Noobie C# question; PONG

I'm trying to program PONG from scratch, by  myself, just to learn how. I have experience in VB, and I was told to program PONG back then to learn how, so  I'm now trying it with C#.

I have a form, with a label (lblBall) which I'm trying to get to move around the screen. My first step was to use a Button click to move the ball (lblBall.Left = lblBall.Left +10). That worked fine.

Now I'm trying to use a timer and loop to keep the ball moving for a few seconds.

I get an error:  Error 1 An object reference is required for the non-static field, method, or property 'WindowsFormsApplication8.Form1.lblBall' c:\users\documents\visual studio 2010\Projects\WindowsFormsApplication8\WindowsFormsApplication8\Form1.cs 42 17 WindowsFormsApplication8

I understand the error (in English), but I don't know how to properly "reference" the "object".

So I guess my question is;

"What is the Syntax for referencing an object?" (obviously I am not a professionl OOP programmer)

All answers to the question are greatly appreciated.

Thanks!

Here's the code, just in case anyone wants to see it.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Timers;

namespace

WindowsFormsApplication8
{
  public partial classForm1 : Form
    {
      private static System.Timers.Timer aTimer;
      public Form1()
        {
            InitializeComponent();
        }

   private void Form1_Load(object sender, EventArgs e)
        {
        }

      privatevoid button1_Click_1(object sender, EventArgs e)
        {
                aTimer = new System.Timers.Timer(1000);
                aTimer.Elapsed += newElapsedEventHandler(OnTimedEvent);
                aTimer.Interval = 500;
                aTimer.Enabled = true;
        }

      private static void OnTimedEvent( object source, ElapsedEventArgs e)
        {
          for (int i = 5; i > 0; i = i - 1)
            {
                lblBall.Left = lblBall.Left + 10;
                lblBall.Top = lblBall.Top + 10;
            }
        }
    }
}

Do I need to make a cross-thread call?

Can I do away with Static? (tried that, I get the error about unsafe cross-thread calls)...

OK, to start with, let's clarify the difference between the reference and the object instance.

lblBall is a reference; it 'points to' an instance of the object. It's natural to think of lblBall as being the object, but it isn't, it's just a note telling you where the object really is. I think of it like a desktop shortcut to a program :  the shortcut isn't actually the EXE file, it just tells us there the file is and looks like the file.

You could do the following code:

Label lblOther = lblBall;
lblOther.Text = "This changes the label text for both lblOther and lblBall";

Here we've created another reference to the label object but we still only have one instance of the label object.  By changing the text property on lblOther, we're also changing it on lblBall because they're both pointing to the same instance of the label object.

Now, if we just did the code:

Label lblDuff;

Then lblDuff is an undefined reference -- we haven't told it what to point to, it's a pointer with nothing to point at.  If we try to use this reference, the software will crash because you're effectively telling it to use something that doesn't exist.

Not sure if this is clear, because I would normally explain this with diagrams.

Now to your error : I would put a breakpoint in line 47, which I'm guessing is where you add 10 to the lblBall.Left parameter.  If you inspected the lblBall variable (by hovering the mouse over it in Visual Studio) you'll probably see it's set to 'null'.  This means it's not pointing to anything.

My usual tactic would be to put a guard conditional around this to make it not crash:

privatestaticvoid OnTimedEvent( object source, ElapsedEventArgs e)
{
    if(lblBall != null)
    {
        for (int i = 5; i > 0; i = i - 1)
        {
            lblBall.Left = lblBall.Left + 10;
            lblBall.Top = lblBall.Top + 10;
        }
    }
}

Then I would start debugging to see why lblBall was being set to null.

If the issue was cross-thread calls, then I think you would get a threading exception, not a reference exception.

Herbie

From a quick glance, I don't think you should be marking anything as static there.

For the cross-thread error, I'm assuming it's "Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on"

Objects that are rendered visually on the form are on GUI threads. The timer method OnTimedEvent runs on a different thread. You cannot access GUI objects directly from separate threads.

You need to "invoke" the method you want on that control.

• Don't use static in this case
• Use the correct timer. It's in the System.Windows.Forms namespace
• The loop is useless

Thank you all! This makes a lot of sense. So I'm doing several things wrong. First of all, the OBJECT is the actual label on the FORM? When I go to the Toolbox Common Controls and double-click Label, it creates an OBJECT? And then the code 'references' it? Am I understanding that correctly? Second; I can't affect the LABEL from the TIMER unless I 'Invoke' it? Do I do that using the LABEL's control InvokeRequired property? And Third; I should use a different TIMER? Instead of using System.Timers; I should be using a timer in System.Windows.Forms;? And finally, I should completely drop the 'static' from everything? Again, thank you all! I'm sure I'll have more Stupid questions soon

When you have Label blah = new Label(); the new is what creates the object. The = creates a reference to the newly created object. So, blah will then be a reference to the newly created object.

