What are Boxing and Unboxing in C#, and why should they be avoided in high-volume code paths?

Short answer

Boxing converts a Value Type into an object on the Heap with additional memory allocation, while Unboxing extracts the original value; repeated use burdens the Garbage Collector and slows performance.

Explanation

When assigning a primitive such as int to an object variable or an interface, the CLR allocates a new object on the Heap and copies the value into it (Boxing). When retrieving it as (int)obj, the type is checked and the value is copied again (Unboxing). Generics, such as List<int> instead of the older ArrayList, eliminate this overhead entirely.

Code example

int val = 42;
object boxed = val; // Boxing: Allocate a new object on the Heap
int unboxed = (int)boxed; // Unboxing: Type-check and extract the value

Common mistakes

• Using non-generic interfaces such as IList with thousands of numeric values, generating millions of unnecessary Boxing operations.

Follow-up questions

• What is the basic difference between Value Types and Reference Types in C#, and how are they stored in memory?

Sources

• Microsoft Learn — Boxing and Unboxing