boxingunboxing

Boxing and unboxing are terms used in programming to describe conversions between value types and reference types. Boxing wraps a value type in an object so it can be used where an object is required; unboxing extracts the original value back to its primitive or value type. These operations are common across languages that distinguish between value and reference types.

In .NET languages like C#, boxing occurs when a value type (for example int, bool, double) is assigned to an object or interface type. This typically involves creating a heap-allocated object and copying the value. Repeated boxing can incur allocations and garbage collection overhead, affecting performance. In Java, boxing refers to converting primitive types to their corresponding wrapper classes (Integer, Boolean, Double), with auto-boxing and unboxing invoked by the compiler in many expressions.

Unboxing requires that the object actually contains a boxed value of the expected type. If not, a runtime exception occurs. In C#, unboxing a non-matching type or a null value can throw InvalidCastException or NullReferenceException. In Java, unboxing a null wrapper produces NullPointerException, and unboxing a wrapper of an incompatible type may throw ClassCastException.

Practical guidance includes limiting boxing in performance-sensitive code, using generics and primitive arrays where possible, and being aware of language-specific features. For example, Java caches small integers (roughly -128 to 127), which can reduce boxing overhead in certain cases. Explicit casts for unboxing can improve readability, but automatic boxing and unboxing should be used judiciously to avoid hidden costs and surprises in runtime behavior.