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Bit Flags Type Converter

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4.61/5 (14 votes)
20 Jun 2006CPOL3 min read 65.1K   393   35   10
An implementation of TypeConverter which allows you to edit bit flag enumerations in the PropertyGrid.

Sample Image

Introduction

I am a component developer at 10Tec Company. 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 PropertyGrid 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 our own TypeConverter which would help to edit bit field 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.

Using the code

To assign a custom type converter to a property, you should use the TypeConverterAttribute attribute:

C#
public class TestClass
{
  …
  [TypeConverter(typeof(FlagsEnumConverter))]
  public FontStyle FontStyle
  {
    get
    {
      …
    }
    set
    {
      …
    }
  }
  …
}

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:

C#
internal class FlagsEnumConverter: EnumConverter
{
  public override PropertyDescriptorCollection 
         GetProperties(ITypeDescriptorContext context, 
         object value, Attribute[] attributes)
  {
    …
  }
  public override bool GetPropertiesSupported(ITypeDescriptorContext context)
  {
    …
  }
}

The TypeConverter has several overridable properties and methods which allow you to customize the appearance of a property in the PropertyGrid. In our type converter, I overrode two methods: GetPropertiesSupported and GetProperties. By using these methods, we inform the PropertyGrid 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):

C#
public override PropertyDescriptorCollection 
       GetProperties(ITypeDescriptorContext context, 
       object value, Attribute[] attributes)
{
  Type myType = value.GetType();
  string[] myNames = Enum.GetNames(myType);
  Array myValues = Enum.GetValues(myType);
  if(myNames != null)
  {
    PropertyDescriptorCollection myCollection = 
           new PropertyDescriptorCollection(null);
    for(int i = 0; i < myNames.Length; i++)
        myCollection.Add(new EnumFieldDescriptor(myType, 
                         myNames[i], context));
    return myCollection;
  }
}

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 the PropertyGrid. The main interests of the EnumFieldDescriptor class are the GetValue and SetValue methods. The GetValue method is called each time the PropertyGrid 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):

C#
public override object GetValue(object component)
{
  return ((int)component & (int)Enum.Parse(ComponentType, Name)) != 0;
}

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:

C#
public override void SetValue(object component, object value)
{
  bool myValue = (bool)value;
  int myNewValue;
  if(myValue)
    myNewValue = ((int)component) | (int)Enum.Parse(ComponentType, Name);
  else
    myNewValue = ((int)component) & ~(int)Enum.Parse(ComponentType, Name);

  FieldInfo myField = component.GetType().GetField("value__", 
                      BindingFlags.Instance | BindingFlags.Public);
  myField.SetValue(component, myNewValue);
  fContext.PropertyDescriptor.SetValue(fContext.Instance, component);
}

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 a value type (non-reference type) and if we unbox it (convert from the object type to an enumeration type), we will obtain a different object, not the one passed with the component parameter. To solve it, I have done a little trick. An enumeration, in its 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.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


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Comments and Discussions

 
QuestionWhy is it necessary to set the value of "value__" using reflection? Pin
Mizan Rahman15-May-12 1:00
Mizan Rahman15-May-12 1:00 
AnswerRe: Why is it necessary to set the value of "value__" using reflection? Pin
Sergey Gorbenko21-May-12 15:51
Sergey Gorbenko21-May-12 15:51 
GeneralDoesn't work Pin
Tim McCurdy6-Jul-07 7:32
Tim McCurdy6-Jul-07 7:32 
GeneralRe: Doesn't work Pin
Sergey Gorbenko30-Jul-07 20:53
Sergey Gorbenko30-Jul-07 20:53 
GeneralRe: Doesn't work Pin
Tim McCurdy31-Jul-07 16:26
Tim McCurdy31-Jul-07 16:26 
GeneralUPDATE Pin
SeriousM22-Dec-06 13:55
SeriousM22-Dec-06 13:55 
GeneralRe: UPDATE Pin
greenxiar10-Aug-07 19:20
greenxiar10-Aug-07 19:20 
GeneralVB.NET Pin
Eric P Schneider23-Oct-06 15:21
Eric P Schneider23-Oct-06 15:21 
Thanks, for the converter.

