Developing a Simple Windows Forms Control

Artículo
10/17/2014

This section walks you through the key steps for authoring a custom Windows Forms control. The simple control developed in this walkthrough allows the alignment of its Text property to be changed. It does not raise or handle events.

To create a simple custom control

Define a class that derives from System.Windows.Forms.Control.

Public Class FirstControl
   Inherits Control
   ...
End Class
[C#]
public class FirstControl:Control{...}

Define properties. (You are not required to define properties, because a control inherits many properties from the Control class, but most custom controls generally do define additional properties.) The following code fragment defines a property named TextAlignment thatFirstControluses to format the display of the Text property inherited from Control. For more information about defining properties, see Properties Overview.

' ContentAlignment is an enumeration defined in the System.Drawing
' namespace that specifies the alignment of content on a drawing 
' surface.
Private alignment As ContentAlignment = ContentAlignment.MiddleLeft

Public Property TextAlignment() As ContentAlignment
  Get
     Return alignment
  End Get
  Set
     alignment = value
     ' The Invalidate method invokes the OnPaint method described 
     ' in step 3.
     Invalidate()
  End Set
End Property
[C#]
// ContentAlignment is an enumeration defined in the System.Drawing
// namespace that specifies the alignment of content on a drawing 
// surface.
private ContentAlignment alignment = ContentAlignment.MiddleLeft;

public ContentAlignment TextAlignment {
   get {
      return alignment;
   }
   set {
      alignment = value;
      // The Invalidate method invokes the OnPaint method described 
      // in step 3.
      Invalidate(); 
   }
}

When you set a property that changes the visual display of the control, you must invoke the Invalidate method to redraw the control. Invalidate is defined in the base class Control.

Override the protected OnPaint method inherited from Control to provide rendering logic to your control. If you do not override OnPaint, your control will not be able to draw itself. In the following code fragment, the OnPaint method displays the Text property inherited from Control with a default alignment.

Public Class FirstControl
   Inherits Control

   Public Sub New()
      ...
   End Sub

   Protected Overrides Sub OnPaint(e As PaintEventArgs)
      MyBase.OnPaint(e)
      e.Graphics.DrawString(Text, Font, New SolidBrush(ForeColor), RectangleF.op_Implicit(ClientRectangle), style)
   End Sub
End Class
[C#]
public class FirstControl : Control{
   public FirstControl() {...}
   protected override void OnPaint(PaintEventArgs e) {
      base.OnPaint(e);
      e.Graphics.DrawString(Text, Font, new SolidBrush(ForeColor), ClientRectangle, style);
   } 
}

The preceding code displays text with the default alignment. The code example at the end of this topic shows how to change the alignment of Text to that specified by the TextAlignment property, defined in step 2 earlier in this section.

Provide attributes for your control. Attributes enable a visual designer to display your control and its properties and events appropriately at design time. The following code fragment applies attributes to the TextAlignment property. In a designer such as Microsoft Visual Studio .NET, the Category attribute (shown in the code fragment) causes the property to be displayed under a logical category. The Description attribute causes a descriptive string to be displayed at the bottom of the Properties window when the TextAlignment property is selected. For more information about attributes, see Design-Time Attributes for Components.
```
<Category("Alignment"), _
Description("Specifies the alignment of text.")> _  
Public Property TextAlignment() As ContentAlignment
   ...
End Class
[C#]
[
Category("Alignment"),
Description("Specifies the alignment of text.")
]
public ContentAlignment TextAlignment {...}
```
(optional) Provide resources for your control. You can provide a resource, such as a bitmap, for your control by using a compiler option (/res for C#) to package resources with your control. At run time, the resource can be retrieved using the methods of the System.Resources.ResourceManager class. For more information about creating and using resources, see the .NET Samples – How To: Resources QuickStart.
Compile and deploy your control. To compile and deploy FirstControl, execute the following steps:
1. Save the code in the following sample to a source file (such as FirstControl.cs or FirstControl.vb).
2. Compile the source code into an assembly and save it in your application's directory. To accomplish this, execute the following command from the directory that contains the source file.
  vbc /t:library /out:[path to your application's directory]/CustomWinControls.dll /r:System.dll /r:System.Windows.Forms.dll /r:System.Drawing.dll FirstControl.vb
  csc /t:library /out:[path to your application's directory]/CustomWinControls.dll /r:System.dll /r:System.Windows.Forms.dll /r:System.Drawing.dll FirstControl.cs
  
