Default CssClass on Custom Control

I am creating a custom ASP.NET textbox control and I am trying to set the default CssClass, but I can't figure out how.
I have added attributes to the controls (OnBlur and OnFocus) those work fine changing the cssClass, but when the page initially loads the textbox doesn't have a default cssClass.

Here's my code

VB Code:

Imports System
Imports System.Web
Imports System.Web.UI
Imports System.Web.UI.WebControls
Imports System.ComponentModel
 
Namespace JHATextBox
 
    Public Class JHATextBox
        Inherits TextBox
 
        'This should be the default cssClass when the page loads.
        Private _cssClass As String = "formBox"
 
        Protected Overrides Sub Render(ByVal writer As HtmlTextWriter)
            Dim strScript As String = ""
 
            If Page.IsClientScriptBlockRegistered("ClientScript") = False Then
                strScript &= "<script language=""javascript"" type=""text/javascript"">" & vbCrLf
                strScript &= vbTab & "function activeField(ptr){" & vbCrLf
                strScript &= vbTab & vbTab & "if((ptr.disabled != true) && (ptr.readOnly != true)) {" & vbCrLf
                strScript &= vbTab & vbTab & vbTab & "ptr.className = 'formBoxSelected';" & vbCrLf
                strScript &= vbTab & vbTab & "}" & vbCrLf
                strScript &= vbTab & "}" & vbCrLf
                strScript &= vbTab & "function inactiveField(ptr) {" & vbCrLf
                strScript &= vbTab & vbTab & "if((ptr.disabled != true) && (ptr.readOnly != true)) {" & vbCrLf
                strScript &= vbTab & vbTab & vbTab & "ptr.className = 'formBox';" & vbCrLf
                strScript &= vbTab & vbTab & "}" & vbCrLf
                strScript &= vbTab & "}" & vbCrLf
                strScript &= "</script>" & vbCrLf
 
                Page.RegisterStartupScript("ClientScript", strScript)
            End If
 
            writer.AddAttribute("onBlur", "void inactiveField(this);")
            writer.AddAttribute("onFocus", "void activeField(this);")
            writer.RenderBeginTag("input")
            writer.RenderEndTag()
 
        End Sub
 
        Public Overrides Property CssClass() As String
            Get
                Return _cssClass
            End Get
            Set(ByVal Value As String)
                _cssClass = Value
            End Set
        End Property
 
    End Class
 
End Namespace

If I just type formBox as the CssClass into the controls properties, it works fine, but I can't set it in the code above for some reason.

Doesn't make sense to me.

If a control doesn't have a cssClass value, it grabs its parent by default.

A few things I would check:

It could be the designer wrote a value on the aspx side when you placed the control on the page.

Your javascript function is resetting it something.

Remember, styles classes are case sensitive.

Trying putting:

VB Code:

<DefaultValueAttribute("formBox")>

in front of your public cssclass property, which will initialize it to that value by default when one hasn't been assigned, when you place the control on a form.