DataBInding question

Question

when i binding the property "Text" of TextBox1 on OjbectTest's property "Value", why when i change the property "Value",the property "Text" of the textBox not changeed? thanks

the code same as
/*the bindinged class
public class OjbectTest
{
public ObjectTest()
{}
private string __value;
public string Value
{
get{return __value;}
set{__value = value;}
}
}

/*in a winFrom*/
ObjectTest obj = new ObjectTest();
this.textBox1.DataBindings.Add("Text", obj, "Value");