How to inherit from TStringField

We would like to create our own field-objects and use them directly in the
field-editor or cast them manually in the code.
Example:
We have created TMyCompanyStringField which inherits from TStringField.

The following declaration doesn't work. We get this error-message:
'FieldForm1.Table1Company should be of type TStringField but is declared as
TMyCompanyStringField.
Correct the declaration?'

type
  TForm1 = class(TForm)
    Table1: TTable;
    DataSource1: TDataSource;
    Table1CustNo: TFloatField;
    Table1Company: TMyCompanyStringField;
    Table1Addr1: TStringField;
    Table1Addr2: TStringField;
    Table1City: TStringField;
    Table1State: TStringField;
    Table1Zip: TStringField;
    Table1Country: TStringField;
    Table1Phone: TStringField;
    Table1FAX: TStringField;
    Table1TaxRate: TFloatField;
    Table1Contact: TStringField;
    Table1LastInvoiceDate: TDateTimeField;
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

Thanks in advance Mikael and Fredrik.