[c#] Only allow specific characters in textbox

How can I only allow certain characters in a Visual C# textbox? Users should be able to input the following characters into a text box, and everything else should be blocked: 0-9, +, -, /, *, (, ).

I've used Google to look up this problem, but the only solutions I'm getting are allowing only alphabetic characters, only numerical or disallowing certain characters. What I want is not disallowing certain characters, I want to disallow everything by default except the characters that I put in the code.

For your validation event IMO the easiest method would be to use a character array to validate textbox characters against. True - iterating and validating isn't particularly efficient, but it is straightforward.

Alternately, use a regular expression of your whitelist characters against the input string. Your events are availalbe at MSDN here: http://msdn.microsoft.com/en-us/library/system.windows.forms.control.lostfocus.aspx

You can probably use the KeyDown event, KeyPress event or KeyUp event. I would first try the KeyDown event I think.

You can set the Handled property of the event args to stop handling the event.

Intercept the KeyPressed event is in my opinion a good solid solution. Pay attention to trigger code characters (e.KeyChar lower then 32) if you use a RegExp.

But in this way is still possible to inject characters out of range whenever the user paste text from the clipboard. Unfortunately I did not found correct clipboard events to fix this.

So a waterproof solution is to intercept TextBox.TextChanged. Here is sometimes the original out of range character visible, for a short time. I recommend to implement both.

using System.Text.RegularExpressions;

private void Form1_Shown(object sender, EventArgs e)
{
    filterTextBoxContent(textBox1);
}


string pattern = @"[^0-9^+^\-^/^*^(^)]";

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    if(e.KeyChar >= 32 && Regex.Match(e.KeyChar.ToString(), pattern).Success) { e.Handled = true; }
}

private void textBox1_TextChanged(object sender, EventArgs e)
{
    filterTextBoxContent(textBox1);
}

private bool filterTextBoxContent(TextBox textBox)
{
    string text = textBox.Text;

    MatchCollection matches = Regex.Matches(text, pattern);
    bool matched = false;

    int selectionStart = textBox.SelectionStart;
    int selectionLength = textBox.SelectionLength;

    int leftShift = 0;
    foreach (Match match in matches)
    {
        if (match.Success && match.Captures.Count > 0)
        {
            matched = true;
            Capture capture = match.Captures[0];

            int captureLength = capture.Length;
            int captureStart = capture.Index - leftShift;
            int captureEnd = captureStart + captureLength;

            int selectionEnd = selectionStart + selectionLength;

            text = text.Substring(0, captureStart) + text.Substring(captureEnd, text.Length - captureEnd);

            textBox.Text = text;

            int boundSelectionStart = selectionStart < captureStart ? -1 : (selectionStart < captureEnd ? 0 : 1);
            int boundSelectionEnd = selectionEnd < captureStart ? -1 : (selectionEnd < captureEnd ? 0 : 1);

            if (boundSelectionStart == -1)
            {
                if (boundSelectionEnd == 0)
                {
                    selectionLength -= selectionEnd - captureStart;
                }
                else if (boundSelectionEnd == 1)
                {
                    selectionLength -= captureLength;
                }
            }
            else if (boundSelectionStart == 0)
            {
                if (boundSelectionEnd == 0)
                {
                    selectionStart = captureStart;
                    selectionLength = 0;
                }
                else if (boundSelectionEnd == 1)
                {
                    selectionStart = captureStart;
                    selectionLength -= captureEnd - selectionStart;
                }
            }
            else if (boundSelectionStart == 1)
            {
                selectionStart -= captureLength;
            }

            leftShift++;
        }
    }

    textBox.SelectionStart = selectionStart;
    textBox.SelectionLength = selectionLength;

    return matched;
}

    private void txtuser_KeyPress(object sender, KeyPressEventArgs e)
    {
        if (!char.IsLetter(e.KeyChar) && !char.IsWhiteSpace(e.KeyChar) && !char.IsControl(e.KeyChar))
        {
            e.Handled = true;
        }
    }

You need to subscribe to the KeyDown event on the text box. Then something like this:

private void textBox1_KeyDown(object sender, System.Windows.Forms.KeyEventArgs e)
{
    if (!char.IsControl(e.KeyChar) 
       && !char.IsDigit(e.KeyChar) 
       && e.KeyChar != '.' && e.KeyChar != '+' && e.KeyChar != '-'
       && e.KeyChar != '(' && e.KeyChar != ')' && e.KeyChar != '*' 
       && e.KeyChar != '/')
    {
        e.Handled = true;
        return;
    }
    e.Handled=false;
    return;
}

The important thing to know is that if you changed the Handled property to true, it will not process the keystroke. Setting it to false will.