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.

I have a combobox that it is populado thus:

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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?