private void showI()
{
 textBox1.Text=i.ToString();
}

   public partial class Form1 : Form
    {
        private UserControl1 uc1 = null;
        private int i = 0;

        public Form1()
        {
            InitializeComponent();
            //初始化用户控件
            uc1 = new UserControl1();
            //监听按钮单击引发的事件
            uc1.AddEvent += new AddEventHandler(uc1_AddEvent);
            uc1.MinusEvent += new MinusEventHandler(uc1_MinusEvent);
        }

        private void ShowI()
        {
            textBox1.Text = i.ToString();
        }

        void uc1_MinusEvent()
        {
            i--;
            ShowI();
        }

        void uc1_AddEvent()
        {
            i++;
            ShowI();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            groupBox1.Controls.Add(uc1);
        }
    }

//以下是UC

    //声明加1和减1委托
    public delegate void AddEventHandler();
    public delegate void MinusEventHandler();

    public partial class UserControl1 : UserControl
    {
        //声明加1和减1事件，即对应委托的实例
        public event AddEventHandler AddEvent = null;
        public event MinusEventHandler MinusEvent = null;

        public UserControl1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (AddEvent != null)
            {
                AddEvent();//触发加1事件
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            if (MinusEvent != null)
            {
                MinusEvent();//触发减1事件
            }
        }
    }

------解决方案--------------------
2 楼说的可以

------解决方案--------------------
可不可以将i设为静态的,
public static int i;
public static bool str;
然后在页面放个timer,在click里写:
if(str == true)
{
   str == false;
   showI();
}
而在用户控件里只需:str = true;

------解决方案--------------------
设置自定义控件属性，赋值给属性
通过委托调用主页面方法，操作属性值
参考