C# – How to prevent the ObjectDisposedException in C# when drawing and application exits

c++exception-handlinggraphics

I'm a CompSci student, and fairly new at C#, and I was doing a "Josephus Problem" program for a class, and I created an Exit button that calls Application.Exit() to exit at anytime, but if C# is still working on painting and the button is pressed it throws an ObjectDisposedExeception for the Graphics object. Is there any way to prevent this?. I was thinking of try{}catch or change a boolean to tell the painting process to stop before exiting, but I want to know if there's another solution.

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It shouldn't be possible for this to happen. If the button is created on the same thread as the window, they share a message pump and the Paint handler cannot be interrupted to handle the exit button. The message that the button has been clicked will be queued up on the thread's message queue until the Paint handler returns.

Generally, you should defer painting to the Paint handler (or override OnPaint) and everywhere else that you need to update the screen, call the control's Invalidate method. That tells Windows that an area needs repainting and, once all other messages have been dealt with, it will generate a WM_PAINT message which ultimately will call OnPaint, which in turn will fire the Paint event.

If animating, use a System.Windows.Forms.Timer to trigger each frame, rather than using a thread. System.Threading.Timer callbacks execute on the threadpool, so they're always on the wrong thread for manipulating the UI.

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – the difference between const and readonly in C#

Apart from the apparent difference of

having to declare the value at the time of a definition for a const VS readonly values can be computed dynamically but need to be assigned before the constructor exits.. after that it is frozen.
const's are implicitly static. You use a ClassName.ConstantName notation to access them.

There is a subtle difference. Consider a class defined in AssemblyA.

public class Const_V_Readonly
{
  public const int I_CONST_VALUE = 2;
  public readonly int I_RO_VALUE;
  public Const_V_Readonly()
  {
     I_RO_VALUE = 3;
  }
}

AssemblyB references AssemblyA and uses these values in code. When this is compiled:

in the case of the const value, it is like a find-replace. The value 2 is 'baked into' the AssemblyB's IL. This means that if tomorrow I update I_CONST_VALUE to 20, AssemblyB would still have 2 till I recompile it.
in the case of the readonly value, it is like a ref to a memory location. The value is not baked into AssemblyB's IL. This means that if the memory location is updated, AssemblyB gets the new value without recompilation. So if I_RO_VALUE is updated to 30, you only need to build AssemblyA and all clients do not need to be recompiled.

So if you are confident that the value of the constant won't change, use a const.

public const int CM_IN_A_METER = 100;

But if you have a constant that may change (e.g. w.r.t. precision).. or when in doubt, use a readonly.

public readonly float PI = 3.14;

Update: Aku needs to get a mention because he pointed this out first. Also I need to plug where I learned this: Effective C# - Bill Wagner

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This is the style that Microsoft tends to use in their examples.

C# – the difference between const and readonly in C#

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