using button in a class

Well there are a lot of ways to do this but maybe this will give you an idea of something you could do.

public partial class Form1 : Form
{
    public Button MyButton
    {
        get { return button1; }
        set { button1 = value; }
    }

    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        MyClass mc = new MyClass(this);
    }
}

class MyClass
{
    Form1 mainForm;

    public MyClass(Form1 form1)
    {
        mainForm = form1;

        //access your button through mainForm
        mainForm.MyButton.Text = "MyText";
    }
}

You could also just pass your Button to your class instead of passing a reference to your form like so

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        MyClass mc = new MyClass(button1);
    }
}

class MyClass
{
    Button myButton;

    public MyClass(Button myButton)
    {
        this.myButton = myButton;

        //access your button
        myButton.Text = "MyText";
    }
}