.NET Framework - DefaultValue attribute on properties of user controls

Asked By m_j_soren

04-Jun-08 07:52 PM

Prior to C# 3.0, I would typically define a property of a custom user control
with pattern A:

=== PATTERN A ==========
private bool myBool = true;
public bool MyBool {
get { return myBool; }
set { myBool = value; }
}
======================

If this property exists in a custom user control, and I then instantiate
this user control in a WinForm, that property shows up in the visual designer
property pane with the default value I have assigned to the private backing
variable (in the above example, "true"). With the advent of C# 3.0, I want to
switch to the much more terse code in pattern B.

=== PATTERN B ==========
public bool MyBool { get; set; }
======================

Which raises the question of where to set the default. My first attempt was
to say "MyBool = true;" in the constructor of the custom user control. With
this technique, instantiating the control in a WinForm again shows my default
value for this property of the control in the property pane of the visual
designer.

I happened across what I thought was an even cleaner approach in the MSDN
documentation--the DefaultValue attribute so I tried pattern C:

=== PATTERN C ==========
[DefaultValue(true)]
public bool MyBool { get; set; }
======================

This compiles but does not work. That is, the MyBool property for this
control shows "false" in the property pane when instantiated on a WinForm.
(Note that my user control is in a library project and the WinForm is in a
WinForm project, both in the same solution. I first compiled the control
library, then dragged the control from the toolbox onto the WinForm so as to
avoid any stale designer-generated code.)

Should pattern C work? If so, what am I missing?

Environment: VS2008 with target=.NET 2.0 for both projects.

06-Jun-08 03:20 AM

Michael,

I do not know whether the pattern should work.

However,  Google helped me find this one.
http://support.microsoft.com/kb/311339

HTH
Kalpesh

04-Jun-08 11:09 PM

Hi Michael,

The pattern B you mentioned is called auto-implemented properties
introduced in C#3.0, which make property-declaration more concise in case
where no additional logic is required in the property accessors.

When you declare an auto-implemented property, the compiler creates a
private, anonymous backing field that is not accessible except through the
property's get and set accessors. Other than this, there isn't any
difference between an auto-implemented property and a normal property(by
the normal property, I mean the pattern A you mentioned).

If we set the value of a property in the constructor of the class or in the
declaration statement, the property is only initialized. Note, the default
value is NOT the initial value. If the value of the property equals to the
default value, the property's value is displayed in a regular font in the
Properties window and will not be serialized in the InitializeComponent
method of the Form; otherwise, shown in bold and will be serialized in the
InitializeComponent method.

To set a default value for a property, we can use the DefaultValueAttribute
on the property. And the default value specified by the
DefaultValueAttribute should equal to the initial value of the property.

A variable of type bool has an initial value of false. In your practice,
you don't set the MyBool property's value explicitly, so its initial value
is false. Then you specify the default value of the MyBool property is true
using the DefaultValueAttribute. When you add the UserControl on a form,
the value of the MyBool property shown in the Properties window is false
and in bold, because it doesn't equals to the specified default value.

Hope I make some clarifications.
If you have anything unclear, please feel free to let me know.

Sincerely,
Linda Liu
Microsoft Online Community Support

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05-Jun-08 10:54 AM

Thanks to both Linda and Kalpesh for useful information on this issue,
notably the distinction between default value and initial value. After
perusing the material, I believe Kalpesh is correct that pattern C does not
do what I want, i.e. initialize the property. So my conclusion, then, is that
pattern B is needed to set an initial value. I should have shown pattern B in
its entirety like this:

=== PATTERN B ==========
public MyObject {
public bool MyBool { get; set; }

public MyObject() { // constructor
MyBool = true;
. . .
}
}
======================

Linda, to followup on one of your comments, is there any particular
advantage for me to add the [DefaultValue(true)] in the code above so that it
does not get serialized in InitializeComponent?

05-Jun-08 09:23 PM

Hi Michael,

Thank you for your quick reply!

in the code above so that it does not get serialized in initializeComponent?

The DefaultValueAttribute is mainly used by designer. If the value of the
property equals to the default value, designer won't serialize this
property in code. This may make the initialization code concise.

Hope this helps.
If you have any question, please feel free to let me know.

Sincerely,
Linda Liu
Microsoft Online Community Support

Delighting our customers is our #1 priority. We welcome your comments and
suggestions about how we can improve the support we provide to you. Please
feel free to let my manager know what you think of the level of service
provided. You can send feedback directly to my manager at:
msdnmg@microsoft.com.

This posting is provided "AS IS" with no warranties, and confers no rights.

06-Jun-08 02:59 AM

Just to add - DefaultValueAttribute is also used by things like
XmlSerializer, and anything that uses System.ComponentModel, where
PropertyDescriptor.ShouldSerializeValue() and ResetValue() respect it,
at least for reflection-based maps.

Obviously we are talking about controls here, so yes: mainly the
designer - but it is supported in the wider context too.

Marc