#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.

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About Sean
Software developer in the Twin Cities area, passionate about .NET technologies. Equally passionate about my own personal projects related to family history and preservation of family stories and photos.

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