private List<string> source = new List<string>()
    {
        "fsadfsadf",
        "fasdfa3rtwetwe",
        "twerkljdflkj",
        "jehrgjkwhe",
        "reiouhe;tg435234",
        "28495uytg4",
        "34y8-tgiy2",
        "43tguj54rjhng-289",
        "2-487g24gkl;j",
        "134f7rhfvsdjk;",
        "247vyhpufdsh"
    };

public MainForm()
{
    InitializeComponent();
            
    comboBox.DataSource = source;
    comboBox.DropDownStyle = ComboBoxStyle.DropDown;    
    comboBox.KeyUp += new KeyEventHandler(comboBox_KeyUp);
}

private void comboBox_KeyUp(object sender, KeyEventArgs e)
{
    string text = comboBox.Text;
    if (!string.IsNullOrEmpty(comboBox.Text) && source.Count(s => s.Contains(comboBox.Text)) > 0)
        comboBox.DataSource = source.Where(s => s.Contains(comboBox.Text)).ToList();
    if (string.IsNullOrEmpty(comboBox.Text))
        comboBox.DataSource = source;
    comboBox.Text = text;
}