Specifying maxlength for multiline textbox

Question

I'm trying to use asp:

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?

user_63245 · Answer

Another way of fixing this for those browsers (Firefox, Chrome, Safari) that support maxlength on textareas (HTML5) without javascript is to derive a subclass of the System.Web.UI.WebControls.TextBox class and override the Render method. Then in the overridden method add the maxlength attribute before rendering as normal.

protected override void Render(HtmlTextWriter writer)
{
    if (this.TextMode == TextBoxMode.MultiLine
        && this.MaxLength > 0)
    {
        writer.AddAttribute(HtmlTextWriterAttribute.Maxlength, this.MaxLength.ToString());
    }

    base.Render(writer);
}

user_56621 · Answer

Here's how we did it (keeps all code in one place):


<% TextBox1.Attributes["maxlength"] = "1000"; %>

Just in case someone still using webforms in 2018..

user_2424608 · Answer

you can specify the max length for the multiline textbox in pageLoad Javascript Event

function pageLoad(){
                     $("[id$='txtInput']").attr("maxlength","10");
                    }

I have set the max length property of txtInput multiline textbox to 10 characters in pageLoad() Javascript function

user_1462295 · Answer

This is the same as @KeithK's answer, but with a few more details. First, create a new control based on TextBox.

using System.Web.UI;
using System.Web.UI.WebControls;

namespace MyProject
{
    public class LimitedMultiLineTextBox : System.Web.UI.WebControls.TextBox
    {
        protected override void Render(HtmlTextWriter writer)
        {
            this.TextMode = TextBoxMode.MultiLine;

            if (this.MaxLength > 0)
            {
                writer.AddAttribute(HtmlTextWriterAttribute.Maxlength, this.MaxLength.ToString());
            }

            base.Render(writer);
        }
    }
}

Note that the code above always sets the textmode to multiline.

In order to use this, you need to register it on the aspx page. This is required because you'll need to reference it using the TagPrefix, otherwise compilation will complain about custom generic controls.

<%@ Register Assembly="MyProject" Namespace="MyProject" TagPrefix="mp" %>

user_1576627 · Answer

Nearly all modern browsers now support the use of the maxlength attribute for textarea elements.(https://caniuse.com/#feat=maxlength)

To include the maxlength attribute on a multiline TextBox, you can simply modify the Attributes collection in the code behind like so:

txtTextBox.Attributes["maxlength"] = "100";

If you don't want to have to use the code behind to specify that, you can just create a custom control that derives from TextBox:

public class Textarea : TextBox
{
    public override TextBoxMode TextMode
    {
        get { return TextBoxMode.MultiLine; }
        set { }
    }

    protected override void OnPreRender(EventArgs e)
    {
        if (TextMode == TextBoxMode.MultiLine && MaxLength != 0)
        {
            Attributes["maxlength"] = MaxLength.ToString();
        }

        base.OnPreRender(e);
    }
}

user_162080 · Answer

MaxLength is now supported as of .NET 4.7.2, so as long as you upgrade your project to .NET 4.7.2 or above, it will work automatically.

You can see this in the release notes here - specifically:

Enable ASP.NET developers to specify MaxLength attribute for Multiline asp:TextBox. [449020, System.Web.dll, Bug]

Specifying maxlength for multiline textbox

19 Answers