  The /t:library compiler option tells the compiler that the assembly you are creating is a library (and not an executable). The /out option specifies the path and name of the assembly. The /r option provides the name of the assemblies that are referenced by your code. In this example, you create a private assembly that only your applications can use. Hence, you have to save it in your application's directory. For more information about packaging and deploying a control for distribution, see Deploying .NET Framework Applications.

The following sample shows the code for FirstControl. The control is enclosed in the namespace CustomWinControls. A namespace provides a logical grouping of related types. You can create your control in a new or existing namespace. In C#, the using declaration (in Visual Basic, Imports) allows types to be accessed from a namespace without using the fully qualified name of the type. In the following example, the using declaration allows code to access the class Control from System.Windows.Forms as simply Control instead of having to use the fully qualified name System.Windows.Forms.Control.

Option Explicit
Option Strict

Imports System
Imports System.ComponentModel
Imports System.Windows.Forms
Imports System.Drawing

Namespace CustomWinControls
   Public Class FirstControl
      Inherits Control
      Private alignment As ContentAlignment = ContentAlignment.MiddleLeft

      <Category("Alignment"), _
      Description("Specifies the alignment of text.")> _
      Public Property TextAlignment() As ContentAlignment
         Get
            Return alignment
         End Get
         Set
            alignment = value
            ' The Invalidate method invokes the OnPaint method.
            Invalidate()
         End Set
      End Property
      
      ' OnPaint aligns text, as specified by the 
      ' TextAlignment property, by passing a parameter
      ' to the DrawString method of the System.Drawing.Graphics object.
      Protected Overrides Sub OnPaint(e As PaintEventArgs)
         MyBase.OnPaint(e)
         Dim style As New StringFormat()
         style.Alignment = StringAlignment.Near
         Select Case alignment
            Case ContentAlignment.MiddleLeft
               style.Alignment = StringAlignment.Near
            Case ContentAlignment.MiddleRight
               style.Alignment = StringAlignment.Far
            Case ContentAlignment.MiddleCenter
               style.Alignment = StringAlignment.Center
         End Select
         ' Call the DrawString method of the System.Drawing class to write   
         ' text. Text and ClientRectangle are properties inherited from
         ' Control.
         e.Graphics.DrawString(Text, Font, New SolidBrush(ForeColor), RectangleF.op_Implicit(ClientRectangle), style)
      End Sub
   End Class
End Namespace
[C#]
namespace CustomWinControls {
   using System;
   using System.ComponentModel;
   using System.Windows.Forms;
   using System.Drawing;
   public class FirstControl : Control {
      private ContentAlignment alignment = ContentAlignment.MiddleLeft;
      
      [
       Category("Alignment"),
       Description("Specifies the alignment of text.")
      ]
      public ContentAlignment TextAlignment {
         get {
            return alignment;
         }
         set {
            alignment = value;
            // The Invalidate method invokes the OnPaint method.
            Invalidate();
         }
      }

      // OnPaint aligns text, as specified by the 
      // TextAlignment property, by passing a parameter
      // to the DrawString method of the System.Drawing.Graphics object.
      protected override void OnPaint(PaintEventArgs e) {
         base.OnPaint(e);
         StringFormat style = new StringFormat();
         style.Alignment = StringAlignment.Near;
         switch (alignment) {
            case ContentAlignment.MiddleLeft:
               style.Alignment = StringAlignment.Near;
               break;
            case ContentAlignment.MiddleRight:
               style.Alignment = StringAlignment.Far;
               break;
            case ContentAlignment.MiddleCenter:
               style.Alignment = StringAlignment.Center;
               break;
         }
         // Call the DrawString method of the System.Drawing class to write   
         // text. Text and ClientRectangle are properties inherited from
         // Control.
         e.Graphics.DrawString(Text, Font, new SolidBrush(ForeColor), ClientRectangle, style);
      }
   }
}

Using the Custom Control on a Form

The following example shows a simple form that uses FirstControl. It creates three instances of FirstControl, each with a different value for the TextAlignment property.

