Question

I’m trying to make an editor for a game which has an event system, with a base class for the events and then another class for each kind which actually do all the stuff.

So, I have a list of BaseEvent which shows in a PropertyGrid, and as a list, the collection editor opens. I prepared a TypeConverter so I have a dropdown with all derived classes which shows in the “Value” property.

Everything is OK, properties from derived classes show as children of “Value”, but as soon as I want to show a property from BaseEvent, the “Value” property disappears, and children appear at the root, so I’m unable to change the type of the event.

Is there a way to make the “Value” property appear at the same time as the BaseEvent properties?

//This allows to get a dropdown for the derived classes
public class EventTypeConverter : ExpandableObjectConverter
{
    public override StandardValuesCollection GetStandardValues(ITypeDescriptorContext context)
    {
        return new StandardValuesCollection(GetAvailableTypes());
    }

    /*
    ...
    */
}

[TypeConverter(typeof(EventTypeConverter))]
public abstract class BaseEvent
{
    public bool BaseProperty; //{ get; set; } If I set this as a property, "Value" disappears
}

public class SomeEvent : BaseEvent
{
    public bool SomeOtherProperty { get; set; }
}

//The object selected in the PropertyGrid is of this type
public class EventManager
{
    public List<BaseEvent> Events { get; set; } //The list that opens the collection editor
}

Answer 1

Finally I found a way to solve this: through the GetProperties method, and a custom PropertyDescriptor:

public override PropertyDescriptorCollection GetProperties(ITypeDescriptorContext context, object value, Attribute[] attributes)
{
    //Get the base collection of properties
    PropertyDescriptorCollection basePdc = base.GetProperties(context, value, attributes);

    //Create a modifiable copy
    PropertyDescriptorCollection pdc = new PropertyDesctiptorCollection(null);
    foreach (PropertyDescriptor descriptor in basePdc)
        pdc.Add(descriptor);

    //Probably redundant check to see if the value is of a correct type
    if (value is BaseEvent)
        pdc.Add(new FieldDescriptor(typeof(BaseEvent), "BaseProperty"));
    return pdc;
}

public class FieldDescriptor : SimplePropertyDescriptor
{
    //Saves the information of the field we add
    FieldInfo field;

    public FieldDescriptor(Type componentType, string propertyName)
        : base(componentType, propertyName, componentType.GetField(propertyName, BindingFlags.Instance | BindingFlags.NonPublic).FieldType)
    {
        field = componentType.GetField(propertyName, BindingFlags.Instance | BindingFlags.NonPublic);
    }

    public override object GetValue(object component)
    {
        return field.GetValue(component);
    }

    public override void SetValue(object component, object value)
    {
        field.SetValue(component, value);
    }
}