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Posted 14 Nov 2011

Avoid redundant calling of container events when adding controls at run-time

, 22 Nov 2011
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Suggestions for adding controls in code
Consider that you have a scenario like this:
  1. You have a primary (or Main) Form in a WinForms Project, 'Form1.'

  2. In this primary Form, you have a Panel, 'Panel1,' meant to serve as a container for Controls to be added at run-time (UserControls, WinForms Controls, whatever).

  3. You wish to set the Dock property of those to-be-added at run-time Controls in some way so that, as they are added, you take advantage of automatic positioning (and you are willing to sacrifice using the Margin property, since setting the Dock property will ignore any Margin setting).

To illustrate: let's use the following code example where two different types of Panels are added in response to a Button Click Event in a Form:
// 'InnerPanel here is a UserControl
// 'LeftInnerPanel is a derived UserControl that inherits from 'InnerPanel
// 'panel1 here is a Panel created at design-time on the Form with AutoScroll = true.
// the container for all run-time added controls of type InnerPanel, LeftInnerPanel
// form scoped variable
private InnerPanel newIP;
// form scoped variable
private int pCount = 0;
// form scoped method, 'Click EventHandler for a Button, 'button1
private void button1_Click(object sender, EventArgs e)
   // use a variation of the InnerPanel control for left-most UserControl
   newIP = (0 == pCount++) ? new LeftInnerPanel() : new InnerPanel();
   newIP.Dock = DockStyle.Left;

If you set the 'Dock property of the control to be added before adding it to its container Control, the 'ReSize, 'ClientSizeChanged, and 'SizeChanged' events of the container Control will not be invoked.

If you set the 'Dock property after adding the Control to its container Control, those same Events will be invoked twice, as each Control is added.

By this time, I hope you are asking the question: "what's the practical value of this?:"

The way I use it is to watch for the change in the Container control when the number of Controls added suddenly exceed the container Control's boundaries, and Scrollbars are automatically turned on, at which point: in an EventHandler for one of those Size-changed type Events:

I do some specific stuff related to visual presentation in the UI to simulate how the added Controls would appear if the 'Margin property wasn't 'wiped out' by the fact the Dock property is set. Very specifically, I adjust the height of the container Panel so the 'bottoms' of the inserted UserControls are not 'clipped.'

Other value ?: perhaps just avoiding firing these various size-related Events when you don't need or want to?

Why are the described Events fired twice if you set the Dock property after adding the Control to its container Control: Ask Microsoft :)


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


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Comments and Discussions

GeneralThanks for your feedback, Johannes, in this scenario the num... Pin
BillWoodruff22-Nov-11 12:28
memberBillWoodruff22-Nov-11 12:28 
GeneralInteresting. I just want to mention that if you want to avoi... Pin
johannesnestler22-Nov-11 7:20
memberjohannesnestler22-Nov-11 7:20 

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