Dragging stuff from the Toolbox just automatically creates a bunch of code, namely the Label blah = new Label() and other code statements which simply makes controls you add children of other controls (e.g. Form1.Children.Add(blah)) . If you browse around the files in your project, you should be able to find where Visual Studio automatically generated code.

Well, you either invoke it as I suggested in my post OR you use the System.Windows.Forms.Timer which should handle stuff like calling methods on objects in the UI thread. Your choice (but Zippy's suggestion is better...you should use the other timer).

Don't use static. You're referring to objects that are not static.

And no, this is by no means a "noob stupid" question. This trips up a lot of people.

Thanks. I'll keep trying.

I also suggest you drop the "noob-stupid" prefix with your posts...unless you want everyone to consistently keep that in the back of their minds as they reply to you, you should just ask your question. It's not uncommon for members here to sometimes say/do/ask something stupid/noobish. Don't be afraid!

What everyone else said.

Rather than the timer you're using, drag one onto the form from the toolbox (it's under the Components category), and handle its "Tick" event. That should keep everything on the same thread so you won't need to do any Invoke-ing.

Thank you! The Components\Timer is just like the Timer from VB6 that I'm familiar with. My new code:

private void button1_Click()
{
if (timer1.Enabled == true)
   timer1.Enabled = false;
else
   timer1.Enabled = true;
}

private void timer1_Tick()
{
lblBall.Left = lblBall.Left + 2;
}

 note: Sent via CellPhone MotoQ9C

Next question...

I have a toggle button that gets the ball to start moving. Now I want it to "bounce" around inside the form borders.

I tried if (lblBall.Left > Form1.Size.Width) but it says

 "Error 1 An object reference is required for the non-static field, method, or property 'System.Windows.Forms.Form.Size.get' C:\Users\BigDaddy\Documents\Visual Studio 2010\Projects\WindowsFormsApplication8\WindowsFormsApplication8\Form1.cs 35 31 WindowsFormsApplication8 "

Does that mean I need to add the line Form Form1 = new Form to create the reference?

if (lblBall.Right > this.Width)

...although you should be using this.ClientSize and this.ClientRectangle instead of this.Width etc since the controls on the form will be constrained to within the form's client rectangle.

EDIT: Whoops, should have refreshed before I replied.

Thanks! So far, so good.

New question #1; is there a way to use an IF statement with multiple requirements without nesting the conditionals? What I mean is, is there an IF (check this) AND (check that) statement?

 if (lblBall.Left < ClientSize.Width) AND (lblBall.Left > 0)

New question #2;  I'm using the bool DIRECTION to control which way the ball moves. True moves right and false moves left.

The problem is that I had to define the bool at the start of my Timer_tick, which is obviously wrong. In VB6 I'd just use a Global variable Dim Direction as Boolean or something like that. Anyways, how (or where) should I declare the boolean for direction?

 private void timer1_Tick(object sender, EventArgs e)
        {       
            bool direction = true;
           
            //direction TRUE moves the ball to the right
            
            
            if (lblBall.Left > ClientSize.Width)
                direction = false;

            if (direction == true)
            lblBall.Left = lblBall.Left + 10;
            else
            lblBall.Left = lblBall.Left - 10;            
        }

The code is almost correct. The ball moves to the right, hits the boundary ClientSize.Width, bounces back, but immediately goes to the right again, since direction is reset to true on every tick.

I tried making the bool public like this

 public void Form1_Load(object sender, EventArgs e)
        {
            lblBall.Left = 100;
            bool direction = true;
        }

But I get the error: Warning 1 The variable 'direction' is assigned but its value is never used.

Thanks!

infinitum3d:

if (condition1) && (condition2) { //AND
}
if (condition1) || (condition2) { //OR
}
 

For the direction, you could either make a member variable or a property of the Form class. You define these outside of methods but inside of the class.

class Form1 {
private bool direction;

private void Form1_Load(object sender, EventArgs e){
direction = false;
}

...

}

If you really want to learn C#/VB.NET, you should be using an object-oriented design for this game, so you'd have a Ball class, a Paddle class, etc. and use the Form simply for displaying the state of those objects.

"If you really want to learn C#/VB.NET, you should be using an object-oriented design for this game, so you'd have a Ball class, a Paddle class, etc. and use the Form simply for displaying the state of those objects."

Thanks. I'll go that way now

@infinitum3d:

Conditionals:

IF( A is true )                 -->  if(a)
IF( A is false)                 --> if(!a)

IF (A is true and B is true) --> if(a && b)
IF (A is true and B is false) --> if(a && !b)
IF (either A or B are true) --> if(a || b) 

private void timer1_Tick(object sender, EventArgs e)
       {            
           if (lblBall.Left > ClientSize.Width)
           {
               // move LEFT
               lblBall.Left = lblBall.Left - 10;            
           }
           else
           {
              // move RIGHT
              lblBall.Left = lblBall.Left + 10;
           }
       }