Casting of int array into double array in immediate window?

That cast is illegal. Just try to compile it and you will see that it doesn't work either.

The following code will perform this conversion:

var d = i.Select(x => (double)x).ToArray();

Unfortunately, you can't use it in the immediate window because it doesn't support lambda expressions.

A solution that doesn't require lambda expressions is the following:

i.Select(Convert.ToDouble).ToArray();

This could work because there is no lambda expression. Thanks to Chris for the idea.

One more way is to use Array.ConvertAll

Array.ConvertAll<int, double>(nums, x => x);