ECMA-334: 11.3.2 Unboxing conversions

Microsoft .NET Framework, ASP.NET, Visual C# (CSharp, C Sharp, C-Sharp) Developer Training, Visual Studio

11.3.2 Unboxing conversions

An unboxing conversion permits an explicit conversion from object or System.ValueType to any non-nullable-value-type or from any interface-type to any non-nullable-value-type that implements the interfacetype. Furthermore, there is an explicit unboxing conversion from System.Enum to any enumeration type. 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.

A nullable-type T? has unboxing conversions from the same set of types as T. An unboxing conversion permits an explicit conversion from object or System.ValueType to any nullable-type or from any interface-type to any nullable-type whose underlying type implements the interface-type. Furthermore, there is an explicit unboxing conversion from System.Enum to any nullable-type whose underlying type is an enumeration type. An unboxing conversion to a nullable-type T? is processed as follows:

• If the source is a null reference, the result is a null value of type T?.

• If the source is a reference to a boxed T, the result is a T? produced by unboxing and wrapping the source.

• Otherwise, a System.InvalidCastException is thrown.

Referring to the imaginary boxing class described in the previous subclause, an unboxing conversion of an object box to a value-type T consists of executing the expression ((T_Box)box).value. [Example: Thus, the statements

object box = 123;
int i = (int)box;

conceptually correspond to

object box = new int_Box(123);
int i = ((int_Box)box).value;

end example]

For an unboxing conversion to a given value-type to succeed at run-time, the value of the source operand shall be a reference to an object that was previously created by boxing a value of that value-type. If the source operand is null, a System.NullReferenceException is thrown. If the source operand is a reference to an incompatible object, a System.InvalidCastException is thrown.