C# WinApp ขอรบกวนเกี่ยวกับ การสุ่มตัวเลข เพื่อนำมา + - x ÷ แล้ว ได้ค่าที่ไม่ติดลบหน่อยครับ

private void button2_Click(object sender, EventArgs e)
        {
            try
            { fncRndNum(); }
            catch (Exception ex)
            { MessageBox.Show(ex.Message.ToString()); }
        }

private void fncRndNum()
        {
            int tmpInt;
            Random rand = new Random();
            int a, b, c;
            try
            {
                c = rand.Next(1, 10);   
                //c = 9;
                
                b = rand.Next(1, 10) * c;
                
                a = rand.Next(1, 10) * (b * c);

                if  (fncChkNum(a + b * c) != true || int.TryParse((a + b * c).ToString(), out tmpInt) != true) 
                { MessageBox.Show("Bad numbers"); }
                if (fncChkNum(a * b + c) != true || int.TryParse((a * b + c).ToString(), out tmpInt) != true)
                { MessageBox.Show("Bad numbers"); }
                if (fncChkNum(a - b * c) != true || int.TryParse((a - b * c).ToString(), out tmpInt) != true)
                { MessageBox.Show("Bad numbers"); }
                if (fncChkNum(a * b - c) != true || int.TryParse((a * b - c).ToString(), out tmpInt) != true)
                { MessageBox.Show("Bad numbers"); }
                if (fncChkNum(a + b / c) != true || int.TryParse((a + b / c).ToString(), out tmpInt) != true)
                { MessageBox.Show("Bad numbers"); }
                if (fncChkNum(a / b + c) != true || int.TryParse((a / b + c).ToString(), out tmpInt) != true)
                { MessageBox.Show("Bad numbers"); }
                if (fncChkNum(a - b / c) != true || int.TryParse((a - b / c).ToString(), out tmpInt) != true)
                { MessageBox.Show("Bad numbers"); }
                if (fncChkNum(a / b - c) != true || int.TryParse((a / b - c).ToString(), out tmpInt) != true)
                { MessageBox.Show("Bad numbers"); }
                else { MessageBox.Show("A=" + a.ToString() + "   B=" + b.ToString() + "    C=" + c.ToString()); }
            }
            catch (Exception ex)
            { MessageBox.Show(ex.Message.ToString(), "Error"); }
        }

        private static Boolean fncChkNum(int pNum)
        {
            Boolean ChkRst;
            try
            {
                if (pNum >= 0) { ChkRst = true; }
                else { ChkRst = false; }
            }
            catch (Exception ex)
            { throw ex; }
            return ChkRst;
        }