Input string was not in a correct format, “double.

问题:

Hi am quite new to visual studios.

This is my code so far to make an example of an atm, i have a text box and i put an amount in and then i have this button where i click credit and it adds the amount to a label called balance and i also have a button named debit which takes the money away from the balance. I'm doing this in wpf c#

so far i have this.

    namespace BankAccounts
     {
     /// <summary>
     /// Interaction logic for MainWindow.xaml
     /// </summary>
     public partial class MainWindow : Window
     {
       public MainWindow()
    {
        InitializeComponent();
    }

    private double totalamount = 0;
    public string balance1;


    private void buttoncredit_Click(object sender, RoutedEventArgs e)
    {
        totalamount = totalamount + double.Parse(textboxamount.Text)

        balance1 = "Your Balance is: £";

        label2.Content = balance1 + totalamount;

        textboxamount.Clear();



    }

    private void buttondebit_Click(object sender, RoutedEventArgs e)
    {
        if (totalamount - double.Parse(textboxamount.Text) < 0)
        {
            MessageBox.Show("Overdraft Limit is not set please contact Customer Services");
        }

        else
        {

            totalamount = totalamount - double.Parse(textboxamount.Text);

            balance1 = " Your Balance is: £";

            label2.Content = balance1 + totalamount;



            textboxamount.Clear();
        }
    }

    private void listboxtransactions_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {



    }
}

}

回答1:

You can't trust your user to type exactly a double value in your textbox.
The Parse method cannot avoid an exception if the input cannot be converted to a double.
Instead the double.TryParse methods give the chance to test if the value typed is effectively a double. Also it seems that you are working with currency values, so perhaps it is better to use a decimal data type and when building the output string use the appropriate formatting to get a correct currency string for your locale. This will also avoid rounding errors intrinsically present in the double/single datatype

private decimal totalamount = 0;
public string balance1;

private void buttoncredit_Click(object sender, RoutedEventArgs e)
{
    decimal temp;
    if(decimal.TryParse(textboxamount.Text, out temp))
    {
        totalamount = totalamount + temp;
        balance1 = "Your Balance is: ";
        label2.Content = balance1 + totalamount.ToString("C");    
        textboxamount.Clear();

    }
    else
        MessageBox.Show("Not a valid amount");
}

private void buttondebit_Click(object sender, RoutedEventArgs e)
{
    decimal temp;
    if(decimal.TryParse(textboxamount.Text, out temp))
    {
        if (totalamount - temp < 0)
        {
             MessageBox.Show("Overdraft Limit is not set please contact Customer Services");
        }
        else
        {
             totalamount = totalamount - temp;
             balance1 = " Your Balance is: ";
             label2.Content = balance1 + totalamount.ToString("C");
             textboxamount.Clear();
        }
    }
    else
        MessageBox.Show("Not a valid amount");
}

回答2:

String in textboxamount.Text can not be parsed as double. To avoid exception you can use double.TryParse instead.

double amount;

if(double.TryParse(textboxamount.Text, out amount))
{
    totalamount += amount;
}

Also, label2 seems to be Label and you must use

label2.Text = balance1 + totalamount;

instead.

回答3:

Two major issues cause this error:

• Any additional text such as whitespace or currency characters.
• Incorrect localization settings causing , and . to be flipped.

Possibly including , but I don't remember offhand whether that is an error condition.

回答4:

The problem is that the value in textboxamount.Text contains something that can't be converted to a double.

The best way to handle this is to use double.TryParse instead:

private void buttoncredit_Click(object sender, RoutedEventArgs e)
{
    double newAmount;
    if(!double.TryParse(textboxamount.Text, out newAmount))
    {
        // The input is wrong - handle that
        MessageBox.Show("Please enter a valid amount");
        textboxamount.Focus();
        return;
    }

    totalamount += newAmount;
    balance1 = "Your Balance is: £";
    label2.Content = balance1 + totalamount;
    // .. Your code...