Boxing/Unboxing

Its been nearly three years since I last blogged and what better topic to start blogging back again than boxing/unboxing.

Most of the literature that I can find online tells me what is a boxing/unboxing operation or what are its disadvantages but none could definitively answer to the question that came up during a recent code review. Whether this code causes boxing/unboxing

class Class1

{

int i = 10;

static void Main(string[] args)

{

Class1 x = new Class1();

int a = x.i; //unboxing??

x.i= 20; //boxing??

}

}

I myself when initially came across the concept of boxing/unboxing used to think that the above code will cause box and unbox operations to be called but that's not the case. The above code does not cause boxing/unboxing.

The idea of boxing unboxing is a little little difficult to grasp and I have found lots of people(including myself) struggling with it. There is an article on type fundamentals by Jeffrey Richter which may help explain things and also there is a blog post by David Cumps which gives a nice visual explanation of boxing/unboxing

To figure out in the C# code if it has any box operation look for the following

1. An instance of a value type is assigned to a variable/field/property of the type object
2. An instance of a value type is cast to an instance of an object type
3. An instance of a value type is passed to a method/property indexer which has object type as a parameter.

To figure out in C# code if it has any Unbox operation check whether an instance of a type object is being cast to an instance of a value type.

Below are the few common cases of boxing and unboxing

int j;

int i = 10; // can be any value type like float, double, Point, DateTime, structures etc.

//The standard example

object o = i; //implicit boxing

o = (object)i; //explicit boxing

j = (int)o; //unboxing

//String.Format and its variations

string.Format("Boxing {0}",i); //implicit boxing

Console.WriteLine("Boxing {0}",i);

StringBuilder strBuild = new StringBuilder();

strBuild.AppendFormat("Boxing {0}",i); //implicit boxing

//Collections

ArrayList list = new ArrayList();

list.Add(i); //implicit boxing

list.BinarySearch(i); //implicit boxing

list.IndexOf(i); //implicit boxing

list.Insert(2,i); //implicit boxing (only for i)

list.LastIndexOf(i); //implicit boxing

list.Remove(i); //implicit boxing

j = (int) list[0]; //unboxing

Hashtable coll = new Hashtable();

coll.Add(i,i); //implicit boxing(twice)

coll.Contains(i); //implicit boxing

coll.ContainsKey(i); //implicit boxing

coll.ContainsValue(i); //implicit boxing

coll.Remove(i); //implicit boxing

j = (int) coll[i]; //implicit boxing for indexer and unboxing for assignment

//DataTable

DataTable dt = new DataTable();

dt.Columns.Add("UntypedColumn");

dt.Columns.Add("IntColumn",typeof(int));

DataRow dr = dt.NewRow();

dr["UntypedColumn"] = i; //implicit boxing

dr["IntColumn"] = i; //implicit boxing

j = (int) dr["UntypedColumn"]; //unboxing

j = (int) dr["UntypedColumn"]; //unboxing