How to use string in VB.NET?

Error	1	'String' is a class type and cannot be used as an expression.	G:\SARA_Project\VMS_SmartCard.NET\SmartCardNET\SmartCardNET\frmSmartCardReader.vb	SmartCardNET

buf = string(256, " ")

<pre>     
        Dim ret As Long
        Dim buf As String
        Dim Add As String
        Dim ReaderPort1, ReaderPort2
        Dim fnum As Integer
        Dim buf1() As Byte
        Dim valueTemp As Integer
        valueTemp = 256

        ret = InitMyKADDLL()
        'Open Reader
        ReaderPort1 = "ACS ACR38USB 0"
        ReaderPort2 = "ACS ACR38USBSAM 0"
        ret = OpenMyKADReader(ReaderPort1, ReaderPort2)

        'Connect Card
        ret = ConnectMyKAD()

        If ret <> 0 Then

            ret = DisconnectMyKAD()
            ret = CloseMyKADReader()
            ret = FreeMyKADDLL()

            MsgBox("Please Insert My-Card before proceed!", vbCritical, "System Message")

            Exit Sub
        End If


        'Read Card ----------------------------------------------------
        'Name

    buf = string(256, " ")
        ret = HolderName(buf)
        If ret = 0 Then
            txtName.Text = Trim(buf)
        End If

Dim site As String = 256,""