C# / .NET Junior (0–2 yrs) Concepts Frequent C# boxing unboxing performance value types

What is boxing and unboxing in C#, and why is it a performance concern?

cs-jun-005

Your answer

Answer as you would in a real interview — explain your thinking, not just the conclusion.

Model answer

Boxing wraps a value type in a heap-allocated object so it can be treated as a reference type. Unboxing extracts the value back. Both require a heap allocation and a copy, which is significantly slower than stack operations. The most common source of accidental boxing is storing value types in non-generic collections (ArrayList, Hashtable) or passing an int where object is expected. With generic collections — List<int>, Dictionary<int, T> — the compiler avoids boxing entirely. For high-throughput hot paths, even a few hundred thousand boxing allocations per second will pressure the GC.

Code example

int x = 42;
object boxed = x;   // boxing: heap allocation
int y = (int)boxed; // unboxing: copy back to stack

// Avoid boxing with generics
var badList = new System.Collections.ArrayList();
badList.Add(42);      // boxes!

var goodList = new List<int>();
goodList.Add(42);     // no boxing

// String.Format boxes — use string interpolation or StringBuilder
string s1 = string.Format("value: {0}", 42);  // boxes 42
string s2 = $"value: {42}";                    // no boxing

Follow-up

How does struct layout (sequential vs auto packing) affect P/Invoke marshalling? How does Nullable<T> avoid boxing compared to casting to object?