Walkthrough: Author a Composite Control with C#

Composite controls provide a means by which custom graphical interfaces can be created and reused. A composite control is essentially a component with a visual representation. As such, it might consist of one or more Windows Forms controls, components, or blocks of code that can extend functionality by validating user input, modifying display properties, or performing other tasks required by the author. Composite controls can be placed on Windows Forms in the same manner as other controls. In the first part of this walkthrough, you create a simple composite control called ctlClock. In the second part of the walkthrough, you extend the functionality of ctlClock through inheritance.

Create the Project

When you create a new project, you specify its name to set the root namespace, assembly name, and project name, and ensure that the default component will be in the correct namespace.

To create the ctlClockLib control library and the ctlClock control

  1. In Visual Studio, create a new Windows Forms Control Library project, and name it ctlClockLib.

    The project name, ctlClockLib, is also assigned to the root namespace by default. The root namespace is used to qualify the names of components in the assembly. For example, if two assemblies provide components named ctlClock, you can specify your ctlClock component using ctlClockLib.ctlClock.

  2. In Solution Explorer, right-click UserControl1.cs, and then click Rename. Change the file name to ctlClock.cs. Click the Yes button when you are asked if you want to rename all references to the code element "UserControl1".


    By default, a composite control inherits from the UserControl class provided by the system. The UserControl class provides functionality required by all composite controls, and implements standard methods and properties.

  3. On the File menu, click Save All to save the project.

Add Windows Controls and Components to the Composite Control

A visual interface is an essential part of your composite control. This visual interface is implemented by the addition of one or more Windows controls to the designer surface. In the following demonstration, you will incorporate Windows controls into your composite control and write code to implement functionality.

To add a Label and a Timer to your composite control

  1. In Solution Explorer, right-click ctlClock.cs, and then click View Designer.

  2. In the Toolbox, expand the Common Controls node, and then double-click Label.

    A Label control named label1 is added to your control on the designer surface.

  3. In the designer, click label1. In the Properties window, set the following properties.

    Property Change to
    Name lblDisplay
    Text (blank space)
    TextAlign MiddleCenter
    Font.Size 14
  4. In the Toolbox, expand the Components node, and then double-click Timer.

    Because a Timer is a component, it has no visual representation at run time. Therefore, it does not appear with the controls on the designer surface, but rather in the Component Designer (a tray at the bottom of the designer surface).

  5. In the Component Designer, click timer1, and then set the Interval property to 1000 and the Enabled property to true.

    The Interval property controls the frequency with which the Timer component ticks. Each time timer1 ticks, it runs the code in the timer1_Tick event. The interval represents the number of milliseconds between ticks.

  6. In the Component Designer, double-click timer1 to go to the timer1_Tick event for ctlClock.

  7. Modify the code so that it resembles the following code sample. Be sure to change the access modifier from private to protected.

    protected void timer1_Tick(object sender, System.EventArgs e)
        // Causes the label to display the current time.
        lblDisplay.Text = DateTime.Now.ToLongTimeString();

    This code will cause the current time to be shown in lblDisplay. Because the interval of timer1 was set to 1000, this event will occur every thousand milliseconds, thus updating the current time every second.

  8. Modify the method to be overridable with the virtual keyword. For more information, see the "Inheriting from a User Control" section below.

    protected virtual void timer1_Tick(object sender, System.EventArgs e)
  9. On the File menu, click Save All to save the project.

Add Properties to the Composite Control

Your clock control now encapsulates a Label control and a Timer component, each with its own set of inherent properties. While the individual properties of these controls will not be accessible to subsequent users of your control, you can create and expose custom properties by writing the appropriate blocks of code. In the following procedure, you will add properties to your control that enable the user to change the color of the background and text.

To add a property to your composite control

  1. In Solution Explorer, right-click ctlClock.cs, and then click View Code.

    The Code Editor for your control opens.

  2. Locate the public partial class ctlClock statement. Beneath the opening brace ({), type the following code.

    private Color colFColor;
    private Color colBColor;

    These statements create the private variables that you will use to store the values for the properties you are about to create.

  3. Enter or paste the following code beneath the variable declarations from step 2.

    // Declares the name and type of the property.
    public Color ClockBackColor
        // Retrieves the value of the private variable colBColor.
            return colBColor;
        // Stores the selected value in the private variable colBColor, and
        // updates the background color of the label control lblDisplay.
            colBColor = value;
            lblDisplay.BackColor = colBColor;
    // Provides a similar set of instructions for the foreground color.
    public Color ClockForeColor
            return colFColor;
            colFColor = value;
            lblDisplay.ForeColor = colFColor;

    The preceding code makes two custom properties, ClockForeColor and ClockBackColor, available to subsequent users of this control. The get and set statements provide for storage and retrieval of the property value, as well as code to implement functionality appropriate to the property.

