Question

I’ve got an enum of types of data that might be shown in a .NET Forms control and I want to provide an interface for consumers of the control to filter some of the types (set some of the flags). A bit field seems the logical way to do this, unfortunately, the enum starts at 0 rather than 1 (0, 1, 2, 4, 8, …) and can’t be changed.

How can I expose this set of flags so that it can be easily configured programmatically or through the Visual Studio designer?

Answer 1

You would need to write a UITypeEditor to do the work, and associate it with the property via [EditorAttribute].

edit now with example – a fairly long one, I’m afraid – but most of the code can be shared between types, fortunately.

You can’t use a single composite enum value because of the zero – so here I’m using a HashSet<T> to hold the selected enums – fairly easy to re-work to List<T> if you have .NET 2.0/3.0, though.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing.Design;
using System.Text;
using System.Windows.Forms;
using System.Windows.Forms.Design;

public class MyControl : UserControl
{
    public MyControl()
    {
        Values = new HashSet<MyEnum>();
    }
    [Editor(typeof(MyEnumSetEditor), typeof(UITypeEditor))]
    [TypeConverter(typeof(MyEnumSetConverter))]
    public HashSet<MyEnum> Values { get; set; }
}

public enum MyEnum
{  // numbers as per the question...
    A = 0, B = 1, C = 2, D = 4, E = 8
}
class MyEnumSetEditor : EnumSetEditor<MyEnum> { }
class MyEnumSetConverter : EnumSetConverter<MyEnum> { }

// from here down is shared between types
abstract class EnumSetConverter<T> : TypeConverter where T : struct
{
    public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
    {
        return destinationType == typeof(string) || base.CanConvertTo(context, destinationType);
    }
    public override object ConvertTo(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value, Type destinationType)
    {
        if(destinationType == typeof(string))
        {
            HashSet<T> set = (HashSet<T>)value;
            if (set == null) return "(null)";

            StringBuilder sb = new StringBuilder();
            foreach (T item in Enum.GetValues(typeof(T)))
            {
                if (set.Contains(item))
                {
                    if (sb.Length > 0) sb.Append(", ");
                    sb.Append(item);
                }
            }
            return sb.ToString();
        }
        return base.ConvertTo(context, culture, value, destinationType);
    }
}

public abstract class EnumSetEditor<T> : UITypeEditor where T : struct
{
    public override UITypeEditorEditStyle GetEditStyle(ITypeDescriptorContext context)
    {
        return UITypeEditorEditStyle.DropDown;
    }
    public override bool IsDropDownResizable
    {
        get { return true; }
    }
    public override object EditValue(ITypeDescriptorContext context, IServiceProvider provider, object value)
    {
        IWindowsFormsEditorService svc = (IWindowsFormsEditorService)
            provider.GetService(typeof(IWindowsFormsEditorService));
        HashSet<T> set = value as HashSet<T>;
        if (svc != null && set != null)
        {
            UserControl ctrl = new UserControl();
            CheckedListBox clb = new CheckedListBox();
            clb.Dock = DockStyle.Fill;
            Button btn = new Button();
            btn.Dock = DockStyle.Bottom;
            foreach (T item in Enum.GetValues(typeof(T)))
            {
                clb.Items.Add(item, set.Contains(item));
            }
            ctrl.Controls.Add(clb);
            ctrl.Controls.Add(btn);
            btn.Text = "OK";
            btn.Click += delegate
            {
                set.Clear();
                foreach (T item in clb.CheckedItems)
                {
                    set.Add(item);
                }
                svc.CloseDropDown();
            };
            svc.DropDownControl(ctrl);
        }

        return value;
    }
}