asp.net - Specifying maxlength for multiline textbox

Question

I'm trying to use asp:

<asp:TextBox ID="txtInput" runat="server" TextMode="MultiLine"></asp:TextBox>

I want a way to specify the maxlength property, but apparently there's no way possible for a multiline textbox. I've been trying to use some JavaScript for the onkeypress event:

onkeypress="return textboxMultilineMaxNumber(this,maxlength)"

function textboxMultilineMaxNumber(txt, maxLen) {
    try {
        if (txt.value.length > (maxLen - 1)) return false;
    } catch (e) { }
    return true;
}

While working fine the problem with this JavaScript function is that after writing characters it doesn't allow you to delete and substitute any of them, that behavior is not desired.

Have you got any idea what could I possibly change in the above code to avoid that or any other ways to get round it?

Answer 1

Use a regular expression validator instead. This will work on the client side using JavaScript, but also when JavaScript is disabled (as the length check will be performed on the server as well).

The following example checks that the entered value is between 0 and 100 characters long:

<asp:RegularExpressionValidator runat="server" ID="valInput"
    ControlToValidate="txtInput"
    ValidationExpression="^[sS]{0,100}$"
    ErrorMessage="Please enter a maximum of 100 characters"
    Display="Dynamic">*</asp:RegularExpressionValidator>

There are of course more complex regexs you can use to better suit your purposes.