CS0120: An object reference is required for the nonstatic field, method, or property 'foo'

Question

Consider:

namespace WindowsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            


        

Answer 1

You start a thread which runs the static method SumData. However, SumData calls SetTextboxText which isn't static. Thus you need an instance of your form to call SetTextboxText.