An object reference is required for the nonstatic field, method, or property

	It looks like you are calling a non static property from a static method. You will need to either make the property static, or create an instance of class
	static void SetTextboxTextSafe(int result)
	{
	label1.Text = result.ToString();
	}
	or
	private static void SumData(object state)
	{
	int result;
	//int[] icount = (int[])state;
	int icount = (int)state;
	for (int i = icount; i > 0; i--)
	{
	result += i;
	System.Threading.Thread.Sleep(1000);
	}
	MyClass objMyClass = new MyClass();
	objMyClass.setTextboxText(result);
	}

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