C# Language Specification

Unboxing conversions

An unboxing conversion permits a reference type to be explicitly converted to a value-type. An unboxing conversion exists from the types object, dynamic and System.ValueType to any non-nullable-value-type, and from any interface-type to any non-nullable-value-type that implements the interface-type. Furthermore type System.Enum can be unboxed to any enum-type.

An unboxing conversion exists from a reference type to a nullable-type if an unboxing conversion exists from the reference type to the underlying non-nullable-value-type of the nullable-type.

A value type S has an unboxing conversion from an interface type I if it has an unboxing conversion from an interface type I₀ and I₀ has an identity conversion to I.

A value type S has an unboxing conversion from an interface type I if it has an unboxing conversion from an interface or delegate type I₀ and either I₀ is variance-convertible to I or I is variance-convertible to I₀ (§13.1.3.2).

An unboxing operation consists of first checking that the object instance is a boxed value of the given value-type, and then copying the value out of the instance. Unboxing a null reference to a nullable-type produces the null value of the nullable-type. A struct can be unboxed from the type System.ValueType, since that is a base class for all structs (§11.3.2).

Unboxing conversions are described further in §4.3.2.

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