C#: What is boxing and unboxing in C#?

Boxing — converting value type to reference type. Unboxing — converting back.

Boxing:

• Value type → object (reference type).
• Allocates on heap.
• Performance overhead.

1int x = 10;           // Value type
2object obj = x;       // Boxing
3// obj now points to heap-allocated int

Unboxing:

• object → value type.
• Requires explicit cast.
• InvalidCastException if wrong type.

1object obj = 10;
2int y = (int)obj;     // Unboxing
3// int z = (string)obj;  // Error!

Performance:

• Boxing/unboxing is expensive.
• Avoid in performance-critical code.
• Use generics instead.

Better alternative:

1// Instead of boxing
2List<int> list = new List<int>();
3list.Add(10);  // No boxing with generics