Boxing and Unboxing in C#
C# is, for the most part, a statically typed language, this means that the compiler will check the type of every expression and you sometimes have to convert between types. The concept of boxing and unboxing is the starting point in C# type system in which a value of any type can be treated as an object.
As mentioned in my previous article C# Difference between Struct and Class, the important difference between a value type and a reference type is that the value type stores its value directly. A reference type stores a reference that points to an object on the heap that contains the value.
Boxing
Boxing is the process of taking a value type, putting it inside a new object on the heap and storing a reference to it on the stack. Boxing is an implicit conversion.
public class Program
{
public static void Main(string[] args)
{
int total = 1976;
object obj = total; // Boxing
total = 2020;
System.Console.WriteLine("The value-type value = {0}", total);
System.Console.WriteLine("The reference-type value = {0}", obj);
}
}
/* Output:
The value-type value = 2020
The reference-type value = 1976
*/
Unboxing
Unboxing is the exact opposite, it takes the item from the heap and returns a value type that contains the value from the heap. Unboxing is an explicit conversion. Attempting to unbox null or to unbox a reference to an incompatible value type causes an Exception.
public class Program
{
public static void Main(string[] args)
{
int total = 1976;
object obj = total; // Boxing
total = (int)obj; // Unboxing
System.Console.WriteLine("The value-type value = {0}", total);
System.Console.WriteLine("The reference-type value = {0}", obj);
}
}
/* Output:
The value-type value = 1976
The reference-type value = 1976
*/
Performance
There are some performance implications and memory requirements with each box and unboxing operation. When using the non-generic collections to store a value type, the boxing and unboxing operations can hurt performance.
Thanks for reading!
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