generate barcode image vb.net 22: Build Your Own Windows Control in C#

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22: Build Your Own Windows Control
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PART IV
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Figure 22-7
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The StudentValidate control
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EXAM TIP Always use properties for your controls (instead of public variables) the Visual Studio NET designer program will only display properties, not variables In the next section, Exposing Properties of the Constituent Control, we will examine the code to not only add new properties but also to expose the existing properties of the constituent control remember, by default, they are hidden from the user of your control
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Once you have set all properties to your initial values, the next step is to decide what you will allow the user of your control to do Exposing Properties of the Constituent Control Let s examine how you can allow the user of your composite control to take advantage of the functionality of its constituent controls For this discussion, add a CheckBox control to the StudentValidate project, as shown in the following illustration
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This CheckBox should remain invisible until the student number has been validated since we don t want the user to have access to it until the validation is performed To accomplish this, you just need to create a public property that exposes the constituent control s Visible property:
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public bool cbValid_Visible { set { if (cbValidVisible == false) cbValidVisible = true; else cbValidVisible = false; } }
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On a request to set the Visible property of the CheckBox called cbValid, this simple code segment will reverse the current value Obviously, in a sophisticated application, the code would be more extensive For our purposes, though, we must expose it in this manner so the user of the StudentValidate control (who could be another developer) can set the Visible property This is what encapsulation is all about users never need to know how the Visible property is set; they just need to know that they can use it EXAM TIP In order to expose the properties of the constituent control, expose them as properties of the composite control
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You can also simply expose a constituent s property without any extra coding:
public Color txtName_BackColor {
22: Build Your Own Windows Control
get { return txtNameBackColor; } set { txtNameBackColor = value; } }
This is the easiest way to allow a user of the control access to a constituent property You can also add properties as we did for our extended control, NewButton Add the private variable declaration and then create the public get and set to access the private data:
private int var; public int NewVariable { get { return var; } set { var = value; } }
PART IV
EXAM TIP Remember that you can add in all the functionality that is required within the get and set code blocks This is the value of using properties instead of public variables Working with Methods and Events There is no practical difference between working with methods in any other class file and working with methods in a custom control Declare your method and provide the body of the method implementation:
public void ControlMethod { // implementation of the method goes here }
Events are a little different An event is the way your control interacts with the rest of the application There are two different ways of creating events: Expose a constituent event (remember, they are private by default) Create a custom event In order to expose a constituent event, the UserControl must raise the event Start by adding an event handler in the UserControl (for example, add a Click event to a button control)
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private void btnValidate_Click (object sender, SystemEventArgs e) { OnClick(e); }
The On before the Click in the method is used because all controls have a method that can be called to raise the event In effect, the OnClick() method raises the Click event from the UserControl In order to create a custom event, you can use delegates, which is the NET standard for using exposing and consuming events in Windows Forms For example, to create the event that should be triggered upon trying the student number too often in our StudentValidate control, you could use this code:
// declare the delegate public delegate void OutOfLuckEventHandler (object sender, OutOfLuckEventArgs e);
Then you could add the event declaration:
public event OutOfLuckEventHandler OutOfLuck;
And then you would raise the event by creating the On method:
public void OnOutOfLuck (OutOfLuckEventArgs e) { // invoke the delegate OutOfLuck (this, e); }
You are not done yet You must now allow the hosting application to use this event You need to add code within the Click event of your Validate button to check the number of times the button has been clicked, and then cause the event to be raised:
OnOutOfLuck (new OutOfLuckEventArgs ( <parameters>);
Finally, the hosting application would use the control s event to respond to the raising of the event:
private void myStudentValidate_OutOfLuck (object sender, OutOfLuckEventArgs e) { // write the code that responds to the 3 click attempts }
If you are still unsure as to how this would work, refer back to 19 to review the handling of events in Visual C# NET Testing the Composite Control Remember that controls are not stand-alone applications they must be added to a hosting application first and then tested from there Follow these steps to test your control: 1 Build your control Select Build | Build from the menus 2 Create a test project Select File | Add Project | New Project
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