Try this:
public Form1()
{
InitializeComponent();
yourTextBox.AutoCompleteMode = AutoCompleteMode.Suggest;
yourTextBox.AutoCompleteSource = AutoCompleteSource.CustomSource;
AutoCompleteStringCollection source = new AutoCompleteStringCollection();
source.AddRange(new string[]
{
"Item1",
"Item2",
"Item3"
});
yourTextBox.AutoCompleteCustomSource = source;
}
Hope this helps.