Developing a custom collection editor

.NET framework has a collection editor, which provides design-time user interface to edit most types of collections. Last month, I needed custom collection editor at design-time, including hidden Add/Remove buttons, custom properties events, collection item validation rules etc...

I have designed a custom collection editor form called "CollectionManager", which  fires custom events as follow:

#Region " Event Handler " Public Event BeforeAddItem As CancelEventHandler Public Event AfterAddItem As EventHandler Public Event BeforeRemoveItem As CancelEventHandler Public Event AfterRemoveItem As EventHandler Public Event SelectedIndexChanged As EventHandler Public Event BeforePropertyGetValue As PropertyDescriptorHandledHandler Public Event BeforePropertySetValue As PropertyDescriptorHandledHandler Public Event AfterPropertyGetValue As PropertyDescriptorHandler Public Event AfterPropertySetValue As PropertyDescriptorHandler #End Region

Now, create a "Columns" property inside usercontrol with EditorAttribute tag as follow.

<Editor(GetType(CustomCollectionEditor), GetType(System.Drawing.Design.UITypeEditor))> _ Public ReadOnly Property Columns() As ColumnCollection Get Return _columns End Get End Property

Editor attribute provides a type of the class to creates an instance, which is inherited UITypeEditor class. Now, implementing CustomCollectionEditor class is really simple.

Now, it requires to override GetEditStyle function. This function indicates PropertyGrid to display design-time editor type for this property. As it's a colleciton editor, it should be UITypeEditorEditStyle.Modal style. Public Overloads Overrides Function GetEditStyle _ (ByVal context As ITypeDescriptorContext) As UITypeEditorEditStyle If (Not (context Is Nothing) And Not (context.Instance Is Nothing)) Then Return UITypeEditorEditStyle.Modal End If Return MyBase.GetEditStyle(context) End Function

Also, it needs to overrides EditValue function, which calls to handle collection's edit request. To implement this EditValue function, it needs to create an instance of custom collection editor form called "CollectionManager". Public Overloads Overrides Function EditValue(ByVal context As ITypeDescriptorContext, _ ByVal provider As IServiceProvider, ByVal value As Object) As Object _context = context _provider = provider _value = value If (Not (context Is Nothing) AndAlso (Not context.Instance Is Nothing) AndAlso (Not provider Is Nothing)) Then Dim edSvc As IWindowsFormsEditorService = CType(provider.GetService( _ GetType(IWindowsFormsEditorService)), IWindowsFormsEditorService) If edSvc Is Nothing Then Return value End If If _collectionEditor Is Nothing Then _collectionEditor = New CollectionManager End If _collectionEditor.Text = "Collection Editor: Column Types" 'implement Custom Collection functions/events handling. 'implement Custom Collection functions/events handling. 'implement Custom Collection functions/events handling. context.OnComponentChanging() If _collectionEditor.ShowDialog() = DialogResult.OK Then context.OnComponentChanged() Return _collectionEditor.Collection Else context.OnComponentChanged() Return value End If End If context.OnComponentChanged() Return MyBase.EditValue(context, provider, value) End Function

When user changes collection value and need to serialise CodeDOM properly, it is important to call context.OnComponentChanging() function and context.OnComponentChanged() functions to notify "forms designer" about these changes.

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