Editing Boxing (computer programming) (section)

===Automatic unboxing===
With automatic unboxing the compiler automatically supplies the extra source code that retrieves the value out of that object, either by invoking some method on that object, or by other means.

For example, in versions of Java prior to J2SE 5.0, the following code did not compile:

<syntaxhighlight lang=Java>
Integer k = new Integer(4);
int l = k.intValue(); // always okay
int m = k;            // would have been an error, but okay now
</syntaxhighlight>

C# doesn't support automatic unboxing in the same meaning as Java, because it doesn't have a separate set of primitive types and object types. All types that have both primitive and object version in Java, are automatically implemented by the C# compiler as either primitive (value) types or object (reference) types.

In both languages, automatic boxing does not downcast automatically, i.e. the following code won't compile:

C#:
<syntaxhighlight lang="csharp">
int i = 42;
object o = i;         // box
int j = o;            // unbox (error)
Console.WriteLine(j); // unreachable line, author might have expected output "42"
</syntaxhighlight>

Java:
<syntaxhighlight lang="java">
int i = 42;
Object o = i;          // box
int j = o;             // unbox (error)
System.out.println(j); // unreachable line, author might have expected output "42"
</syntaxhighlight>