To compile and run this sample

Save the code in the following example to a source file (SimpleForm.cs or SimpleForms.vb).
Compile the source code into an executable assembly by executing the following command from the directory that contains the source file.
vbc /r:CustomWinControls.dll /r:System.dll /r:System.Windows.Forms.dll /r:System.Drawing.dll SimpleForm.vb
csc /r:CustomWinControls.dll /r:System.dll /r:System.Windows.Forms.dll /r:System.Drawing.dll SimpleForm.cs

CustomWinControls.dll is the assembly that contains the class FirstControl. This assembly must be in the same directory as the source file for the form that accesses it (SimpleForm.cs or SimpleForms.vb).
Execute SimpleForm.exe using the following command.

SimpleForm

Option Explicit
Option Strict

Imports System
Imports System.Windows.Forms
Imports System.Drawing
Imports CustomWinControls

Class SimpleForm
   Inherits Form
   Private leftControl As FirstControl
   Private centerControl As FirstControl
   Private rightControl As FirstControl
   
   Protected Overloads Overrides Sub Dispose(disposing as Boolean)
      MyBase.Dispose(disposing)
   End Sub
   
   
   Public Sub New()
      leftControl = New FirstControl()
      With leftControl
         .Text = "Left"
         .Location = New Point(50, 50)
         .Size = New Size(50, 50)
      End With
      Controls.Add(leftControl)
      
      centerControl = New FirstControl()
      With centerControl
         .TextAlignment = ContentAlignment.MiddleCenter
         .Text = "Center"
         .Location = New Point(125, 50)
         .Size = New Size(50, 50)
      End With
      Controls.Add(centerControl)
      
      rightControl = New FirstControl()
      With rightControl
         .TextAlignment = ContentAlignment.MiddleRight
         .Text = "Right"
         .Location = New Point(200, 50)
         .Size = New Size(50, 50)
      End With
      Controls.Add(rightControl)
   End Sub

   <STAThread()> _
   Public Shared Sub Main()
      Dim myForm As New SimpleForm()
      myForm.Text = "Uses FirstControl"
      myForm.Size = New Size(400, 150)
      Application.Run(myForm)
   End Sub
End Class
[C#]
using System;
using System.Windows.Forms;
using System.Drawing;
using CustomWinControls;

class SimpleForm : Form {
   private FirstControl left;
   private FirstControl center;
   private FirstControl right;
   
   protected override void Dispose(bool disposing) {
      base.Dispose(disposing);
   }

   public SimpleForm() : base() {
      left = new FirstControl();
      left.Text = "Left";
      left.Location = new Point(50, 50);
      left.Size = new Size(50, 50);
      Controls.Add(left);
      
      center = new FirstControl();
      center.TextAlignment = ContentAlignment.MiddleCenter;
      center.Text = "Center";
      center.Location = new Point(125, 50);
      center.Size = new Size(50, 50);
      Controls.Add(center);
      
      right = new FirstControl();
      right.TextAlignment = ContentAlignment.MiddleRight;
      right.Text = "Right";
      right.Location = new Point(200, 50);
      right.Size = new Size(50, 50);
      Controls.Add(right);
   }
   
   [STAThread]
   public static void Main(string[] args) {
      Form form = new SimpleForm();
      form.Text = "Uses FirstControl";
      form.Size = new Size(400, 150);
      Application.Run(form);
   }
}

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Developing a Simple Windows Forms Control

Using the Custom Control on a Form

See Also

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