C#

class Class1
{
    public void ChangeTextctl ( string text , System.Windows.Forms.Control ctl )
    {
        ctl.Text = text;
    }
}

Class1 cls = new Class1();
cls.ChangeTextctl("Test", button1);

// Assumes form already contains a Label (label1).
public partial class WinForm : Form
{
    private Calc calc;
    public WinForm()
    {
        InitializeComponent();
        calc = new Calc();
        calc.CalcResult += new EventHandler<CalcResultEventArgs>(calc_CalcResult);
        calc.Add(3.14159265f);
    }

    void calc_CalcResult(object sender, CalcResultEventArgs e)
    {
        label1.Text = e.ResultAsString;
    }
}

public class Calc
{
    public event EventHandler<CalcResultEventArgs> CalcResult;

    public float Value
    {
        get;
        private set;
    }

    public void Add(float number)
    {
        Value += number;
        OnCalcResult(new CalcResultEventArgs(Value.ToString()));
    }

    protected virtual void OnCalcResult(CalcResultEventArgs e)
    {
        EventHandler<CalcResultEventArgs> eh = CalcResult;
        if (eh != null)
            eh(this, e);
    }
}

public class CalcResultEventArgs : EventArgs
{
    public CalcResultEventArgs(string resultAsString)
    {
        ResultAsString = resultAsString;
    }

    public string ResultAsString
    {
        get;
        private set;
    }
}