Automatically generate AlphaNumeric Unique number in code behind VB.Net

Hi Experts, The code below generates Customer ID automatically:

Private Sub autogenerated()
        Dim connStr As String = ConfigurationManager.ConnectionStrings("SY_InventoryConnectionString").ConnectionString
        Dim con As New SqlConnection(connStr)
        con.Open()
        Try
            Dim cmd As New SqlCommand("SELECT COUNT(CustID) as Tot FROM Customer", con)

            Dim dr As SqlDataReader

            dr = cmd.ExecuteReader()
            While dr.Read()
                Dim i As Integer = Convert.ToInt32(dr("tot"))

                If i > 0 Then
                    Dim j As Integer = i + 1

                    txtcustid.Text = "C0" + j.ToString()
                Else
                    txtcustid.Text = "C01"

                End If
            End While
            con.Close()
        Catch ex As Exception
        End Try

 End Sub

The problem is, when i delete a row that is before the last inserted row i get the following error: Violation of PRIMARY KEY constraint 'PK_Customer'. Cannot insert duplicate key in object 'dbo.Customer'. The statement has been terminated. Note: Customer ID data type is Varchar(50)

Thanks in advance.