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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.

peter.murray.rust
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2 Answers2

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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.

Marc Gravell
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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.

bruno conde
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  • I was under the impression that List existed long before Type Erasure in Java, or am I misunderstanding what you're saying? – RHSeeger Oct 20 '09 at 20:55
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    @RHSeeger, List is a generic Type. Generics in Java where/are implemented with Type Erasure. So this implies that List didn't existed before Type Erasure (at least with the Type parameters). Notice that, List will be translated to the raw type List. – bruno conde Oct 20 '09 at 21:09