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While developing one of our components, iGrid.NET, I came across a problem: the PropertyGrid control does not provide you with a comfortable-enough editor for enumerations marked with the Flags attribute (enumerations that can be treated as bit fields; that are a set of flags). The property grid treats them as usual enumerations and allows you to select a single field from the drop-down list or enter a value as text. So I decided to implement own TypeConverter which would help to edit bit fields properties.
There can be a few solutions to this problem, for example we could show a check list box in the drop-down list. My solution is to show flags as sub-properties.
To assign a custom type converter to a property, you should use the TypeConverterAttribute attribute:
Public Class TestClass
<TypeConverter(GetType(FlagsEnumConverter))> _
Public Property FontStyle() As FontStyle
Get
End Get
Set(ByVal value As FontStyle)
End Set
End Property
End Class
The parameter passed to the TypeConverterAttribute attribute should be inherited from the TypeConverter class or one of its descendents.
Our type converter is named FlagsEnumConverter and is inherited from the standard .NET EnumConverter class:
Friend Class FlagsEnumConverter
Inherits EnumConverter
Public Overloads Overrides Function GetProperties(ByVal context As ITypeDescriptorContext, ByVal value As
Object, ByVal attributes As Attribute()) As PropertyDescriptorCollection
End Function
Public Overloads Overrides Function GetPropertiesSupported(ByVal context As ITypeDescriptorContext) As Boolean
End Function
End Class
TypeConverter has several overridable properties and methods which allow you to customize the appearance of a property in the property grid. In our type converter I overrode two methods: GetPropertiesSupported and GetProperties. By using these methods we inform the property grid that our property is complex (has several nested properties) and return descriptors of these nested properties (we show a single nested Boolean property for each bit flag):
Public Overrides Function GetProperties(ByVal context As ITypeDescriptorContext, ByVal value As Object, ByVal attributes As Attribute()) As PropertyDescriptorCollection
Dim myType As Type = value.GetType
Dim myNames As String() = Enum.GetNames(myType)
Dim myValues As Array = Enum.GetValues(myType)
If Not (myNames Is Nothing) Then
Dim myCollection As PropertyDescriptorCollection = New PropertyDescriptorCollection(Nothing)
Dim i As Integer = 0
While i < myNames.Length
myCollection.Add(New EnumFieldDescriptor(myType, myNames(i), context))
System.Math.Min(System.Threading.Interlocked.Increment(i), i - 1)
End While
Return myCollection
End If
End Function
Each bit flag (nested property) is represented with the EnumFieldDescriptor class inherited from the standard .NET class named SimplePropertyDescriptor. This is a base class which allows you to represent a custom property in property grid. The main interest of the EnumFieldDescriptor class is the GetValue and SetValue methods. The GetValue method is called each time when the property grid displays a custom property (a bit flag) and returns the value of the custom property (in our case it is a Boolean value which indicates whether the bit flag is included in the enumeration property value):
Public Overrides Function GetValue(ByVal component As Object) As Object
Return Not ((CType(component, Integer) And CType(Enum.Parse(ComponentType, Name), Integer)) = 0)
End Function
In the code snippet above ComponentType is the type of the enumeration, and Name is the name of the enumeration field (bit flag).
The SetValue method is more complicated, it is called when the user modifies a custom property. In our case it sets a particular bit flag to the enumeration value:
Public Overrides Sub SetValue(ByVal component As Object, ByVal value As Object)
Dim myValue As Boolean = CType(value, Boolean)
Dim myNewValue As Integer
If myValue Then
myNewValue = CType(component, Integer) Or CType(Enum.Parse(ComponentType, Name), Integer)
Else
myNewValue = CType(component, Integer) And Not CType(Enum.Parse(ComponentType, Name), Integer)
End If
Dim myField As FieldInfo = component.GetType.GetField("value_", BindingFlags.Instance Or BindingFlags.Public)
myField.SetValue(component, myNewValue)
fContext.PropertyDescriptor.SetValue(fContext.Instance, component)
End Sub
In this method we accept a boxed enumeration value (the object type component parameter) which we should modify. The main problem is that an enumeration is value type (non-reference type) and if we unbox it (convert from the object type to an enumeration type), we will obtain different object, not the one passed with the component parameter. To solve it, I have done a little trick. Enumeration in core stores its actual value in a private parameter named value_ (you can trace it with a disassembler or decompiler). I have accessed this parameter by using reflection and set its value without unboxing.
NOTE: THIS ARTICLE IS CONVERTED FROM C# TO VB.NET USING A CONVERSION TOOL. ORIGINAL ARTICLE CAN BE FOUND ON C# CORNER (http://www.c-sharpcorner.com/).
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