Principal.cs

using System.Windows.Forms;

// OrtizOL
namespace R709AutocompletadoTextBoxComboBox
{
    public partial class Principal : Form
    {
        public Principal()
        {
            InitializeComponent();

            Inicializar();
        }

        private void Inicializar()
        {
            cbxAutocompletado1.AutoCompleteCustomSource.AddRange(
                new string[]
                {
                    "Avión", "Ave", "Atrás", "Angela", "Atila", "Ala",
                    "Azúcar", "Argelia", "Antes", "Azteca", "Asociación",
                    "Argentina", "Antonio", "Antónimo", "Arpa", "Alexander"
                });
            cbxAutocompletado1.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
            cbxAutocompletado1.AutoCompleteSource = AutoCompleteSource.CustomSource;


            cbxAutocompletado2.Items.AddRange(
                new string[]
                {
                    "Avión", "Ave", "Atrás", "Angela", "Atila", "Ala",
                    "Azúcar", "Argelia", "Antes", "Azteca", "Asociación",
                    "Argentina", "Antonio", "Antónimo", "Arpa", "Alexander"
                });
            cbxAutocompletado2.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
            cbxAutocompletado2.AutoCompleteSource = AutoCompleteSource.ListItems;


            cbxAutocompletado3.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
            cbxAutocompletado3.AutoCompleteSource = AutoCompleteSource.AllUrl;
        }
    }
}