How to limit the number of characters allowed in a textbox?

This did it for me. I did not keep the MultiLine property.

<asp:TextBox ID="txtBodySMS" runat="server" Rows="10" MaxLength="2" Width="100%"></asp:TextBox>

`MaxLength` does not apply to ASP.NET to Textboxes with `TextMode="MultiLine"`. An easy way to do this **and keep your `MultiLine` mark up** would be to add:

onkeypress="return this.value.length<=10"

with 10 being the limit. Like so:

<asp:TextBox ID="txtBodySMS" runat="server" Rows="10" onkeypress="return this.value.length<=10" TextMode="MultiLine" Width="100%"></asp:TextBox>

 

**EDIT Option One (Server side)** The above example has some issues pointed out over the years. The underlying problem is a multi-line `textBox` is rendered as a text-area and `MaxLength` is removed. We can also fix in the C# or VB asp.net code behind of the page by forcing the re-attachment of `MaxLength` instead of attaching `onkeypress`.

protected void Page_Load(object sender, EventArgs e)
{
txtBodySMS.Attributes.Add("maxlength", "10");
}

**EDIT Option Two (Client side):** Here's a simple jquery solution. Include the following script in your head instead of attaching `onkeypress` or editing the server side code.


$(document).ready(function () {
$(".MultiLineLimit").on('change keydown paste input', function () {
this.value = (this.value.length <= 10 ? this.value : this.value.substring(0, 10));
});
});


Now just add the `MultiLineLimit`class to any of your `<asp:TextBox>` textbox of `TextMode="MultiLine"`.

<asp:TextBox ID="txtBodySMS" CssClass="MultiLineLimit" runat="server" Rows="10" TextMode="MultiLine" Width="100%"></asp:TextBox>

Remember in either scenario client side validation can be removed. If your storing this in a database it should always be validated server side as well.

It seems to be a curious oversight in the design of ASP.NET (leaving aside the anomaly with textareas) that you can set the MaxLength property of a textbox but there seems to be no way provided to enforce this limit server-side without explicitly adding a validator. That's a nuisance if you have lots of fields and just want to be sure that no-one is subventing the limits of the form by injecting their own POST data. So I wrote this, which seems to do the trick - obviously you might not want to force Response.End

Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load

Dim colControls As Collection = New Collection
getControlList(colControls, Me)
For Each ctlNextPlease As Control In colControls
If TypeOf (ctlNextPlease) Is TextBox Then
Dim ctlTextBox As TextBox = ctlNextPlease
If Len(Request(ctlTextBox.ID)) > ctlTextBox.MaxLength Then
Response.Write("The value <i>" & Request(ctlTextBox.ID) & "</i> submitted for <b>" & ctlTextBox.ID & "</b> exceeds the permitted maximum length (" & ctlTextBox.MaxLength & ") for that value")
Response.End()
End If
End If
Next
...
End Sub

Public Shared Sub getControlList(ByRef colControls As Collection, ByVal rootControl As Control)
colControls.Add(rootControl)
If rootControl.Controls.Count > 0 Then
For Each controlToSearch As Control In rootControl.Controls
getControlList(colControls, controlToSearch)
Next
End If
End Sub

```xml
<asp:TextBox ID="txtBodySMS" runat="server" Rows="10" MaxLength="220"
TextMode="MultiLine" Width="100%">
</asp:TextBox>
```