文本框上的C＃退格立即创建错误

Question

I am creating an event where if someone types into a text box it will show an error using this code:

try
{
    dblCostSqFt = double.Parse(txtCost.Text);
}
catch
{
    MessageBox.Show("Error. You must enter valid numbers. Please correct.");
    txtCost.Select();
    return;
}

The issue with this is if I input a backspace it will throw that error message immediately I would like to make it to where that doesn't happen.

Answer 1

You're working with userinput here. Therefore i'd suggest to use Double.TryParse()

• If you've got a string, and you expect it to always be a double (say, if some web service is handing you a double in string format), you'd use Double.Parse() .

• If you're collecting input from a user, you'd generally use Double.TryParse() , since it allows you more fine-grained control over the situation when the user enters invalid input.

With tryparse() your code will be something like this:

if (!double.TryParse(txtCost.Text, out var dblCostSqFt))
{
    MessageBox.Show("Error. You must enter valid numbers. Please correct.");
    txtExample.Select(0, txtCost.Text.Length);
    return;
}

To make the example complete and address the issue one could simply check if the Text is not null or empty by using String.IsNullOrEmpty() making the whole code:

// makes sure your app isn't crashing upon backspaces.
if(string.IsNullOrEmpty(textCost.Text))
{
    // Personally i'd indicate the user nothing is typed in (yet).
    return;
}
if (!double.TryParse(txtCost.Text, out var dblCostSqFt))
{
    // The user filled in something that can't be parse to doubles.
    MessageBox.Show("Error. You must enter valid numbers. Please correct.");
    txtExample.Select(0, txtCost.Text.Length);
    return;
}
// All is great; Do stuff with dblCostSqFt.

Answer 2

Assuming you are using WPF for your UI without straying too far from what you have I would use something like below (as LarsTech suggested, use TryParse to test if the value can be converted). Note the if block surrounding the core code within the function, you can avoid execution entering the if block by checking if the key pressed was backspace. I also added a check for the enter key as many users would press the enter key to close the MessageBox, which would cause the event to trigger once again.

private void txtExample_KeyUp(object sender, KeyEventArgs e)
{
    if (e.Key != Key.Back && e.Key != Key.Enter)
    {
        double dblCostSqFt = 0;

        if (!double.TryParse(txtExample.Text, out dblCostSqFt))
        {
            MessageBox.Show("Error. You must enter valid numbers. Please correct.");
            txtExample.Select(0, txtExample.Text.Length);
        }
    }
}

Never rely on exception handling to control the workflow of your application, exceptions have a ton of overhead and it is typically a bad practice in general.

You can accomplish the same thing in WinForms as well using the following...

private void txtExample_KeyUp(object sender, KeyEventArgs e)
{
    if (e.KeyCode != Keys.Back && e.KeyCode != Keys.Enter)
    {
        double dblCostSqFt = 0;

        if (!double.TryParse(txtExample.Text, out dblCostSqFt))
        {
            MessageBox.Show("Error. You must enter valid numbers. Please correct.");
            txtExample.Select();
        }
    }
}

It looks like you are using WinForms because your textbox.Select function call does not supply any arguments, only WinForms supports an overload of the select function without any arguments.