Trying to create a form calculator using C#, unsure of "System.FormatException" error

Question

namespace Calculator
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Calculate_Click(object sender, EventArgs e)
        {
            int num1 = Convert.ToInt32(Number1.Text);
            int num2 = Convert.ToInt32(Number2.Text);
            int result = 0;
            string resultString = Convert.ToString(result);

            if (Addition.Checked == true)
            {
                result = num1 + num2;
                resultBox.Text = resultString;
            }
           else if (Subtraction.Checked == true)
            {
                result = num1 - num2;
                resultBox.Text = resultString;
            }
            else if (Multiplication.Checked == true)
            {
                result = num1 * num2;
                resultBox.Text = resultString;
            }
            else
            {
                resultBox.Text = "Error, no parameter selected";
            }
        }
    }
}

I'm pretty sure that most of it is right, it seems to be the converting that is tripping me up. I'm new to C# (first day!) so i'm a bit confused. Also first post on this website, so sorry for any formatting problems.

Add a breakpoint and debug. You can see where the code generates the error. — Luud van Keulen, Sep 27 '16 at 20:25
The error is on the Convert.ToInt32 but i'm unsure how to rectify the error. As far as I know I'm converting the string to an int, calculating the sum and then reconverting it back to a string — Sean, Sep 27 '16 at 20:31
When you want to parse text to an integer its a good idea to use tryParse. You can find more information about that [here](https://msdn.microsoft.com/en-us/library/f02979c7(v=vs.110).aspx) — Luud van Keulen, Sep 27 '16 at 20:33
In addition to using `TryParse`, you need to use your `ResultString` AFTER your calculation. See my answer below. — Icemanind, Sep 27 '16 at 20:39

score 2 · Accepted Answer · answered Sep 27 '16 at 20:34

It sounds like an issue with trying to convert to a number. Instead of using Convert.ToInt32(), try TryParse() instead:

int num1;
int num2;
bool isNum1Valid = int.TryParse(Number1.Text, out num1);
bool isNum2Valid = int.TryParse(Number2.Text, out num2);

if (!isNum1Valid)
{
    // num1 is invalid. Throw an error message or something
}

if (!isNum2Valid)
{
    // num2 is invalid. Throw an error message or something
}

Your other issue in your code is that your converting ResultString before you are parsing the result. Use this line AFTER you do your calculation:

string resultString = result.ToString();

score 1 · Answer 2 · edited May 23 '17 at 12:16

1

Icemanind's solution is very good, but you can consider not allowing the user to enter incorrect number from the beginning (something like a numeric text box). Several solutions are shown here:

private void FirstNumber_KeyPress(object sender, KeyPressEventArgs e)
{
    e.Handled = !char.IsDigit(e.KeyChar) && !char.IsControl(e.KeyChar);
}

Also, you should consider overflowing (* may easily get past Int32.MaxValue for numbers large enough), as it may lead to unexpected results, as shown here:

try
{
    checked
    {
        // may be used for all operations

        int product = num1 * num2;
    }
}
catch(OverflowException ex)
{
    resultBox.Text = "Integer operation overflow";
}

OR

Declare product as Int64 to make sure it accommodates all possible results.

Thus, you can be almost certain that you have valid numbers in your text boxes.

edited May 23 '17 at 12:16

Community

1
1

answered Sep 27 '16 at 20:55

Alexei - check Codidact

22,016
16
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164

This is good. Only issue is it can still allow for empty values (unless the textbox has a default value of 0 or something) – Icemanind Sep 27 '16 at 21:01
Yes, checking for empty is still needed. Also, parsing trials are needed, since user may paste the numbers from somewhere (unless pasting is disabled). So, this should be regarded as a complementary answer to make the interface more friendly. – Alexei - check Codidact Sep 27 '16 at 21:08

hogarth45 · Answer 3 · 2016-09-27T20:36:10.403

0

Be sure to check for null or empty values before attempting to convert.

if(String.IsNullOrEmptry(Number2.Text))
  num2 = 0;
else
  num2 = Convert.ToInt32(Number2.Text);

You are also converting 'result' to 'resultString' before you are setting a the answer to 'result.' You should move

resultBox.Text = resultString;

To below the if-else block.

edited Sep 27 '16 at 20:36

answered Sep 27 '16 at 20:28

hogarth45

3,387
1
22
27

Zulqarnain Jalil · Answer 4 · 2016-09-27T20:39:20.517

you should place string resultString = Convert.ToString(result); after calculation

private void Calculate_Click(object sender, EventArgs e)
        {

            int num1 = Number1.Text==""?0:Convert.ToInt32(Number1.Text);
            int num2 = Number2.Text==""?0:Convert.ToInt32(Number2.Text);
            int result = 0;


            if (Addition.Checked == true)
            {
                result = num1 + num2;
            }
           else if (Subtraction.Checked == true)
            {
                result = num1 - num2;
            }
            else if (Multiplication.Checked == true)
            {
                result = num1 * num2;
            }
            else
            {
                resultBox.Text = "Error, no parameter selected";
            }
        string resultString = Convert.ToString(result);
            resultBox.Text = resultString;
        }

Trying to create a form calculator using C#, unsure of "System.FormatException" error

4 Answers4