| title | ms.date | ms.prod | ms.technology | ms.topic | f1_keywords | dev_langs | helpviewer_keywords | ms.assetid | caps.latest.revision | author | ms.author | translation.priority.ht | translation.priority.mt | ||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Boxing and Unboxing (C# Programming Guide) |
2015-07-20 |
.net |
|
article |
|
|
|
8da9bbf4-bce9-4b08-b2e5-f64c11c56514 |
34 |
BillWagner |
wiwagn |
|
|
Boxing and Unboxing (C# Programming Guide)
Boxing is the process of converting a value type to the type object or to any interface type implemented by this value type. When the CLR boxes a value type, it wraps the value inside a System.Object and stores it on the managed heap. Unboxing extracts the value type from the object. Boxing is implicit; unboxing is explicit. The concept of boxing and unboxing underlies the C# unified view of the type system in which a value of any type can be treated as an object.
In the following example, the integer variable i is boxed and assigned to object o.
[!code-cscsProgGuideTypes#14]
The object o can then be unboxed and assigned to integer variable i:
[!code-cscsProgGuideTypes#15]
The following examples illustrate how boxing is used in C#.
[!code-cscsProgGuideTypes#47]
Performance
In relation to simple assignments, boxing and unboxing are computationally expensive processes. When a value type is boxed, a new object must be allocated and constructed. To a lesser degree, the cast required for unboxing is also expensive computationally. For more information, see Performance.
Boxing
Boxing is used to store value types in the garbage-collected heap. Boxing is an implicit conversion of a value type to the type object or to any interface type implemented by this value type. Boxing a value type allocates an object instance on the heap and copies the value into the new object.
Consider the following declaration of a value-type variable:
[!code-cscsProgGuideTypes#17]
The following statement implicitly applies the boxing operation on the variable i:
[!code-cscsProgGuideTypes#18]
The result of this statement is creating an object reference o, on the stack, that references a value of the type int, on the heap. This value is a copy of the value-type value assigned to the variable i. The difference between the two variables, i and o, is illustrated in the following figure.
It is also possible to perform the boxing explicitly as in the following example, but explicit boxing is never required:
[!code-cscsProgGuideTypes#19]
Description
This example converts an integer variable i to an object o by using boxing. Then, the value stored in the variable i is changed from 123 to 456. The example shows that the original value type and the boxed object use separate memory locations, and therefore can store different values.
Example
[!code-cscsProgGuideTypes#16]
Unboxing
Unboxing is an explicit conversion from the type object to a value type or from an interface type to a value type that implements the interface. An unboxing operation consists of:
-
Checking the object instance to make sure that it is a boxed value of the given value type.
-
Copying the value from the instance into the value-type variable.
The following statements demonstrate both boxing and unboxing operations:
[!code-cscsProgGuideTypes#21]
The following figure demonstrates the result of the previous statements.
For the unboxing of value types to succeed at run time, the item being unboxed must be a reference to an object that was previously created by boxing an instance of that value type. Attempting to unbox null causes a xref:System.NullReferenceException. Attempting to unbox a reference to an incompatible value type causes an xref:System.InvalidCastException.
Example
The following example demonstrates a case of invalid unboxing and the resulting InvalidCastException. Using try and catch, an error message is displayed when the error occurs.
[!code-cscsProgGuideTypes#20]
This program outputs:
Specified cast is not valid. Error: Incorrect unboxing.
If you change the statement:
int j = (short) o;
to:
int j = (int) o;
the conversion will be performed, and you will get the output:
Unboxing OK.
C# Language Specification
[!INCLUDECSharplangspec]
Related Sections
For more information:
C# Language Specification
[!INCLUDECSharplangspec]

