This should work:

C#

// this is used to store the PDF in memory
MemoryStream m = new MemoryStream();
// which document to open
string file = Server.MapPath("form.pdf");
PdfReader reader = new PdfReader(file);
// then create a PdfStamper from the created reader to modify the form fields
PdfStamper outStamper = new PdfStamper(reader, m);
string name = outStamper.AcroFields.GetField("name");

VB.NET
' this is used to store the PDF in memory
Dim m As MemoryStream = New MemoryStream()
' which document to open
Dim file As String = Server.MapPath("form.pdf")
Dim reader As PdfReader = New PdfReader(file)
' then create a PdfStamper from the created reader to modify the form fields
Dim outStamper As PdfStamper = New PdfStamper(reader, m)
Dim name As String = outStamper.AcroFields.GetField("name")