异常处理c#

我在这方面相对较新，一直在绞尽脑汁试图让我的程序正常工作，但它就是不会。我在Visual Studio 2012 c#中工作在一个表单应用程序。

我需要它在用户输入值大于0但小于10,000时产生明显的错误消息。当用户输入非数值时，它还必须产生一个明显的错误消息，当用户根本没有输入任何值时，它必须产生一个明显的错误消息。

到目前为止，我所编写的代码在用户输入非数字值或根本没有输入任何值时产生一个明显的错误消息，但是当用户输入的值低于或超过所需范围时，它不会触发错误消息。

这就好像编译器忽略了我为第一个异常/溢出异常编写的代码，只识别第二个也是最后一个异常的代码。我的代码没有编码错误。看来我的问题出在逻辑上。

请帮助我，如果你可以。我的代码是下面谢谢!

    private void btnCalculate_Click(object sender, System.EventArgs e)
    {
        try
        {
            {
                decimal subtotal = Convert.ToDecimal(txtSubtotal.Text);
                decimal discountPercent = .25m;
                decimal discountAmount = subtotal * discountPercent;
                decimal invoiceTotal = subtotal - discountAmount;
                lblDiscountPercent.Text = discountPercent.ToString("p1");
                lblDiscountAmount.Text = discountAmount.ToString("c");
                lblTotal.Text = invoiceTotal.ToString("c");
            }
        }
        catch (OverflowException)
        {
            decimal subtotal = Convert.ToDecimal(txtSubtotal.Text);
            if (subtotal <= 0)
            {
                MessageBox.Show("Subtotal must be greater than $0.00. ", "Error Entry");
                txtSubtotal.Focus();
            }
             if (subtotal >= 10000)
            {
                MessageBox.Show("Subtotal must be less than $10000.00. ", "Error Entry");
                txtSubtotal.Focus();
            }
        }
        catch (Exception)
        {
            if (txtSubtotal.Text == "")
            {
                MessageBox.Show("Subtotal is a required field. ", "Error Entry");
            }

            else
            {
                MessageBox.Show(
                 "Please enter a valid Number for the subtotal field.", "Error Entry");
                txtSubtotal.Focus();
            }

            }           
        }


    private void btnExit_Click(object sender, System.EventArgs e)
    {
        this.Close();
    }
    private void txtSubtotal_TextChanged(object sender, EventArgs e)
    {
    }
}

}

   public class iCnF()
   {
    public static System.Boolean IsNumeric(System.Object Expression) 
      {
        if (Expression == null || Expression is DateTime)
            return false;
        if (Expression is Int16 || Expression is Int32 || Expression is Int64 || Expression is Decimal || Expression is Single || Expression is Double || Expression is Boolean)
            return true;
        try 
        {
            if(Expression is string)
                Double.Parse(Expression as string);
            else
                Double.Parse(Expression.ToString());
                return true;
        } catch {} // just dismiss errors but return false
            return false;
     }
}

   private void txtSubtotal_KeyDown(object sender, KeyEventArgs e)
   {
    if (e.KeyCode == Keys.Enter || e.KeyCode == Keys.Tab)
    {
        if (txtSubtotal.Text.Length > 0)
        {
            bool sd = iCnF.IsNumeric(txtSubtotal.Text);
            if (sd == false)
            {
               MessageBox.Show("Subtotal must be a numeric value. ", "Error Entry");
               txtSubtotal.Clear();
               txtSubtotal.Focus();
            }
            else
            {
                decimal subtotal = Convert.ToDecimal(txtSubtotal.Text);
                if (subtotal <= 0)
                {
                    MessageBox.Show("Subtotal must be greater than $0.00. ", "Error Entry");
                    txtSubtotal.Focus();
               }
               if (subtotal >= 10000)
               {
                   MessageBox.Show("Subtotal must be less than $10000.00. ", "Error Entry");
                   txtSubtotal.Focus();
               }
           }
       }
       else
       {
           MessageBox.Show("Subtotal must not be empty. ", "Error Entry");
           txtSubtotal.Focus();
       }
   }
  }  

  private bool IsPageValid()  
  {
           string errorMessage = string.Empty;               
           bool isValid = true;
           if (subtotal <= 0)
           {
           errorMessage+="<li>"+"<b>Subtotal must be greater than $0.00. ", "Error Entry"+ "</b><br/>";
           txtSubtotal.Focus();
           isValid=false;
           }
   }

 protected void btnSubmit_Click(object sender, EventArgs e)
    {
            ClearMessage();
            if (IsPageValid().Equals(true))
            {
             // allow next action to happen.
            }
     }

try {
    decimal subtotal = Convert.ToDecimal(txtSubtotal.Text);
    try {
        if(x<0 || x>10000){
            throw new OverFlowException("");
        }
        //Do what ever you want
    }
    catch(Exception ex){
        // catch overflow
    }
}
catch(Exception ex){
    // catch nonnumeric value
}