  4. On the File menu, click Save All to save the project.

Test the Control

Controls are not stand-alone applications; they must be hosted in a container. Test your control's run-time behavior and exercise its properties with the UserControl Test Container. For more information, see How to: Test the Run-Time Behavior of a UserControl.

To test your control

  1. Press F5 to build the project and run your control in the UserControl Test Container.

  2. In the test container's property grid, locate the ClockBackColor property, and then select the property to display the color palette.

  3. Choose a color by clicking it.

    The background color of your control changes to the color you selected.

  4. Use a similar sequence of events to verify that the ClockForeColor property is functioning as expected.

    In this section and the preceding sections, you have seen how components and Windows controls can be combined with code and packaging to provide custom functionality in the form of a composite control. You have learned to expose properties in your composite control, and how to test your control after it is complete. In the next section you will learn how to construct an inherited composite control using ctlClock as a base.

Inherit from a Composite Control

In the previous sections, you learned how to combine Windows controls, components, and code into reusable composite controls. Your composite control can now be used as a base upon which other controls can be built. The process of deriving a class from a base class is called inheritance. In this section, you will create a composite control called ctlAlarmClock. This control will be derived from its parent control, ctlClock. You will learn to extend the functionality of ctlClock by overriding parent methods and adding new methods and properties.

The first step in creating an inherited control is to derive it from its parent. This action creates a new control that has all of the properties, methods, and graphical characteristics of the parent control, but can also act as a base for the addition of new or modified functionality.

To create the inherited control

  1. In Solution Explorer, right-click ctlClockLib, point to Add, and then click User Control.

    The Add New Item dialog box opens.

  2. Select the Inherited User Control template.

  3. In the Name box, type ctlAlarmClock.cs, and then click Add.

    The Inheritance Picker dialog box appears.

  4. Under Component Name, double-click ctlClock.

  5. In Solution Explorer, browse through the current projects.


    A file called ctlAlarmClock.cs has been added to the current project.

Add the Alarm Properties

Properties are added to an inherited control in the same way they are added to a composite control. You will now use the property declaration syntax to add two properties to your control: AlarmTime, which will store the value of the date and time the alarm is to go off, and AlarmSet, which will indicate whether the alarm is set.

To add properties to your composite control

  1. In Solution Explorer, right-click ctlAlarmClock, and then click View Code.

  2. Locate the public class statement. Note that your control inherits from ctlClockLib.ctlClock. Beneath the opening brace ({) statement, type the following code.

    private DateTime dteAlarmTime;
    private bool blnAlarmSet;
    // These properties will be declared as public to allow future
    // developers to access them.
    public DateTime AlarmTime
            return dteAlarmTime;
            dteAlarmTime = value;
    public bool AlarmSet
            return blnAlarmSet;
            blnAlarmSet = value;

Add to the Graphical Interface of the Control

Your inherited control has a visual interface that is identical to the control it inherits from. It possesses the same constituent controls as its parent control, but the properties of the constituent controls will not be available unless they were specifically exposed. You may add to the graphical interface of an inherited composite control in the same manner as you would add to any composite control. To continue adding to your alarm clock's visual interface, you will add a label control that will flash when the alarm is sounding.

To add the label control

  1. In Solution Explorer, right-click ctlAlarmClock, and then click View Designer.

    The designer for ctlAlarmClock opens in the main window.

  2. Click the display portion of the control, and view the Properties window.


    While all the properties are displayed, they are dimmed. This indicates that these properties are native to lblDisplay and cannot be modified or accessed in the Properties window. By default, controls contained in a composite control are private, and their properties are not accessible by any means.


    If you want subsequent users of your composite control to have access to its internal controls, declare them as public or protected. This will allow you to set and modify properties of controls contained within your composite control by using the appropriate code.

  3. Add a Label control to your composite control.

  4. Using the mouse, drag the Label control immediately beneath the display box. In the Properties window, set the following properties.

    Property Setting
    Name lblAlarm
    Text Alarm!
    TextAlign MiddleCenter
    Visible false

Add the Alarm Functionality

In the previous procedures, you added properties and a control that will enable alarm functionality in your composite control. In this procedure, you will add code to compare the current time to the alarm time and, if they are the same, to flash an alarm. By overriding the timer1_Tick method of ctlClock and adding additional code to it, you will extend the capability of ctlAlarmClock while retaining all of the inherent functionality of ctlClock.

