The array_length attribute should only be used in bindings, never in Vala code. Can you explain why this is an issue?

I have added the array_length attribute for try to resolve the problem.

The copy of the arrays seems ok.

But the original code doesn't work:

using Sqlite;
 
class Dba
{
	Database db;
	int rc;
	public string[] db_result;
	public Dba()
	{
		rc = Database.open ("list.db", out db);
	}
		
	public string[] get_all_list ()
	{
		string sql = "SELECT * FROM list";
		db.exec (sql, select_callback);
		db = null;
		return db_result;
	}

  [Callback]
  private int select_callback (int n_columns, string[] values, string[] column_names)
  {                
    db_result = values;
    return 0;
  }
}


In the generated C code, the callback function pass -1 for the length of the array:

static gint _dba_select_callback_sqlite3_callback (gpointer self, gint n_columns, char** values, char** column_names) {
	return dba_select_callback (self, n_columns, values, -1, column_names, -1);
}

The SQLite API is a bit funky in places, which means the vala bindings are a bit akward, too. The problem here is that the callback function in sqlite takes void*,int,char**,char**. The first parameter is taken up with the object (this in vala, self in the generated C). The next parameter is actually the length of both of the next *two* arrays of strings. This is unusual--it would generally be one length parameter per array. Vala doesn't support handling this automatically, so the sqlite bindings mark both arrays as not having a length parameter, and let you deal with the length (n_columns) yourself. A -1 length in vala basically means that the true length is unknown (usually that translates to a null terminated array, but not in this case).

Try inserting "values.length = column_names.length = n_columns;" at the beginning of your callback function. After that, the length property should be what you would expect.

An argument could be made that the n_columns should be treated as the length of values, but column_names would still be -1. I prefer consistency, though, so I would leave it as is.