<asp:Button ID="bttnCounter" runat="server" Text="Click Me" OnClick="ButtonClick"/>

Counter counter = new Counter();

    protected void Page_Load(object sender, EventArgs e)
    {
        bttnCounter.Click += new EventHandler(this.ButtonClick);
    }

    public void ButtonClick(Object sender, EventArgs e)
    {
        counter.CountUp();
        output.Text = "The count is " + counter.CurrentCount;
    }
}

public class Counter
{
    public int CurrentCount { get; private set; }

    public Counter()
    {
        CurrentCount = 0;
    }


    public void CountUp()
    {
        CurrentCount++;
    }
}

this.ViewState["someVar"] = yourObject;

YourClass yourObject = (YourClass)this.ViewState["someVar"];