textbox และ static C#.NET มือใหม่มากวอนช่วยดู ขอบพระคุณมากๆนะครับ

public struct MyStruct
        {
            public float Salary;
            public float TimeFront;
            public float TimeBack;
            public int Day;
            public double dOTSum;
        }
        private ArrayList MyRecord = new ArrayList();

        double dot; // Ot Variable
        double dduty; // Duty Variable      
 public static string dSum (string dSum)
        {
           
            double dTime;
            dTime = fTime2 - fTime1;

            dSum = ((((fSalary / 240) * dTime) * iDate) * dot) + dduty;

            return dSum;
        }
        private void btnAdd_Click(object sender, EventArgs e)
        {
            object oTime1 = this.txtTime1.Text;
            object oTime2 = this.txtTime2.Text;
            object oSalary = this.txtSalary.Text;


            float fTime1 = Convert.ToSingle(txtTime1.Text);  //
            float fTime2 = Convert.ToSingle(txtTime2.Text);  // Convert Float to String
            float fSalary = Convert.ToSingle(txtSalary.Text);//

            int iDate = (int)nudDate.Value;

            MyStruct structX;
            structX.Salary = fSalary;
            structX.TimeFront = fTime1;
            structX.TimeBack = fTime2;
            structX.Day = iDate;
            MyRecord.Add(structX);  // Add Data into MyStruct

            string strShow = "เงินเดือน : " + structX.Salary + " บาท ตั้งแต่เวลา : " + structX.TimeFront + " ถึง" + structX.TimeBack + " นาฬิกา จำนวน " + structX.Day + " วัน";
            lstRecord.Items.Add(strShow);
            txtSalary.Text = "";
            txtTime1.Text = "";
            txtTime2.Text = "";

            
            MyRecord.Add(dSum);
            
        }