little help needed in this problem vb 2010

For Each digit In (TextBox4.Text + TextBox5.Text + TextBox6.Text).ToCharArray()
     i += CInt(digit)

Private Function AddDigits(s As String) As Integer
    Dim result As Integer = 0
    For Each c As Char In s
        If [Char].IsDigit(c) Then
            result += AscW(c - "0"C)
        End If
    Next
    Return result
End Function

Dim str As String = "123324::34"
Dim res As Integer = 0
Do
    res = AddDigits(str)
    str = res.ToString()
Loop While res >= 10
myTextBox.Text = str

Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
    Dim Day As String = "1"
    Dim Month As String = "3"
    Dim Year As String = "1986"
    Dim Holder As Integer = 0
 
    For Each num As Char In (Day + Month + Year)
        Holder += Val(num)
    Next
 
    MessageBox.Show(AddString(Holder))
End Sub
 
Function AddString(number As String) As String
        Dim i As Integer = 0
 
        For Each num As Char In number
            i += Val(num)
        Next
 
        If i >= 10 Then
            Return AddString(i) 
        End If
 
        Return i.ToString()
End Function

'
' This is a Very Simple Solution applicable for any Number, It Based on Simple Maths
' It so Easy, No need to Work with Loops or Goto's
'
' Any Number (Mod) 9 = Sum Of Digits
'
' 12 Mod 9 = 3
' 21 Mod 9 = 3
' 28 Mod 9 = 1 (2+8 = 1+0 = 1)
' One Validation of Any Number / 9 = 0 then Sum of Digit is 9
' 27 Mod 9 = 0
' Hence Sum of Digit is 9
'
Function SumOfDigits(DOB As Long) As Integer
'One Line Calculation
SumOfDigits =  DOB Mod 9
 
'One more Validation
If SumOfDigits = 0 AND DOB <> 0 Then SumOfDigits = 9
 
'Return Result
Return SumOfDigits
End Function