ComboBox Databinding problem

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C#

i have been searching alot with no answer
i have acombobox that have a datasource and let it be table A (table A is a table in a datacontext dx)
i want to save selected value of this combo in field B in table C
that is done by binding selected value of this combo to field B in CbindingSource
and it is saved to the database through datacontext.submitChanges()
but the problem that when i open the form again the selectedvalue cant be retrieved and the dx.combobox selects the first element
how i can handle this issue? should i program it manually?

let me explain i hope that will clarify more
Table A has two attributes ID And Value
combobox.DataSource=dx.A;
combobox.DisplayMember=Value;
combobox.ValueMember=ID
combobox.SelectedValue is bound in design to cbindingSource.B;
Knowing That B Field is the same type as ValueMember
when i submit changes ID value will be stored in database in Field B through the binding
but i dont know how to make the reverse operation which is :
when running the program how to let the selected item in combobox be the
one that has ID Value that is stored in field B (maybe that is through
combobox.SelectedValueChange event )