在文本框 C#/winforms 上只允许 {'delete', 'backspace', one '.', '-', numbers '0 to 9'}

Question

I am a beginner programmer and I am trying to modify the following code in order to allow only {'delete', 'backspace', one '.', one '-' sign in the beginning of input, numbers '0 to 9'} on a textbox C#/winforms:

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    if (!char.IsControl(e.KeyChar) && !char.IsDigit(e.KeyChar) &&
        (e.KeyChar != '.'))
    {
            e.Handled = true;
    }

    // only allow one decimal point
    if ((e.KeyChar == '.') && ((sender as TextBox).Text.IndexOf('.') > -1))
    {
        e.Handled = true;
    }
} 

any help please?

Answer 1

The question is very similar to this one except you have the added requirement of allowing -

So try this

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    if (!char.IsControl(e.KeyChar))  //bypass control keys
    {
        int dashIndex = textBox1.Text.IndexOf('-');
        int dotIndex = textBox1.Text.IndexOf('.');
        int selectionStart = textBox1.SelectionStart;
        int selectionEnd = selectionStart + textBox1.SelectionLength;

        if (char.IsDigit(e.KeyChar))
        {
            if (dashIndex == 0 && selectionStart == 0 && selectionEnd == 0)
                e.Handled = true;
        }
        else if (e.KeyChar == '-')
        {
            if (dashIndex == -1)
            {
                if (selectionStart != 0)
                    e.Handled = true;
            }
            else
                e.Handled = !(selectionStart <= dashIndex && selectionEnd > dashIndex);
        }
        else if (e.KeyChar == '.')
        {
            if (dotIndex == -1)
                e.Handled = false;
            else
                e.Handled = !(selectionStart <= dotIndex && selectionEnd > dotIndex);
        }
        else
            e.Handled = true;
    }
}

Answer 2

It seems to me that you want to enable the operator to edit a possibly negative number with exactly one decimal point.

Your proposal is to check every input character as soon as it is typed, and allow or forbid the character. This gives a rather clumsy user interface.

• Operator types 9876
• Oh no, this is the incorrect number, I need -76"
• The operator uses the arrow keys to go back to before the 7, type the minus sign (intermediate: 98-76), use the arrow key to go one to the left, and delete delete to remove the 98 (intermediate: 9-76, -76)

Copy paste other values "Price €76.12" and remove unwanted characters (76.12) is impossible.

The proper interface would be, to allow the operator everything, until he signals you that he finished editing the number. Usually this is an OK button, or if you want the enter button. Subscribe to the proper event, and handle the input only then:

public void OnButtonOk_Clicked(object sender, ...)
{
    string inputText = this.textBox1.Text;
    this.ProcessInput(inputText);
}

// or if you want: react on enter:
public void OnKeyPress(object sender, ...)
{
    // check if enter
    bool enterPressed = ...
    if (enterPressed)
    {
        this.ProcessInput(this.textBox1.Text);
    }
}

public void ProcessInput(string inputText)
{
    // check if the text can be converted to a double:
    if (double.TryParse(inputText, double result))
    {
         // text ok: do what you need to do
    }
    else
    {
         // text not ok: warn the operator
    }
}

Answer 3

The below method checks if the Character entered is permitted:

private bool NotPermittedChar(char character)
{
    return !char.IsDigit(character) && 
        !character.Equals('-') && 
        !character.Equals('.') && 
        !char.IsControl(character);
}

Then the below KeyPress method code will perform the relevant checks and conditions you require:

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    if (this.NotPermittedChar(e.KeyChar))
    {
        e.Handled = true;
        return;
    }

    if (this.textBox1.Text.Contains(".") && e.KeyChar.Equals('.'))
    {
        e.Handled = true; //// Don't allow more than 1 decimal place
        return;
    }

    if (this.textBox1.SelectionStart == 0 && e.KeyChar.Equals('.'))
    {
        e.Handled = true; //// Don't allow . to be at the beginning
        return;
    }

    if (this.textBox1.SelectionStart != 0 && e.KeyChar.Equals('-'))
    {
        e.Handled = true; //// Don't allow - in other index than the beginning
        return;
    }
}

I added some comments at the side of the e.Handled so you know which condition is being checked, it all seems to work according to my testing.