静态对象解除及关联事件的问题

C# code

public class Employee : System.ComponentModel.INotifyPropertyChanged{    private string _name;    public string Name    {        get{return _name;}        set{if(_name != value){        _name =value;        SendPropertyChanged("Name:"+value);        }}    }    public event System.ComponentModel.PropertyChangedEventHandler PropertyChanged;        protected virtual void SendPropertyChanged(String propertyName)        {            if ((this.PropertyChanged != null))            {                this.PropertyChanged(this, new System.ComponentModel.PropertyChangedEventArgs(propertyName));            }        }}public class ClassTest{        public ClassTest()    {              Gloal.Em.PropertyChanged -= Employee_PropertyChanged;           Gloal.Em.PropertyChanged += Employee_PropertyChanged;    }    private void Employee_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)    {        Console.WriteLine(e.PropertyName);    }}public class Gloal{   public static Employee Em;}static class Program{   [STAThread]   static void Main(string[] args)   {    Gloal.Em = new Employee{Name = "a"};    ClassTest t = new ClassTest();    Gloal.Em.Name = "b";        ClassTest t1 = new ClassTest();    Gloal.Em.Name = "c";        ClassTest t2 = new ClassTest();    ClassTest t12 = new ClassTest();    Gloal.Em.Name = "d";   }｝

输出结果：
Name:b
Name:c
Name:c
Name:d
Name:d
Name:d
Name:d

-= 为啥不起作用呢?

[解决办法]

C# code

public ClassTest()    {              Gloal.Em.PropertyChanged -= Employee_PropertyChanged;           Gloal.Em.PropertyChanged += Employee_PropertyChanged;    }    private void Employee_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)    {        Console.WriteLine(e.PropertyName);    }