Implicit array casting in C#

Question

Hi,

I have the following classes with an implicit cast operator defined:

class A
{
    ...
}
class B
{
    private A m_a;

    public B(A a)
    {
        this.m_a = a;
    }

    public static implicit operator B(A a)
    {
        return new B(a);
    }
}

Now, I can implicitly cast A to B.

But why can't I implicitly cast A[] to B[] ?

static void Main(string[] args)
{
    // compiles
    A a = new A();
    B b = a;

    // doesn't compile
    A[] arrA = new A[] {new A(), new A()};
    B[] arrB = arrA;
}

Thanks, Malki.

Answer 1

Array covariance only works for reference types and in the inheritance hierarchy (note that it's not a representation-changing conversion: just a set of pointers with identical size interpreted differently.) It will not work for value types and user defined conversions.

Answer 2

As Mehrdad Afshari mentioned, you're out of luck doing this implicitly. You'll have to get explicit, and it'll involve an array copy. Thankfully, you can probably do it with a one-liner:

arrB = arrA.Cast<B>().ToArray();

Although if you only want to iterate arrB in a foreach statement, you can avoid the copy by omitting ToArray()

Answer 3

Imagine for a moment if Arrays used the same syntax as other collections in .Net, and what you're trying to compare is an Array<A> with an Array<B>. You wouldn't compare a List<A> to a List<B>. Well, that's essentially what you're trying.

I'd recommend using a simple extension method to get the result you want, you'll need to change your syntax slightly to say 'B[] arrB = arrA.ToBArray();`

static class ArrayCast {
    public static B[] ToBArray(this A[] source) {
        var result = new B[source.Length];
        for (int i = 0;i < source.Length;i++)
            result[i] = source[i];
        return result;
    }
}