1.定义ListItem类 

 public class ListItem

    {

        private string _key = string.Empty;

        private string _value = http://www.mamicode.com/string.Empty;

        public ListItem(string pKey, string pValue)

        {

            _key = pKey;

            _value = http://www.mamicode.com/pValue;

        }

        public override string ToString()

        {

            return this._value;

        }

        public string Key

        {

            get

            {

                return this._key;

            }

            set

            {

                this._key = value;

            }

        }

        public string Value

        {

            get

            {

                return this._value;

            }

            set

            {

                this._value = http://www.mamicode.com/value;

            }

        }

    }

2.使用 

ListItem listItem1 = new ListItem("1","中国");

ListItem listItem2 = new ListItem("1","美国");

ListItem listItem3 = new ListItem("1","英国");

comboBox1.Items.Add(listItem1);

comboBox1.Items.Add(listItem2);

comboBox1.Items.Add(listItem3);

3.取值 

string id = ((ListItem)comboBox1.SelectedItem).Key;

string value = http://www.mamicode.com/((ListItem)comboBox1.SelectedItem).Value;

4.默认选中项 
comboBox1.SelectedIndex = 0;//设置第一项为默认选择项 
comboBox1.SelectedItem = listItem1//设置指定的项为默认选择项 

c#(winform)中自定义ListItem类方便ComboBox添加Item项