#657 – Boxing Makes a Copy of An Object

When you pass a value-typed object to a function that accepts a parameter of type object, the value-typed object is boxed.  That is, it is copied into a temporary reference-typed object, which is actually passed into the function.  This boxing happens automatically.

If we look at an object that represents a boxed value type, we’ll see that its reported type is the value type.

        static void Test(object o)
        {
            Console.WriteLine(string.Format("Value = {0}, Type = {1}", o, o.GetType()));
        }

        static void Main()
        {
            int x = 12;
            Test(x);
        }

However, if we try changing the object within the method, the original int that was passed in is unaffected.

        static void Test(object o)
        {
            Console.WriteLine(string.Format("Value = {0}, Type = {1}", o, o.GetType()));
            o = 100;
        }

        static void Main()
        {
            int x = 12;
            Test(x);
            Console.WriteLine(string.Format("After call, x = {0}", x));
        }

The boxed object is passed into the function using value type semantics.