Field validation in editable pdf while filling data - itext

Question

I have editable pdf with fields name, roll number , DOB and city, I want to validate the fields like, for example when I enter the value in roll number field, if name is empty then it should alert "name is empty" and similar to roll number and DOB and city.
DOB - Need to validate mm/dd/yyyy format.
I am able to get fields and values using below iTextSharp code, but how can validate and and show alert to user.
iTextSharp.text.pdf.PdfReader reader = new iTextSharp.text.pdf.PdfReader(filename);
using (MemoryStream outputStream = new MemoryStream())
{
//PdfStamper stamper = new PdfStamper(reader, );
using (PdfStamper stamper = new PdfStamper(reader, outputStream, '\0', true))
{
PdfWriter writer = stamper.Writer;
AcroFields acroFields = reader.AcroFields;
AcroFields.Item dateField = acroFields.GetFieldItem("D");
iTextSharp.text.pdf.PdfAction pdfAction = iTextSharp.text.pdf.PdfAction.JavaScript("app.alert('hello');", writer);
iTextSharp.text.pdf.PdfDictionary widgetRefDict = (iTextSharp.text.pdf.PdfDictionary)iTextSharp.text.pdf.PdfReader.GetPdfObject(dateField.GetWidgetRef(0));
iTextSharp.text.pdf.PdfDictionary actionDict = widgetRefDict.GetAsDict(iTextSharp.text.pdf.PdfName.AA);
if (actionDict == null)
{
actionDict = new iTextSharp.text.pdf.PdfDictionary();
// add the newly created action dict
widgetRefDict.Put(iTextSharp.text.pdf.PdfName.AA, actionDict);
}
actionDict.Put(iTextSharp.text.pdf.PdfName.V, pdfAction);
stamper.Close();
reader.Close();
}
byte[] content = outputStream.ToArray();
// Write out PDF from memory stream.
using (FileStream fs = File.Create("d:\\Page1-output.pdf"))
{
fs.Write(content, 0, (int)content.Length);
}
}