Manage Nullable Values in a Bound Combobox

Question

I have a BO called Agent and another called AgentList that is defined as:

Inherits ComponentModel.BindingList(Of Agent)  

Agent list contains a method called "CreateAgentList" that returns a AgenteList object populated from DB.

To populate a combobox and bind it I use the following code:

    cmbAgent.DataSource = AgenteList.CreateAgentList()
    cmbAgent.DisplayMember = "AgentCode"
    cmbAgent.ValueMember = "Id"
    cmbAgent.DataBindings.Add(New Binding("ValueMember", AgenteBindingSource, 
         "AgenteDiRiferimento", False, DataSourceUpdateMode.OnPropertyChanged))

How can I have a no-text first item in the combobox associated to Null value member?

N.B. The purpose is to let the user not chose any Agent. ReferenceAgent field in DB is a Nullable Integer FK to the Agent table. The bind is used to display the correct agent every time a user surf in the agents list and the program loads the data of the selected Agent in the form.

Answer 1

Besides than storing null values in the Agent field of the DB I just dropped the FK that wasn't mandatory being the input of the combobox already driven and not letting the user put wrong info and replacing the "null id" with 0 (my Id field is an identity that starts from 1, so there're no problems doing it).

The code to populate my combobox and to bind it during form loading is the following:

    Dim cmb_Items As New Dictionary(Of Integer, String)
    cmb_Items.Add(0, "")

    For Each item As Agent In AgentList.CreateAgentList()
        cmb_Items.Add(item.Id, item.AgentCode)
    Next

    cmbAgent.DataSource = New BindingSource(cmb_Items, Nothing)
    cmbAgent.ValueMember = "Key"
    cmbAgent.DisplayMember = "Value"
    cmbAgent.DataBindings.Add(New Binding("SelectedValue", AgentBindingSource, "Agent", False, DataSourceUpdateMode.OnPropertyChanged))

Not exactly the solution I had in mind in first place, but it works like a charm!