Set default value in custom control
===

I am creating custom web control(textbox) which allows only integer data type in textbox.
On button submit event if there is no data in textbox, custom control should set TextBox1.Text=0, this is what i want. This is code which i wrote.

    public class MasIntTextBox : TextBox
    {
        private RequiredFieldValidator req;
        private RegularExpressionValidator regex;
        public string ValGrp { get; set; }
        public string IsRequired { get; set; }//give true/yes or false/no values only

        protected override void OnInit(EventArgs e)
        {
            //this.CssClass = "inp-form";
            if (IsRequired == "true" || IsRequired == "yes")
            {
                req = new RequiredFieldValidator();
                req.ControlToValidate = this.ID;
                req.ErrorMessage = "Enter Numeric Value";
                req.Display = ValidatorDisplay.Dynamic;
                if (!string.IsNullOrEmpty(ValGrp))
                    req.ValidationGroup = ValGrp;
                Controls.Add(req);
            }
            regex = new RegularExpressionValidator();
            regex.ControlToValidate = this.ID;
            regex.ErrorMessage = "Numeric Value Only";
            regex.Display = ValidatorDisplay.Dynamic;
            regex.ValidationExpression = "^\\d+(\\.\\d+)?$";
            if (!string.IsNullOrEmpty(ValGrp))
                regex.ValidationGroup = ValGrp;
            Controls.Add(regex);
            //base.OnInit(e);
        }

        protected override void Render(HtmlTextWriter w)
        {
            base.Render(w);
            if (IsRequired == "true" || IsRequired == "yes")
                req.RenderControl(w);
            regex.RenderControl(w);
        }
    }
I checked few events or methods for this but they got fired after btnA_click event.
I am kind of newbie in this.
Any event or method i am missing? Any suggestion accepted.
Thanks in Advance...