Question

I am working on the following program which deals with threads and thread priorities.

I have constructed a windows form containing a rich textbox (DisplayTextBox) and two buttons (StartButton for running app and ExitButton for closing app).

In the form, I am creating several threads and running them one after another. The method used by each thread is found in the Threading class. The method in question is PrintOnScreen().

In this method, I am appending the thread name as well as its priority to str of type StringBuilder. Then I want to display the contents of str in DisplayTextBox (which is in Form.cs).

However, I am getting a “NullReferenceException was unhandled: Object reference not set to an instance of an object” error. The line where the error crops up is the following:

DisplayTextBox.Text = Convert.ToString(str);

Can you please help me to solve this error? Thanks.

Edit

Thanks to all for your help. In order to solve the problem, I copied the PrintOnScreen method to the Form.cs class and discarded Threading.cs.

I later used the code given by Anand and placed it below t2.Join(). Now it works like a charm.

Answer 1

The problem occurs in the constructor of your form. You declare DisplayText again as local member, so that the field of your form isn’t initialized. Change your constructor like this:

private void Form1_Load(object sender, EventArgs e)
{
    DescTextBox.Visible = false;
    DisplayTextBox = new RichTextBox();
    DisplayTextBox.Location = new Point(15, 31);
    DisplayTextBox.Height = 258;
    DisplayTextBox.Width = 303;
    panel1.Controls.Add(DisplayTextBox);
} 

and it should work like you expect.

Edit: But beware of the problems you will get, when you want to change UI elements from a background worker thread. In those cases you will have to use the invoke pattern.