c# - メソッドにエラーチェックを入れる方法は?

Question

これらのエラーチェックをメソッドに入れるにはどうすればよいですか? ユーザーがプログラムをクラッシュさせることはできません。数値は 0 から 1,000,000 の間でなければなりません。これは私がこれまでに持っているものです。

空のフィールドがある場合は、次のようなエラー メッセージが表示されます。不足しているフィールドに入力してください。

    private void btnCalculate_Click(object sender, EventArgs e)
    {
        try
        {
            // changes value that is in text box and switches it into a decimal
            decimal firstNumber = Convert.ToDecimal(txtOperand1.Text);
            decimal secondNumber = Convert.ToDecimal(txtOperand2.Text);
            string mathSymbol = txtOperator.Text;   // already a string variable
            decimal result = 0;

            //calls the method
            result = calculate(firstNumber, secondNumber, mathSymbol, result);



            if (mathSymbol != "/" && mathSymbol != "*" && mathSymbol != "+" && mathSymbol != "-")
            {
                txtOperator.Text = ""; 
                MessageBox.Show("ERROR invalid operator");

            }



            if (firstNumber <= 0 || firstNumber >= 100000 || secondNumber <= 0 || secondNumber >= 1000000)
            {
                txtResult.Text = ""; 
                MessageBox.Show("Numbers must be greater than 0 and less than 1,000,000");
                txtOperand1.Text = "";
                txtOperand2.Text = "";
                txtOperator.Text = "";                
            }

        }
        catch (FormatException)
        {
            MessageBox.Show("Please enter values.", "ERROR");
        }
        catch (Exception op)
        {
            MessageBox.Show(op.Message, "ERROR");
        }
    }

    // method calculate - amount = to calculate then calculate = to result
    private decimal calculate(decimal num1, decimal num2, string symbol, decimal amount)
       {    
            if (symbol == "+")      // checks if user enters a + then adds numbers
                amount = num1 + num2;

            else if (symbol == "-") // checks if user enters a - then minus numbers
                amount = num1 - num2;

            else if (symbol == "*") // checks if user enters a * then multiplies numbers
                amount = num1 * num2;

           else if (symbol == "/")  // checks if user enters a / then divides numbers
                amount = num1 / num2;

            txtResult.Text = amount.ToString("f4");

           return amount;
       }






    private void btnExit_Click(object sender, EventArgs e)
    {
        this.Close();   // closes the form if exit button is pressed
    }

    private void clearResult(object sender, EventArgs e)
    {
        txtResult.Text = " "; // clears results if txtOperand1 is changed
    }

    private void clear_result(object sender, EventArgs e)
    {
        txtResult.Text = " "; // clears results if txtOperator is changed
     }

    private void ClearResults3(object sender, EventArgs e)
    {
        txtResult.Text = " "; // clears results if txtOperand2 is changed
    }
}