Button2.Text = UserControlTest1.Txt;//从用户控件里面取给页面

namespace WebAppControlTest
{
    public partial class UserControlTest : System.Web.UI.UserControl
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            TextBox1.Text = "Hello World!";
        }


        public string Txt
        {
            get
            {
                return TextBox1.Text;
            }
            set
            {
                TextBox1.Text = value;
            }
        }

    }
}

    public partial class WebForm1 : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            //去用户控件找文本框
            TextBox text = UserControlTest1.FindControl("TextBox1") as TextBox;
            //赋值给用户控件的文本框
            text.Text = "GoodBye World!";
        }

        protected void Button2_Click(object sender, EventArgs e)
        {
            Button2.Text = UserControlTest1.Txt;//从用户控件里面取给页面
        }
    }