C＃，后台工作者类

Question

when i compile this code i get his error, object reference set to null, and the error location is in Dowork, argumenttest.valueone = 8;

public partial class Form1 : Form
{
    BackgroundWorker bgw1 = new BackgroundWorker();
    public Form1()
    {
        InitializeComponent();
        // bgw1.RunWorkerAsync(test1);

        test test1 = new test
        {
            valueone = 5,
            valuetwo = 10
        };
        bgw1.RunWorkerAsync(test1);
    }

    class test
    {

        public int valueone { get; set; }
        public int valuetwo { get; set; }
    }

    private void bgw1_DoWork(Object sender, DoWorkEventArgs e)
    {
        test argumenttest = e.Argument as test;
        Thread.Sleep(10);

        argumenttest.valueone = 8;
        argumenttest.valuetwo = 10;

        e.Result = argumenttest;
    }

    private void bgw1_RunWorkerCompleted(Object sender, RunWorkerCompletedEventArgs e)
    {
        test test12 = e.Result as test;
        button1.Text = test12.valueone.ToString();// +test.valuetwo.ToString();
        //this.Text = test.valueone.ToString() + " "+ test.valuetwo.ToString();
    }

    private void button1_Click(object sender, EventArgs e)
    {

        bgw1.DoWork += bgw1_DoWork;
        bgw1.RunWorkerCompleted += bgw1_RunWorkerCompleted;
        //bgw1.RunWorkerAsync(test);

    }
}

Answer 1

There are two possible ways argumenttest ends up as null :

1. argumenttest was sent as null into the RunWorkerAsync .

2. e.Argument as test; e.Argument is something not compliant with test , and the as operator makes it null .

It hard to see which one, since your code example above is quite messed up.

EDIT

Can you confirm that your code is exactly as decyclone edited it? In that case, it looks fine, and should have worked as far as I can see.

Set a breakpoint on the first line of the DoWork method, and when debugging you should be easily able to see if 1. or 2. is the problem by hovering over e.Argument .

Answer 2

One problem that I see is that you don't set the event handler before you run the worker, so these 2 lines

    bgw1.DoWork += bgw1_DoWork;
    bgw1.RunWorkerCompleted += bgw1_RunWorkerCompleted;

have to be called before

    bgw1.RunWorkerAsync(test1);

Answer 3

You should subscribe to DoWork and RunCompleted in Form load or before you make a call to RunWorkerAsync.

bgw1.DoWork += bgw1_DoWork;
bgw1.RunWorkerCompleted += bgw1_RunWorkerCompleted;

move the above lines to Form_Load from Button Click event handler.

And move bgw1.RunWorkerAsync(test1); to button click handler from Form Load method.