How can I use "AcroExch.App"

[gao ming via DotNetMonster.com]

Now ,I want to add form field to existing pdf file .
below is vb code :
Set AcroExchAVDoc = CreateObject("AcroExch.AVDoc")
bOK = AcroExchAVDoc.Open("C:\mydoc.pdf", "")
If (bOK) Then
Set AFormAut = CreateObject("AFormAut.App")
Set Fields = AFormAut.Fields
End If
Set Field = Fields.Add("Name", "text", 0, 100, 650, 450, 675)
Field.SetBorderColor "RGB", 0.7, 0.5, 0.3, 0
Field.TextSize = "12"
Field.DefaultValue = "Enter your full name here"

I can't find "AcroExch.App".
How can I convert it to c#?
best regards