I have a combobox that it is populado thus:

---------

cbTipo.Items.Add(new DictionaryEntry("Pessoa F¡sica", "F"));

cbTipo.Items.Add(new DictionaryEntry("Pessoa Jur¡dica", "J"));

cbTipo.Items.Add(new DictionaryEntry("Prospect", "P"));

cbTipo.Items.Add(new DictionaryEntry("Internacional", "I"));

---------

This textbox is bind with a field string of the Business Object that in the data base keeps value “F”, “J”, “P”, “I”.

When I select an option of the Combobox gives an error of cast (Unable you cast object of type “System.DBNull” to type “System.String”.)

You can help me?

Also, if the SelectedValue is wrong, then you might need to use the DataSource property of the ComboBox.  Instead of adding the items directly, create a DictionaryEntry[] and set it as the data source:

DictionaryEntry[] dataSource = new DictionaryEntry[] {
    new DictionaryEntry("Pessoa F¡sica", "F"),
    new DictionaryEntry("Pessoa Jur¡dica", "J"),
    new DictionaryEntry("Prospect", "P"),
    new DictionaryEntry("Internacional", "I") };
cbTipo.DataSource = dataSource;
cbTipo.DisplayMember = "Key";
cbTipo.ValueMember = "Value";

That should get you fixed.