Allow only numbers and negative values

Question

I have a textbox which needs only accept numbers (can be decimal values) and negative values.

Currently I have something like this in KeyPress event

   if (!char.IsControl(e.KeyChar) && !char.IsDigit(e.KeyChar) && e.KeyChar != '.')
    {
        e.Handled = true;                
    }

What else I should do in order to allow negative values?

Thanks

Answer 1

if (!char.IsControl(e.KeyChar) && (!char.IsDigit(e.KeyChar)) 
        && (e.KeyChar != '.')  && (e.KeyChar != '-'))
    e.Handled = true;

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

// only allow minus sign at the beginning
if (e.KeyChar == '-' && (sender as TextBox).Text.Length > 0)
    e.Handled = true;

As LB correctly mentioned in the comments this won't allow some advanced notations like 3E-2 , but for simple numbers it will do the trick.

Answer 2

I thought that the textbox had a property where you could set the input on what is being inserted. Though I am currently not able to check this.

otherwise as an alternative you could try parsing your input to a double when submitting the value. Something like:

double myDouble;
try
{
    myDouble = double.parse(textbox.Text)
}
catch (Exception e)
{
    MessageBox.Show("Input is incorrect", "Error")
}

it's probably not the best work-around, but it might just do the trick.

Answer 3

Hook up the Validating event, like so:

private void myTextBox_Validating(object sender, CancelEventArgs event) {
    decimal d;
    if(!decimal.TryParse(myTextBox.Text, out d) {
        event.Cancel = true;
        //this.errorProvider1.SetError(myTextBox, "My Text Box must be a negative number."); //optional
        return;
    }

    if(d >= 0) {
        event.Cancel = true;
        //this.errorProvider1.SetError(myTextBox, "My Text Box must be a negative number."); //optional
        return;
    }
}

Answer 4

@DennisTraub's works, however there are some corner cases that it omits. For example, if the text in the textbox is "-11" and the user has the cursor at the start of the text, he or she may enter another character, such that the text could be "1-11" or ".-11". Here is an extension of his answer that seems to work for me.

TextBox textBox = sender as TextBox;
// If the text already contains a negative sign, we need to make sure that 
//    the user is not trying to enter a character at the start
// If there is already a negative sign and the negative sign is not selected, the key press is not valid
// This allows the user to highlight some of the text and replace it with a negative sign
if ((textBox.Text.IndexOf('-') > -1) && textBox.SelectionStart == 0 && !textBox.SelectedText.Contains('-'))
{
    e.Handled = true;
}
// Do not accept a character that is not included in the following
if (!char.IsControl(e.KeyChar) && !char.IsDigit(e.KeyChar) && e.KeyChar != '.' && e.KeyChar != '-')
{
    e.Handled = true;
}
// Only allow one decimal point
if ((e.KeyChar == '.') && (textBox.Text.IndexOf('.') > -1))
{
    e.Handled = true;
}            
// The negative sign can only be at the start
if ((e.KeyChar == '-'))
{
    // If the cursor is not at the start of the text, the key press is not valid
    if (textBox.SelectionStart > 0)
    {
        e.Handled = true;
    }
    // If there is already a negative sign and the negative sign is not selected, the key press is not valid
    // This allows the user to highlight some of the text and replace it with a negative sign
    if (textBox.Text.IndexOf('-') > -1 && !textBox.SelectedText.Contains('-'))
    {
        e.Handled = true;
    }
}

I have broken some stuff out that could be combined into one line. But basically you also need to check if there is already a negative sign in the text, and if the user has the cursor at the start of the text.

Answer 5

Something like:

var regex = new Regex("^[-]?\d+(\.\d+)?$", RegexOptions.Compiled);
Match m = regex.Match(textbox.Text + e.KeyChar);
e.Handled = m.Success;

Edit : It allows any real number now

Answer 6

// Boolean flag used to determine when a character other than a number is entered. 
        private bool nonNumberEntered = false;

        // Handle the KeyDown event to determine the type of character entered into the control. 
        private void textBox1_KeyDown(object sender, System.Windows.Forms.KeyEventArgs e)
        {
            // Initialize the flag to false.
            nonNumberEntered = false;

            // Determine whether the keystroke is a number from the top of the keyboard. 
            if (e.KeyCode < Keys.D0 || e.KeyCode > Keys.D9)
            {
                // Determine whether the keystroke is a number from the keypad. 
                if (e.KeyCode < Keys.NumPad0 || e.KeyCode > Keys.NumPad9)
                {
                    // Determine whether the keystroke is a backspace. 
                    if(e.KeyCode != Keys.Back)
                    {
                        // A non-numerical keystroke was pressed. 
                        // Set the flag to true and evaluate in KeyPress event.
                        nonNumberEntered = true;
                    }
                }
            }
            //If shift key was pressed, it's not a number. 
            if (Control.ModifierKeys == Keys.Shift) {
                nonNumberEntered = true;
            }
        }

        // This event occurs after the KeyDown event and can be used to prevent 
        // characters from entering the control. 
        private void textBox1_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
        {
            // Check for the flag being set in the KeyDown event. 
            if (nonNumberEntered == true)
            {
                // Stop the character from being entered into the control since it is non-numerical.
                e.Handled = true;
            }
        }

Answer 7

Try this Regular Expression

"/^(?!0*[.,]0*$|[.,]0*$|0*$)\d+[,.]?\d{0,2}$/" 

and use System.Text.RegularExpressions Namespace

See the example here : http://msdn.microsoft.com/en-us/library/system.text.regularexpressions.regex.aspx