Dataset and multiple forms problem

I'm really stuck with this problem and I think it's quite simple but my knowledge of c# just in't enough, so any help wolud be appriciated. I have a class (called broker) that contains the dataset and methods to manipulate the data in the dataset. It looks something like this: public class Broker { public MainDataSet ds=null; public Broker() { ds=new MainDataSet(); } public void FillDataSet() { //Do something it's not important what } } Next I have a form (called maniform) that uses methods from broker class. Something like this (just the stuff important for my problem): public class MainForm : System.Windows.Forms.Form { public MainForm() { InitializeComponent(); b=new Broker(); } public Broker b=null; private void Form1_Load(object sender, System.EventArgs e) { b.FillDtatSet(); } private void Something() { //Do something } } And in the end I have another form called modform that contains a combobox. Combobox has an on change event that needs to call a method from the mainform. I tried something like this: public void ReturnNewIndex(object sender, System.EventArgs e) { MainForm mfrm=new MainForm(); int index=NameCombo.SelectedIndex; mfrm.Something(index); } This is the problem: if I create instance of the mainform (MainForm mfrm=new MainForm()) in the modform dataset is recreated (Mainform new--> mainform calls constructor and creates new broker instance--> new broker instance calls its broker and recreates the dataset and the dataset is now empty). How to do this so that the dataset is not recreated and I can access methods from the main form? Please help I don't know who to ask and I have only a few days to solve the problem.