Bit Flags Type Converter

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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:

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:

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):

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):

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:

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.