c# – 为什么一个事件是空的？ (你调用的对象是空的)

我创建了一个委托事件(SetStatusEvent)来执行此操作.当我在EventHandler(usbforProcessExited)之外调用此事件时似乎很好但是当我从usbforProcessExited调用它时它会出错 –

object reference not set to an instance of an object

主要形式

public partial class main : Form
{
    b rsSet = new b();

    public main()
    {
        InitializeComponent();
        rsSet.SetStatusEvent += new RemoteS.SetStatus(updateStatus);
    }

    private void button1_Click(object sender,EventArgs e)
    {
        rsSet.FormatUSB();
    }

    private delegate void UpdateStatus(int i,string str,Color clr);

    private void SetStatus(int i,Color clr)
    {
        this.progressBar1.Value = i;
        this.lbl_status.ForeColor = clr;
        this.lbl_status.Text = str;
    }

    private void updateStatus(int i,String msg,Color color)
    {
        object[] p = GetInokerPara(i,msg,color);
        BeginInvoke(new UpdateStatus(SetStatus),p);
    }

    private object[] GetInokerPara(int progress,string msg,Color color)
    {
        object[] para = new object[3];
        para[0] = progress;
        para[1] = msg;
        para[2] = color;

        return para;
    }
}

b级

class b
{
    public delegate void SetStatus(int i,Color color);
    public event SetStatus SetStatusEvent;

    System.Diagnostics.Process usbfor = new System.Diagnostics.Process();

    public void FormatUSB()
    {

        usbfor.StartInfo.FileName = @"usbformat.bat";
        usbfor.EnableRaisingEvents = true;
        usbfor.Exited += new EventHandler(usbforProcessExited);
        usbfor.Start();
    }

    public void usbforProcessExited(object sender,EventArgs f)
    {
        SetStatusEvent(100,"DONE",Color.Green); //ERROR HERE! (object reference not set to an instance of an object
    }
}

问题出在哪儿？