c#中委托、事件中的Object reference not set to an instance of an object

[qaz2008]

代码如下：
public delegate void DClickHandler(object sender, DClickEventArgs ev);
public event DClickHandler DClick;
public delegate void UIChangedHandler(object sender, EventArgs e);
public event UIChangedHandler UIChanged;

private void AddStick(){
stick = new Stick();
stick.id = System.Math.Abs((int)DateTime.Now.Ticks);
stick.x = x;
stick.y = y;
stick.width = t_width;
stick.height = t_height;
StickSetFrm stickSetFrm = new StickSetFrm(stick);
stickSetFrm.ShowDialog();
if (stick.yes)
{
if (para.array==null)
{
para.array = new List<Stick>();
}
para.array.Add(stick);
}
UIChanged(this, new EventArgs());/////////出错的地方
}
为什么呐？

[zgke]

使用了为空的事件

if(UIChanged!=null)UIChanged(this, new EventArgs());

[qaz2008]

我加过，但加上这一句就就永远没有动作（画图形）输出！！
郁闷！！

[阿非]

你缺少一个赋值的过程

[qaz2008]

但是在，release状态下去会有图形

[qaz2008]

UIChanged(this, new EventArgs());
不是对它的赋值调用吗？

[阿非]

这句是调用，不是赋值。

赋值 是类似

ClassName.UIChanged += new ClassName.UIChangedHandler(MethodName);

于这样的一句话

[xhy_ym]

顶一下

[klint_khl1]

public delegate void DClickHandler(object sender, DClickEventArgs ev);
public delegate void UIChangedHandler(object sender, EventArgs e);

public class Draw
{
public event UIChangedHandler UIChanged;
public event DClickHandler DClick;
private void AddStick(){
stick = new Stick();
stick.id = System.Math.Abs((int)DateTime.Now.Ticks);
stick.x = x;
stick.y = y;
stick.width = t_width;
stick.height = t_height;
StickSetFrm stickSetFrm = new StickSetFrm(stick);
stickSetFrm.ShowDialog();
if (stick.yes)
{
if (para.array==null)
{
para.array = new List <Stick>();
}
para.array.Add(stick);
}
UIChanged+=new UIChangedHandler(DrawMap);
if(UIChanged!=null)
UIChanged(this, new EventArgs());/////////出错的地方
}

private void DrawMap(object sender, EventArgs e)
{//画图行
}
}

[wuyq11]

UIChanged+=new UIChangedHandler(DrawMap);

class Class1
{
static void Main(string[] args)
{
Student s1=new Student();
Student s2=new Student();
s1.RegisterOK +=new Student.DelegateRegisterOkEvent(Student_RegisterOK);
s2.RegisterOK +=new Student.DelegateRegisterOkEvent(Student_RegisterOK);
s1.Register();
s2.Register();
Console.ReadLine();
}
static void Student_RegisterOK()
{
Console.WriteLine("Hello");
}
}
class Student
{
public delegate void DelegateRegisterOkEvent();
public event DelegateRegisterOkEvent RegisterOK;
public string Name;
public void Register()
{
Console.WriteLine("Register");
RegisterOK();
}
}

[qaz2008]

谢谢！！我来改改代码！