What is the difference between boxing/unboxing and type casting?

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Boxing refers to a conversion of a non-nullable-value type into a reference type or the conversion of a value type to some interface that it implements (say int to IComparable<int>). Further, the conversion of an underlying value type to a nullable type is also a boxing conversion. (Caveat: Most discussions of this subject will ignore the latter two types of conversions.)

For example,

int i = 5;
object o = i;

converts i to an instance of type object.

Unboxing refers to an explicit conversion from an instance of object or ValueType to a non-nullable-value type, the conversion of an interface type to a non-nullable-value type (e.g., IComparable<int> to int). Further, the conversion of a nullable type to the underlying type is also an unboxing conversion. (Caveat: Most discussion of this subject will ignore the latter two types of conversions.)

For example,

object o = (int)5;
int i = (int)o;

converts the integer boxed in o to an instance of type int.

A type cast is an explicit conversion of an expression to a given type. Thus

(type) expression

explicitly converts expression to an object of type type.

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