Just for easier maint. of some users here is a VB.NET version (seems to work for me):

Imports System
Imports System.ComponentModel
Imports System.Reflection
Imports System.Windows.Forms

Friend Class FlagsEnumConverter
Inherits EnumConverter

Protected Class EnumFieldDescriptor
Inherits SimplePropertyDescriptor
Private fContext As ITypeDescriptorContext

Public Sub New(ByVal componentType As Type, ByVal name As String, ByVal context As ITypeDescriptorContext)
MyBase.New(componentType, name, GetType(Boolean))
fContext = context
End Sub

Public Overloads Overrides Function GetValue(ByVal component As Object) As Object
Return Not ((CType(component, Integer) And CType([Enum].Parse(ComponentType, Name), Integer)) = 0)
End Function

Public Overloads 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

Public Overloads Overrides Function ShouldSerializeValue(ByVal component As Object) As Boolean
Return Not (CType(GetValue(component), Boolean) = GetDefaultValue())
End Function

Public Overloads Overrides Sub ResetValue(ByVal component As Object)
SetValue(component, GetDefaultValue)
End Sub

Public Overloads Overrides Function CanResetValue(ByVal component As Object) As Boolean
Return ShouldSerializeValue(component)
End Function

Private Function GetDefaultValue() As Boolean
Dim myDefaultValue As Object = Nothing
Dim myPropertyName As String = fContext.PropertyDescriptor.Name
Dim myComponentType As Type = fContext.PropertyDescriptor.ComponentType
Dim myDefaultValueAttribute As DefaultValueAttribute = CType(Attribute.GetCustomAttribute(myComponentType.GetProperty(myPropertyName, BindingFlags.Instance Or BindingFlags.Public Or BindingFlags.NonPublic), GetType(DefaultValueAttribute)), DefaultValueAttribute)
If Not (myDefaultValueAttribute Is Nothing) Then
myDefaultValue = myDefaultValueAttribute.Value
End If
If Not (myDefaultValue Is Nothing) Then
Return Not ((CType(myDefaultValue, Integer) And CType([Enum].Parse(ComponentType, Name), Integer)) = 0)
End If
Return False
End Function

Public Overloads Overrides ReadOnly Property Attributes() As AttributeCollection
Get
Return New AttributeCollection(New Attribute() {RefreshPropertiesAttribute.Repaint})
End Get
End Property
End Class

Public Sub New(ByVal type As Type)
MyBase.New(type)
End Sub

Public Overloads Overrides Function GetProperties(ByVal context As ITypeDescriptorContext, ByVal value As Object, ByVal attributes As Attribute()) As PropertyDescriptorCollection
If Not (context Is Nothing) Then
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
If Not (CType(myValues.GetValue(i), Integer) = 0) AndAlso Not (myNames(i) = "All") Then
myCollection.Add(New EnumFieldDescriptor(myType, myNames(i), context))
End If
System.Math.Min(System.Threading.Interlocked.Increment(i), i - 1)
End While
Return myCollection
End If
End If
Return MyBase.GetProperties(context, value, attributes)
End Function

Public Overloads Overrides Function GetPropertiesSupported(ByVal context As ITypeDescriptorContext) As Boolean
If Not (context Is Nothing) Then
Return True
End If
Return MyBase.GetPropertiesSupported(context)
End Function

Public Overloads Overrides Function GetStandardValuesSupported(ByVal context As ITypeDescriptorContext) As Boolean
Return False
End Function
End Class



Schneider

GeneralNice Pin
leppie20-Jun-06 10:26
leppie20-Jun-06 10:26 
GeneralExcellent! Pin
Alexey A. Popov20-Jun-06 8:23
Alexey A. Popov20-Jun-06 8:23 

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