Deep Dive Into Boxing And Unboxing

Now we understand the memory organization. Let’s see the value types and reference types in .NET Framework.

System.Object class

In .NET framework all the value types and reference types are derived from System.Object class. This is the parent class which is derived by all the available data types in .NET Framework. That means all the data types are the specialized classes of System.Object class. All the available data types and their hierarchy in .NET Framework is as follows:



Here all reference types are classes and value types are enumerations and structures.

Explicit and Implicit casting

When we work with data types there are situations when we need some sort of conversion to store any computational result. .NET Architecture allows type conversion among different primitive types. Sometimes it is quite possible that our result is too large or too small to store in the specific data type. Here Explicit or Implicit casting comes in handy.

Implicit Casting is done by the compiler. This is sometimes called Widening Data Type Conversion because we try to store the smaller result to the larger data type. For example:





Variable marks is of int type, when we want to store the result in float type then some information is lost. But still compiler is doing implicit conversion for us. It is storing int result in the float data type.

Explicit Casting is done by the programmer. This is called Narrowing Data Type Conversion because some information may loss in the conversion, storing the large result to the smaller data types. For example:





Here we are casting explicitly marks variable to float type so that we can get whole result after division.

Boxing and Unboxing

Although we can convert from one data type to another data type, .NET Architecture also allows us to convert form value type to reference type and conversely. For instant we can store int type to object type (Boxing) and object type to int type (Unboxing).

Boxing

As we know that Object is parent class for all the data types so it can refer to any reference and value type. Whenever we store a value type to a reference type (Object) what runtime does for us is, it create a memory on heap, copies the value of value type into that memory location, and stores the reference to that location on the stack. This is called Implicit Conversion.



For example:





Here implicit cast is performed and int data type is converted to object, widening the age variable. But still it is Int32 structure. Because what is done by the runtime, a reference is created pointing to 10 in the heap.

Unboxing

Once we store the value type in the reference type we may want to use this value later in our application. We may attempt to cast back the object type to the value type. But we’ll get a compile time error. And it does make sense that int is a value type it can’t store a reference to a memory location. So we have to explicitly caste reference type to value type to get that value back. This is called Explicit Conversion.



For example:



Here we are storing a reference type in value type. But this will not compile and shows the following error.



Because Age is and object type. Here an explicit conversion is required and we have to cast Age back into int type.



Now if we compile and run this code it will compile and shows following output.



Boxing is used when we want to compare two objects.

For example: We have a Human class:



A Teacher class inheriting Human class:



If we want to convert Teacher into object type we can use is operator to check if Teacher is an object type.





We can also use as operator to cast a type to another type and check compatibility:



And we can cast back Object into Teacher object as follows:



This will produce following output:



Sometimes boxing is necessary (to check compatibility and equality of different objects) but we should avoid this because it involves a lot of overhead of creating a new memory location on heap copy value and then assign its reference on stack. It can slow down our application becausethe processor has to do extra work of memory management and allocation. This delay can’t be noticed in small applications but when we work with large projects this can slow down application performance.