自动装箱和拆箱在Java和C＃中的表现不同

Question

I am manually converting code from Java (1.6) to C# and finding some difficulty with the behaviour of primitives (int and double). In C# it appears that almost all conversions happen automatically

        List<double> list1 = new List<double>();  // legal, C#
        double d0 = 3.0;
        list1.Add(d0);                            // legal, C#
        Double dd = 2.3f;                         // legal, C#
        list1.Add(dd);                            // legal, C#
        List<Double> list2 = new List<Double>();  // legal, C#
        double d1 = 3.0;
        list2.Add(d1);                            // legal, C#
        list2.Add(2.0);                           // legal, C#
        double d2 = list2.get(0);                 // legal, C#

but in Java only some are allowed

        List<double> list1 = new ArrayList<double>();  // illegal, Java
        List<Double> list2 = new ArrayList<Double>();  // legal, Java
        double d1 = 3.0;
        list2.add(d1);                                 // legal, Java
        list2.add(2.0);                                // legal, Java
        double d2 = list2.get(0);                      // legal, Java

I'd be grateful for a systematic analysis of the differences and any underlying rationale.

Answer 1

In C#, double and Double are exactly the same thing (as long as you haven't created your own type called Double , which would be stupid).

double is defined as an alias to global::System.Double . As such, there is no boxing here.

In java, Double is a boxed double , with type-erasure being a key part of the generics implementation.

Answer 2

In your C# example there is no boxing or unboxing (and autoboxing) happening. double is just an alias for the struct Double .

In Java, the boxing is necessary. Because of type erasure , you can't create a List<double> , only List<Double> . At compile time, List<?> becomes List<Object> and boxing/unboxing will need to take place so you can add a native type variable to a List<Object> or assign a native variable to an element of the List.