#1,062 – Unboxing Conversions

If you’ve converted a value-typed object to a reference-typed object by boxing it, you can later unbox the object, converting it back to a value type.  Unboxing is an explicit conversion and requires a cast operator.

Below are some examples of unboxing conversions.

        public interface IArea
        {
            double CalcArea();
        }

        public struct MyPoint : IArea
        {
            public double X;
            public double Y;

            public MyPoint(double x, double y)
            {
                X = x;
                Y = y;
            }

            public double CalcArea()
            {
                return X * Y;
            }
        }

        static void Main(string[] args)
        {
            int i1 = 12;
            object o = i1;  // Boxing - implicit

            // Unbox from object to value type
            int i2 = (int)o;  // Boxing - explicit conversion

            // Unbox from dynamic
            dynamic d = i1;
            int i3 = (int)d;

            // Boxing, implicit, creates new copy
            MyPoint pt = new MyPoint(2.0, 3.0);
            IArea area = pt;

            // Unboxing, explicit,
            // from interface to value type,
            // creates new copy again
            MyPoint pt2 = (MyPoint)area;

            // Unbox to nullable type
            int? i4 = (int?)o;
            o = null;
            int? i5 = (int?)o;  // also works

            // Boxing, implicit, to ValueType
            // (creates copy)
            ValueType vty = pt;

            // Unboxing, creates copy
            MyPoint pt3 = (MyPoint)vty;
        }