CollectionEditor and objects that implement an interface

microsoft.public.dotnet.framework.windowsforms

26 Oct 2007 5:42 PM

David

Hello,

I am using Visual Studio 2005 (VB.NET) and I have encountered a problem that
I am hoping someone can help me with.

I am trying to get the CollectionEditor of a PropertyGrid control to allow
the editing of a collection of objects that implement a custom interface
(ITextTransform). The ITextTransform interface has a single method defined.

[begin code]

Public Interface ITextTransform
     Function Execute(ByVal Input As String) As String
End Interface

[end code]

Classes that implement ITextTransform are used to transform an input string.
Each class implements the Execute() method of ITextTransform and adds
additional properties unique to the type of string transformation that is
taking place (e.g A ReplaceText class adds Replace and ReplaceWith
properties).

I have created the following custom CollectionEditor class to only allow
objects that implement the ITextTransform interface to be created.

[begin code]

Public Class CustomCollectionEditor
    Inherits CollectionEditor

    Public Sub New(ByVal newType As Type)
        MyBase.new(newType)
    End Sub

    ' Allow the Collection Editor to only create instances of classes that
implement
    ' the ITextTransform interface, by using Reflection to dynamically build
the allowed Type
    ' array.
    Protected Overrides Function CreateNewItemTypes() As System.Type()
        Dim ITextTransformType As Type = GetType(ITextTransform)
        Dim AllTypes As Type() = ITextTransformType.Assembly.GetTypes

        Dim AllowedNewTypes As New ArrayList

        For Each TempType As Type In AllTypes
            If ITextTransformType.IsAssignableFrom(TempType) = True And
TempType.IsInterface = False Then _
                AllowedNewTypes.Add(TempType)
        Next

        CreateNewItemTypes = AllowedNewTypes.ToArray(GetType(Type))

        AllowedNewTypes = Nothing
    End Function

    Protected Overrides Function CanSelectMultipleInstances() As Boolean
        Return False
    End Function

    Protected Overrides Function GetDisplayText(ByVal value As Object) As
String
        Return value.GetType.Name
    End Function
End Class

[end code]

The property that exposes the collection of ITextTransform implemented
objects in the class that is being bound to the PropertyGrid control has had
the Editor attribute set, so that the CustomCollectionEditor class is used.

[begin code]

    <Editor(GetType(CustomCollectionEditor), GetType(UITypeEditor))> _
    Public ReadOnly Property Items() As List(Of ITextTransform)
        Get
            Return pItems
        End Get
    End Property

[end code]

When I open the collection editor and click the Add button, the object that
is being created is of type ITextTransform, the interface, and *NOT* of the
object that is implementing the interface.

However, if I add a property declaration to the interface, and implement it
in all classes that use the interface, then everything works as it should:

[begin code]

Public Interface ITextTransform
    Function Execute(ByVal Input As String) As String
    Property NonsenseProperty as Boolean
End Interface

[end code]

While the code now works, I don't really want an unnecessary property like
that.

Is this a bug in the CollectionEditor code, that it cannot handle objects
that implement interfaces if the interface does not have one or more
properties defined?

...Or I am doing something really stupid?

You can download the full code at
http://rapidshare.com/files/65366563/CollectionEditiorProblem.zip.html

Thanks for any help!

David.