ShouldSerialize and Reset Methods
ShouldSerialize and Reset are optional methods that you can provide for a property, if the property does not a have simple default value. If the property has a simple default value, you should apply the DefaultValueAttribute and supply the default value to the attribute class constructor instead. Either of these mechanisms enables the following features in the designer:
The property provides visual indication in the property browser if it has been modified from its default value.
The user can right-click on the property and choose Reset to restore the property to its default value.
The designer generates more efficient code.
Note Either apply the DefaultValueAttribute or provide ResetPropertyName and ShouldSerializePropertyName methods. Do not use both.
The ResetPropertyName method sets a property to its default value, as shown in the following code fragment.
Public Sub ResetMyFont()
MyFont = Nothing
End Sub
[C#]public void ResetMyFont() {
MyFont = null;
}
Note If a property does not have a Reset method, is not marked with a DefaultValueAttribute, and does not have a default value supplied in its declaration, the Reset option for that property is disabled in the context menu of the Properties window of the Windows Forms designer in Microsoft Visual Studio .NET.
Designers such as Visual Studio .NET use the ShouldSerializePropertyName method to check whether a property has changed from its default value and write code into the form only if a property is changed, thus allowing for more efficient code generation. For example:
'Returns true if the font has changed; otherwise, returns false.
' The designer writes code to the form only if true is returned.
Public Function ShouldSerializeMyFont() As Boolean
Return Not (thefont Is Nothing)
End Function
[C#]// Returns true if the font has changed; otherwise, returns false.
// The designer writes code to the form only if true is returned.
public bool ShouldSerializeMyFont() {
return thefont != null;
}
A complete code example follows.
Option Explicit
Option Strict
Imports System
Imports System.Windows.Forms
Imports System.Drawing
Public Class MyControl
Inherits Control
' Declare an instance of the Font class
' and set its default value to Nothing.
Private thefont As Font = Nothing
' The MyFont property.
Public Property MyFont() As Font
' Note that the Font property never
' returns null.
Get
If Not (thefont Is Nothing) Then
Return thefont
End If
If Not (Parent Is Nothing) Then
Return Parent.Font
End If
Return Control.DefaultFont
End Get
Set
thefont = value
End Set
End Property
Public Function ShouldSerializeMyFont() As Boolean
Return Not (thefont Is Nothing)
End Function
Public Sub ResetMyFont()
MyFont = Nothing
End Sub
End Class
[C#]using System;
using System.Windows.Forms;
using System.Drawing;
public class MyControl : Control {
// Declare an instance of the Font class
// and set its default value to null.
private Font thefont = null;
// The MyFont property.
public Font MyFont {
// Note that the MyFont property never
// returns null.
get {
if (thefont != null) return thefont;
if (Parent != null) return Parent.Font;
return Control.DefaultFont;
}
set {
thefont = value;
}
}
public bool ShouldSerializeMyFont() {
return thefont != null;
}
public void ResetMyFont() {
MyFont = null;
}
}
In this case, even when the value of the private variable accessed by the MyFont
property is a null reference (Nothing in Visual Basic), the property browser does not display null (Nothing); instead, it displays the Font property of the parent (if it is not a null reference (Nothing)) or the default Font value defined in Control. Thus the default value for MyFont
cannot be simply set, and a DefaultValueAttribute cannot be applied to this property. Instead, the ShouldSerialize and Reset methods have to be implemented for the MyFont
property.
See Also
Properties in Windows Forms Controls | Defining a Property | Property-Changed Events