Passing dynamic data with callbacks

pragma solidity ^0.4.7;


contract usingCallback {
    function __callback(uint id, string s) {}
}



contract A {

    usingCallback C;
    uint queryId;

    function A () {}

    function getString() returns(uint _queryId) {
        
        C = usingCallback(msg.sender);
        C.__callback(queryId++, 'hello World');
        _queryId = queryId;
    }

}

contract B is usingCallback {

    A a;

    string s;
    uint queryId;

    function B (address _a) {
        a = A(_a);
    }

    function test () {

        queryId = a.getString();

    }

    
    function __callback(uint id, string _s) {
        if (id != queryId) {
            throw;
        }
        s = _s;
    }
    
}