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

It looks like you are calling a non static member (a property or method, specifically setTextboxText) from a static method (specifically SumData). You will need to either:

1. Make the called member static also:

   static void setTextboxText(int result)
   {
       // Write static logic for setTextboxText.  
       // This may require a static singleton instance of Form1.
   }
   

2. Create an instance of Form1 within the calling method:

   private static void SumData(object state)
   {
       int result = 0;
       //int[] icount = (int[])state;
       int icount = (int)state;
   
       for (int i = icount; i > 0; i--)
       {
           result += i;
           System.Threading.Thread.Sleep(1000);
       }
       Form1 frm1 = new Form1();
       frm1.setTextboxText(result);
   }
   

   Passing in an instance of Form1 would be an option also.

3. Make the calling method a non-static instance method (of Form1):

   private void SumData(object state)
   {
       int result = 0;
       //int[] icount = (int[])state;
       int icount = (int)state;
   
       for (int i = icount; i > 0; i--)
       {
           result += i;
           System.Threading.Thread.Sleep(1000);
       }
       setTextboxText(result);
   }
   

More info about this error can be found on MSDN.

For this case, where you want to get a Control of a Form and are receiving this error, then I have a little bypass for you.

Go to your Program.cs and change

Application.Run(new Form1());

to

public static Form1 form1 = new Form1(); // Place this var out of the constructor
Application.Run(form1);

Now you can access a control with

Program.form1.<Your control>

Also: Don't forget to set your Control-Access-Level to Public.

And yes I know, this answer does not fit to the question caller, but it fits to googlers who have this specific issue with controls.

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.