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Using Design Time Attributes
Introduction
Developing a nice custom control is just one part of the story. As a control
author you should also pay attention about the experience of other developers
who will be using your control. In most of the real world cases developers use
Visual Studio as the IDE for developing .NET applications. You can enhance the
experience of other developers using your control by providing proper designer
support. For example, you can control how your control properties and events are
displayed in property window and toolbox. A set of attributes often called as
Design Time Attributes allow you to accomplish this.
Common Design Time Attributes
The following sections explain the common design time attributes that allow
you to change how the control behaves in the property window and toolbox. Most
of these attributes reside in System.ComponentModel namespace. In the
following sections we use the
GraphicButton control that
we created earlier in this series.
Deciding whether a property will be visible in the property window
By default all the public properties of a custom control are displayed in the
property window. Sometimes you may want that developers should set a property
only via code and not via property window. The [Browsable] attribute allows you
to control just that. The usage of this attribute is as follows:
[Browsable(false)]
public string ImageUrl
{
get { return ViewState["imgurl"] as string; }
set { ViewState["imgurl"] = value; }
}
The value of false indicates that the ImageUrl property will not be displayed
in the property window.
Adding description to a property
Have a look at the screen capture below:
Can you notice a one line description of the Text property at the bottom
portion of the property window? Such description can be added using
[Description] attribute.
[Description("Specifies the caption of the graphic button")]
public string Text
{
get { return ViewState["text"] as string; }
set { ViewState["text"] = value; }
}
Grouping related properties
The property windows allows you list properties either alphabetically or
grouped in categories. Grouping properties by category makes it simple to locate
them as per their functionality. Have a look below where the ImageUrl and Text
properties are grouped together under Control Values category.
The [Category] attribute allows you to specify category for your property.
[Category("Control Value")]
public string Text
{
get { return ViewState["text"] as string; }
set { ViewState["text"] = value; }
}
Enabling data binding for a property
You may want to use your custom control as a child control of some data bound
control (say DataList). Further, you may want to data bind some of the control
properties. In other words you want to list your control properties in the data
bindings editor as shown below:
The [Bindable] attribute enables this behavior for you.
[Bindable(true)]
public string Text
{
get { return ViewState["text"] as string; }
set { ViewState["text"] = value; }
}
Making a property read only
At times you may want that a control property be listed in the property
window but it should be read only. This behavior can enabled using [ReadOnly]
attribute.
[ReadOnly(true)]
public string Text
{
get { return ViewState["text"] as string; }
set { ViewState["text"] = value; }
}
Setting a property of multiple control instances
When you select multiple instance of the same control and set a property in
the property window all the instances bear the same property value. You may not
always this default behavior. You can use [MergableProperty] attribute to
indicate this to the IDE.
[MergableProperty(true)]
public string Text
{
get { return ViewState["text"] as string; }
set { ViewState["text"] = value; }
}
Refreshing other properties when a property value changes
Some times your properties are interdependent in that if value of one
property changes then it causes value of some other property to change. In such
cases you would like to refresh the property window when a property value
changes. The [RefreshProperties] attribute allows you to indicate such a
behavior.
[RefreshProperties(RefreshProperties.All)]
public string Text
{
get { return ViewState["text"] as string; }
set { ViewState["text"] = value; }
}
The [RefreshProperties] attribute accepts a parameter of type
RefreshProperties enumeration. The three possible enumeration values are None,
All and Repaint.
Setting a default property and event
When yu drag and drop a control instance on the web form you may want to show
a particular property (or event) selected by default in the property window.
This can be accomplished with the help of [DefaultProperty] and [DefaultEvent]
attributes respectively.
[DefaultEvent("Click")]
[DefaultProperty("Text")]
public class GraphicButton :
WebControl, IPostBackEventHandler
...
Notice that unlike other attributes we discussed so far the [DefaultProperty]
and [DefaultEvent] attributes are class level attributes. Both of these
attributes take a string parameter indicating the name of the default property
and event respectively.
Changing icon displayed in the toolbox
By default a newly created custom control is shown in the toolbox as shown
below (see the gear icon):
You may want to change the default icon displayed along with your control.
You do this by adding a bitmap file to your custom control project and naming it
same as the custom control class. For example, for our
BinaryIntellect.UI.GraphicButton control you would add a bitmap with name
BinaryIntellect.UI.GraphicButton.bmp. The bitmap must be 16x16 pixels and should
use no more than 16 colors.
There is another way to set toolbox bitmap - the [ToolboxBitmap] class level
attribute.
[ToolboxBitmap(@"C:\BinaryIntellect.UI.GraphicButton.bmp")]
The [ToolboxBitmap] attribute takes the full path of the bitmap file.
Remember that the [ToolboxBitmap] attribute resides in System.Drawing namespace.
The following figure shows how a custom bitmap looks like in the toolbox.
This article covered the basics of design time support. You can further add
features such as custom pickup lists and smart tags. I will cover them in my
next articles.