Custom Controls and Design-Time Support: Part 2/2 - Filtering Control Class Members
(Page 3 of 7 )
From time to time, you might want to hide a control event or property from the design-time view of the developer.
For example, every control provides a Text property, because every control inherits from the base System.Windows.Forms.Control class.
However, not all controls can effectively use the Text property. For example, the TreeView control doesn't actually use its Text property - you can set it in code, but it won't modify the user interface. For this reason, the Text property is not displayed in the Properties window.
If you are defining a property, you can use the Browsable attribute to keep it from appearing in the Properties window. However, consider the TreeView control, which provides a Nodes collection.
The DirectoryTree inherits the Nodes property, and allows it to be modified at design-time, even though the list of nodes is built automatically at runtime based on the Drive property.
This conflict means that there are really two ways the developer can alter the Nodes collection-indirectly through the Driver property, or directly through the Nodes collection, which is sure to lead to a great deal of confusion.
Unfortunately, because the TreeView.Nodes property is not overridable, you can't use the Browsable attribute. However, you can create a custom designer that ensures it won't appear at design-time.
To use filtering with the DirectoryTree, create a custom designer class that derives from ControlDesigner:
public class DirectoryTreeDesigner : ControlDesigner
{
protected override void PostFilterProperties(
System.Collections.IDictionary properties)
{
properties.Remove("Nodes");
}
} This designer overrides the PostFilterProperties() method, and removes the Nodes property from the properties collection.
The next step is to link the custom designer to the DirectoryTree control. To do this, use the Designer attribute, and specify the appropriate designer type.
[DefaultEvent("DirectorySelected"),
DefaultProperty("Drive"),
Designer(typeof(DirectoryTreeDesigner))]
public class DirectoryTree : TreeView
{ Note that the Nodes property is still accessible in code. This allows clients to perform other useful tasks (like enumerating through the collection of nodes) at their discretion. However, it can't be designed, and it won't be serialized.
Next: Designer Verbs >>
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