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Explain boxing and unboxing with examples.

Anubhav Sharma 526 10 Jun 2025

Explain boxing and unboxing with examples in C# languages.

c# c#
Updated 13 Jun 2025
Anubhav Sharma
Anubhav Sharma

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The Anubhav portal was launched in March 2015 at the behest of the Hon'ble Prime Minister for retiring government officials to leave a record of their experiences while in Govt service .

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1 Answers

Ravi Vishwakarma
Ravi Vishwakarma 12 Jun 2025
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What is Boxing and Unboxing in C#?

Boxing and Unboxing are concepts in C# related to value types (int, bool, struct, etc.) and reference types (object, class, etc.).

Definitions

Concept Description
Boxing Converting a value type to an object (or to any interface type it implements)
Unboxing Extracting the value type from an object

Why Does It Happen?

C# treats value types and reference types differently:

  • Value types are stored on the stack
  • Reference types are stored on the heap
  • To store a value type in a variable of type object, it must be boxed (copied to the heap).

Example: Boxing

int number = 42;           // value type (stack)
object obj = number;       // BOXING (copy 'number' into heap)

What happens:

  • number is a value type.
  • When assigned to object obj, it's boxed.
  • A new object is created on the heap with a copy of number.

Example: Unboxing

object obj = 42;           // boxing
int number = (int)obj;     // UNBOXING
  • Unboxing extracts the value type from the object.
  • You must cast explicitly.
  • If the object doesn't contain the expected type, it throws InvalidCastException.

Example with Error:

object obj = 42;
double num = (double)obj; // ❌ InvalidCastException

Even though both int and double are value types, the boxed int cannot be unboxed to a double.

Performance Impact

  • Boxing and unboxing is relatively expensive, especially in loops.
  • Frequent boxing/unboxing can lead to GC pressure and heap fragmentation.

Example with Collection

Before generics:

ArrayList list = new ArrayList();
list.Add(10);              // boxing
int x = (int)list[0];      // unboxing

With generics (avoids boxing):

List<int> list = new List<int>();
list.Add(10);              // no boxing
int x = list[0];           // no unboxing

Summary

Operation Description Explicit? Heap/Stack
Boxing Value → Object (value to reference) No Moves to heap
Unboxing Object → Value (reference to value) Yes Copies to stack
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