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.