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.
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