C# – Redrawing during drag and drop

cdrag and dropnet

I would like trigger the drag target control to redraw itself (via either invalidate or refresh) during the DragEnter and DragLeave events. The code looks something like:

protected override void OnDragEnter (DragEventArgs drgargs)
{
  //-- Set a property that affects drawing of control, then redraw
  this.MyProperty = true;
  this.Refresh(); //-- Does nothing???      
}

protected override void OnDragLeave (EventArgs e)
{
  //-- Set a property that affects drawing of control, then redraw
  this.MyProperty = false;
  this.Refresh(); //-- Does nothing???      
}

It doesn't actually redraw the control, the OnPaint method is not called by the Refresh() within these events. Is there any way to do this? I must not be understanding something here.

UPDATE: the answer provided by jasonh doesn't actually work. When using Invalidate() or Invalidate(rect) the control does not actually update. This is being calls during a drag and drop action. Any other ideas? Can you trigger a redraw of a control during a drag and drop? Thanks!

UPDATE 2: I created a sample project and could not get this to not work. Sigh… I finally tracked it down to some code in the OnPaint that was causing the problem. So, this turned out to be more of me not understanding how the debugger worked (it never hit break points in the OnPaint…still don't know why). Invalidate(), Refresh() both work. JasonH gets the answer as it was ultimately correct and also showed how to invalidate just a portion of a control…I didn't know about that.

Thanks for all your help!

Best Answer

Call this.Invalidate() to get the Form/Control to redraw itself. If you know the specific region, then call one of the overloaded methods to specify what to invalidate. For example:

Rectangle toInvalidate = new Rectangle(drgargs.X - 50, drgargs.Y - 50, 50, 50);
this.Invalidate(toInvalidate);

That would invalidate an area 50 pixels around the area the drag target is.

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

Best Answer

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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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