[Vala] How to define a const string array in Vala?

[PCMan <pcman tw gmail com>]

I C language, I can do this:
const char* strs[] = {
  "string1",
  "string2",
  "string3",
  NULL
};

In Vala, I cannot find a way to do the same.
Defining a string[] will cause copy of every constant string.
If I use unowned, like this:

unowned string strs[] = {
  ...
};

This did not generate the same result.
Every element of the string array gets copied.

So, If I need an array containing constant unowned strings, how to define
it in Vala?
Besides, in C language, we can write "const char* const *".
In Vala, I cannot find equivalent stuff.
Help is needed.

Thanks a lot!