To override the timer1_Tick method of ctlClock

  1. In the Code Editor, locate the private bool blnAlarmSet; statement. Immediately beneath it, add the following statement.

    private bool blnColorTicker;
  2. In the Code Editor, locate the closing brace (}) at the end of the class. Just before the brace, add the following code.

    protected override void timer1_Tick(object sender, System.EventArgs e)
        // Calls the Timer1_Tick method of ctlClock.
        base.timer1_Tick(sender, e);
        // Checks to see if the alarm is set.
        if (AlarmSet == false)
            // If the date, hour, and minute of the alarm time are the same as
            // the current time, flash an alarm.
            if (AlarmTime.Date == DateTime.Now.Date && AlarmTime.Hour ==
                DateTime.Now.Hour && AlarmTime.Minute == DateTime.Now.Minute)
                // Sets lblAlarmVisible to true, and changes the background color based on
                // the value of blnColorTicker. The background color of the label
                // will flash once per tick of the clock.
                lblAlarm.Visible = true;
                if (blnColorTicker == false)
                    lblAlarm.BackColor = Color.Red;
                    blnColorTicker = true;
                    lblAlarm.BackColor = Color.Blue;
                    blnColorTicker = false;
                // Once the alarm has sounded for a minute, the label is made
                // invisible again.
                lblAlarm.Visible = false;

    The addition of this code accomplishes several tasks. The override statement directs the control to use this method in place of the method that was inherited from the base control. When this method is called, it calls the method it overrides by invoking the base.timer1_Tick statement, ensuring that all of the functionality incorporated in the original control is reproduced in this control. It then runs additional code to incorporate the alarm functionality. A flashing label control will appear when the alarm occurs.

    Your alarm clock control is almost complete. The only thing that remains is to implement a way to turn it off. To do this, you will add code to the lblAlarm_Click method.

To implement the shutoff method

  1. In Solution Explorer, right-click ctlAlarmClock.cs, and then click View Designer.

    The designer opens.

  2. Add a button to the control. Set the properties of the button as follows.

    Property Value
    Name btnAlarmOff
    Text Disable Alarm
  3. In the designer, double-click btnAlarmOff.

    The Code Editor opens to the private void btnAlarmOff_Click line.

  4. Modify this method so that it resembles the following code.

    private void btnAlarmOff_Click(object sender, System.EventArgs e)
        // Turns off the alarm.
        AlarmSet = false;
        // Hides the flashing label.
        lblAlarm.Visible = false;
  5. On the File menu, click Save All to save the project.

Use the Inherited Control on a Form

You can test your inherited control the same way you tested the base class control, ctlClock: Press F5 to build the project and run your control in the UserControl Test Container. For more information, see How to: Test the Run-Time Behavior of a UserControl.

To put your control to use, you will need to host it on a form. As with a standard composite control, an inherited composite control cannot stand alone and must be hosted in a form or other container. Since ctlAlarmClock has a greater depth of functionality, additional code is required to test it. In this procedure, you will write a simple program to test the functionality of ctlAlarmClock. You will write code to set and display the AlarmTime property of ctlAlarmClock, and will test its inherent functions.

To build and add your control to a test form

  1. In Solution Explorer, right-click ctlClockLib, and then click Build.

  2. Add a new Windows Forms Application project to the solution, and name it Test.

  3. In Solution Explorer, right-click the References node for your test project. Click Add Reference to display the Add Reference dialog box. Click the tab labeled Projects. Your ctlClockLib project will be listed under Project Name. Double-click the project to add the reference to the test project.

  4. In Solution Explorer, right-click Test, and then click Build.

  5. In the Toolbox, expand the ctlClockLib Components node.

  6. Double-click ctlAlarmClock to add a copy of ctlAlarmClock to your form.

  7. In the Toolbox, locate and double-click DateTimePicker to add a DateTimePicker control to your form, and then add a Label control by double-clicking Label.

  8. Use the mouse to position the controls in a convenient place on the form.

  9. Set the properties of these controls in the following manner.

    Control Property Value
    label1 Text (blank space)
    Name lblTest
    dateTimePicker1 Name dtpTest
    Format Time
  10. In the designer, double-click dtpTest.

    The Code Editor opens to private void dtpTest_ValueChanged.

  11. Modify the code so that it resembles the following.

    private void dtpTest_ValueChanged(object sender, System.EventArgs e)
        ctlAlarmClock1.AlarmTime = dtpTest.Value;
        ctlAlarmClock1.AlarmSet = true;
        lblTest.Text = "Alarm Time is " +
  12. In Solution Explorer, right-click Test, and then click Set as StartUp Project.

  13. On the Debug menu, click Start Debugging.

    The test program starts. Note that the current time is updated in the ctlAlarmClock control, and that the starting time is shown in the DateTimePicker control.

  14. Click the DateTimePicker where the minutes of the hour are displayed.

  15. Using the keyboard, set a value for minutes that is one minute greater than the current time shown by ctlAlarmClock.

    The time for the alarm setting is shown in lblTest. Wait for the displayed time to reach the alarm setting time. When the displayed time reaches the time to which the alarm is set, the lblAlarm will flash.

  16. Turn off the alarm by clicking btnAlarmOff. You may now reset the alarm.

This article has covered a number of key concepts. You have learned to create a composite control by combining controls and components into a composite control container. You have learned to add properties to your control, and to write code to implement custom functionality. In the last section, you learned to extend the functionality of a given composite control through inheritance, and to alter the functionality of host methods by overriding those methods.

See also