public partial class Form3 : Form
    {
        MyClass myClass = new MyClass();
        public Form3()
        {
            InitializeComponent();
        }

        private void Form3_Activated(object sender, EventArgs e)
        {
            myClass.Add("汽车");
            myClass.Add("客车");
            this.comboBox1.Items.AddRange(myClass.MyType.ToArray());
        }
    }
    public class MyClass
    {
        private List<string> myType;
        public MyClass()
        {
            myType = new List<string>();
        }
        public List<string> MyType
        {
            get { return this.myType; }
            set { this.myType = value; }
        }
        public void Add(string name)
        {
            if (!myType.Contains(name))
            {
                myType.Add(name);
            }
        }
        public void Remove(string name)
        {
            if (myType.Contains(name))
            {
                myType.Remove(name);
            }
        }
    }