C#: What is Boxing?

My friend asked me about C# boxing at the end of last year, and I promised to explain in a blog article this year. So, this is the article for him.

Value Type and Reference Type

Before I explain the C# boxing, I need to explain that C# has two categories of types.

• Value Types
• Reference Types

Value Types

Value Types is a type that an instance of the type contains the value. If I pass the value as a method argument or assign it to a new variable, the copy of the instance is created.

Following code demonstrates that the x and y values are independent objects, because the instance of x is copied to the y value as the new instance.

int x = 1;
int y = x;
y += 1;
Console.WriteLine($"x: {x}");
Console.WriteLine($"y: {y}");

x: 1
y: 2

The instances of Value types are stored in the stack area.

Reference Types

Reference Types is a type that an instance of the type contains the refence to the object. If I pass the value as a method argument or assign it to a new variable, both variable points to the same object.

Following code demonstrates that the x and y variables are pointing to the same object in memory after assigning it.

char[] x = { 't', 'e', 's', 't' };
Console.WriteLine($"x: {new string(x)}");
char[] y = x;
y[0] = 'n';
Console.WriteLine($"x: {new string(x)}");
Console.WriteLine($"y: {new string(y)}");

x: test
x: nest
y: nest

The instances of Reference types are stored in managed heap area.

Boxing and Unboxing

Boxing is a process to move the value type instance from the stack to the managed heap by converting it to an object type.

Unboxing is a process to extract the value type instance from the managed heap to the stack.

How it happens

Boxing happens when we assign a value type instance to System.Object. Unboxing happens when we convert a reference type instance to a value type.

int x = 1; // boxing 
object xobj = x;
int y = (int)xobj; // unboxing

Open the compiled dll from ILDASM tool, then we can clearly see it.

What's the issue?

According to Microsoft official documentation, it has a performance penalty: Performance: Boxing and Unboxing

The ArrayList document also explains about it.

Summary

It may be important to know how C# works under the hood, but there is not much we can do. We simply need to aware of this behavior and follow the best practice to code. I hope this article helps my colleague.