Anti Injection Snippet

Within the code behind, where do we insert the following anti injection snippet:
Public Function strLong(ByVal pstrText As String) As String
        Dim strTemp As String = Nothing
        Dim lngI As Long = 0
        strTemp = ""
        For lngI = 1 To Strings.Len(pstrText)
            If Strings.InStr(1, "1234567890", Strings.Mid(pstrText, lngI, 1)) > 0 Then
                strTemp = strTemp & Strings.Mid(pstrText, lngI, 1)
            End If
        Next
        Return Strings.Left(strTemp, 20)
 &nbs ...