Try:
Private Shared validData As String = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789 @.-_"
Private Shared Function IsValid(c As Char) As Boolean
Return validData.Contains(c)
End Function
Private Shared Function ContainsAllValid(s As String) As Boolean
For Each c As Char In s
If Not IsValid(c) Then
Return False
End If
Next
Return True
End Function