Input string was not in a correct format

Question

Hey All!

When the "Loan Amount" textbox is left empty in my form [seen below] .NET throws an exception [also listed below].

Any help is appreciated! [I put my code below as well] I looked at some of the other postings related to the issue and still couldn't figure it out which is why I decided to post.

using System;  
using System.Collections.Generic;  
using System.Linq;  
using System.Web;  
using System.Web.UI;  
using System.Web.UI.WebControls;  
  
namespace CIS2222_Lab2  
{  
    public partial class InterestCalculator : System.Web.UI.Page  
    {  
        protected void Page_Load(object sender, EventArgs e)  
        {  
  
        }  
  
        protected void BtnCalculateInterest_Click(object sender, EventArgs e)  
        {  
            double calculatedInterest;  
            double loanAmt = Convert.ToDouble(txtLoanAmt.Text);  
            double interestRate = (Convert.ToDouble(radioInterestRate.Text)) / 100;  
            int numOfYears = Convert.ToInt32(listNumOfYears.Text);  
  
            calculatedInterest = loanAmt * interestRate * numOfYears;  
  
            lblResult.Text = String.Format("Total interest for a loan of {0:c} at {1:0%} interest for {2} years is {3:c}",  
                            loanAmt, interestRate, numOfYears, calculatedInterest);  
        }  

Answer 1

Either use Data Annotations for validating prior to using or old school assertion e.g.

if (double.TryParse(txtLoanAmt.Text, out var loanAmt) &&   
    double.TryParse(radioInterestRate.Text, out var interestRate) &&   
    int.TryParse(listNumOfYears.Text, out var numOfYears)) {  
    // do work  
    var calculatedInterest = loanAmt * interestRate * numOfYears;  
}  
else  
{  
    // have invalid value(s)  
}  

Or double assertion

if (  
    !string.IsNullOrWhiteSpace(txtLoanAmt.Text) &&   
    double.TryParse(txtLoanAmt.Text, out var loanAmt))  
{  
      
}  

Answer 2

Thanks Karen.

Based on what you said, I made one little tweak to the code in line 20, which seemed to make it work [tweaked code below].

What do you think?

using System;  
using System.Collections.Generic;  
using System.Linq;  
using System.Web;  
using System.Web.UI;  
using System.Web.UI.WebControls;  
  
namespace CIS2222_Lab2  
{  
    public partial class InterestCalculator : System.Web.UI.Page  
    {  
        protected void Page_Load(object sender, EventArgs e)  
        {  
  
        }  
  
        protected void BtnCalculateInterest_Click(object sender, EventArgs e)  
        {  
            double calculatedInterest;  
            **double.TryParse(txtLoanAmt.Text, out double loanAmt);**  
            double interestRate = (Convert.ToDouble(radioInterestRate.Text)) / 100;  
            int numOfYears = Convert.ToInt32(listNumOfYears.Text);  
  
            calculatedInterest = loanAmt * interestRate * numOfYears;  
  
            lblResult.Text = String.Format("Total interest for a loan of {0:c} at {1:0%} interest for {2} years is {3:c}",  
                            loanAmt, interestRate, numOfYears, calculatedInterest);  
        }