1.7 Unboxing and Boxing Conversions

📌 1. What Is Boxing?

• Boxing is the process of converting a value type (like int, bool, struct) into an object (reference type).

• The value gets wrapped in a heap object.

Example

int num = 42;
object obj = num; // Boxing
Console.WriteLine(obj); // 42

• Here, the int (value type, stored on the stack) is boxed into an object on the heap.

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📌 2. What Is Unboxing?

• Unboxing is the reverse — converting an object back to a value type.

• Requires an explicit cast, and the object must hold the correct type, otherwise an exception occurs.

Example

object obj = 42;   // Boxing
int num = (int)obj; // Unboxing
Console.WriteLine(num); // 42

⚠️ If the object doesn’t contain the right type, an InvalidCastException is thrown:

object obj = 42;
string str = (string)obj; // ❌ InvalidCastException

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📌 3. Performance Considerations

• Boxing is costly because:

  1. It allocates memory on the heap.

  2. Copies the value type into that memory.

• Unboxing is costly because:

  1. It checks the object type at runtime.

  2. Copies the value back from the heap to the stack.

Example of heavy boxing (bad practice)

ArrayList list = new ArrayList();
list.Add(1);  // Boxing
list.Add(2);  // Boxing
list.Add(3);  // Boxing
 
int x = (int)list[0]; // Unboxing

✅ Instead, use generics (List<int>) to avoid boxing:

List<int> list = new List<int> { 1, 2, 3 };
int x = list[0]; // No boxing/unboxing

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📌 4. Summary Table

Conversion	Definition	Requires Cast?	Example
Boxing	Value type → object (heap)	No	object o = 42;
Unboxing	Object → value type (stack)	Yes	int n = (int)o;

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📌 5. Analogy

• Boxing: like putting a gift inside a box — the value type is “wrapped” into an object.

• Unboxing: like taking the gift out — you need to know what’s inside, otherwise you grab the wrong thing and get an error.

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✅ Summary:

• Boxing wraps a value type into an object (implicit).

• Unboxing extracts the value type back from the object (explicit, requires cast).

• Avoid boxing/unboxing in performance-critical code — use generics instead.

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👉 Do you want me to also cover nullable value types (int?) vs boxing in this section, or keep nullables for later when we expand on advanced types?