如果值为null,则将字符串设置为空

NULL 例如,不是每个电话号码都有分机。这就是我目前的代码:

Structure CustomerContact
    Public _name As String
    Public _email As String
    Public _fax As String
    Public _phone1 As String
    Public _phone2 As String
    Public _phone3 As String
    Public _ext1 As String
    Public _ext2 As String
    Public _ext3 As String
    Public _type1 As String
    Public _type2 As String
    Public _type3 As String
End Structure

    Dim contactData As DataTable = CustomerDBFunctions.GetCustomerContacts(Customer)
    For Each row As DataRow In contactData.Rows
        If contacts.Count < 1 Then
            contacts.Add(New CustomerContact With {
                            ._name = row.Item("FullName").ToString() & " (" & row.Item("ContactType").ToString() & ")",
                            ._email = row.Item("Email").ToString(),
                            ._fax = row.Item("Fax").ToString(),
                            ._phone1 = row.Item("Phone").ToString(),
                            ._ext1 = row.Item("Extension").ToString(),
                            ._type1 = row.Item("PhoneType").ToString()})
        End If
    Next

现在,当数据库中的值为 因为它不能给字符串赋值。我想在出现空值的情况下,将变量的值设置为 "" 相反。我只是不知道该怎么编码。