Hi Arnold!

>If the controlsource is an expression.The column was became readonly and can't write to it's controlsource - table.myfield
>Any idea ?

Other idea is create other field in your cursor (type Logical) like above and use it in your grid, e.g.:

* Supose that your get your datas with SQL Statement and save them in Cursor.
Select *, Iif(FieldChar == "X", .T., .F.) as NewField From MyTable

* In CheckBox's Valid metod put this
If this.Value
   Replace FieldChar With "X"
Else
   Replace FieldChar With ""
EndIf

I hope this help you.
Bye!