it implements. To convert a base class to a class that inherits from it, the conversion must be explicit in order for the conversion statement to compile. Similarly, to convert an interface instance to a class that implements it, the conversion must be explicit in order for the conversion statement to compile. In either case, the runtime environment throws a conversion exception if the value to convert is not an instance of the target type or any of its derived types.