Question

An object has some properties , now at runtime — when a condition is met .. I want to add new properties to this object .

“DynamicObject” cant be ustilised since i wont be knowing the property Names beforehand .

I came acroos PropertyBuilder http://msdn.microsoft.com/en-us/library/system.reflection.emit.propertybuilder.aspx

But i cant find help regarding how to use the propertBuilder for adding properties to an existing object of a defined-existing class.

Answer 1

You cannot add real (reflection) properties to an object or type at runtime.

If the context here is data-binding, then you can all artificial properties, by implementing one or more of ICustomTypeDescriptor, TypeDescriptionProvider, TypeConverter, ITypedList – and providing your own PropertyDescriptors for the extra properties.

• ICustomTypeDescriptor is per-object and tied to that object
• TypeDescriptionProvider is per-object or per-type, and is separate to the object
• TypeConverter is per-type and is used in particular by PropertyGrid
• ITypedList is used by a list (IList) to describe the properties of the child objects

Example:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Windows.Forms;
static class Program
{
    [STAThread]
    static void Main()
    {
        Application.EnableVisualStyles();
        FooConverter.AddProperty("Time", typeof(DateTime));
        FooConverter.AddProperty("Age", typeof(int));
        using (var grid = new PropertyGrid { Dock = DockStyle.Fill, SelectedObject = new Foo() })
        using (var form = new Form { Controls = { grid } })
        {
            Application.Run(form);
        }
    }
}
class FooConverter : ExpandableObjectConverter
{
    private static readonly List<Tuple<string, Type>> customProps = new List<Tuple<string, Type>>();
    public static void AddProperty(string name, Type type)
    {
        lock (customProps) customProps.Add(Tuple.Create(name, type));
    }
    public override PropertyDescriptorCollection GetProperties(ITypeDescriptorContext context, object value, System.Attribute[] attributes)
    {
        var orig = base.GetProperties(context, value, attributes);
        lock(customProps)
        {
            if(customProps.Count == 0) return orig;

            PropertyDescriptor[] props = new PropertyDescriptor[orig.Count + customProps.Count];
            orig.CopyTo(props, 0);
            int i = orig.Count;
            foreach (var prop in customProps)
            {
                props[i++] = new SimpleDescriptor(prop.Item1, prop.Item2);
            }
            return new PropertyDescriptorCollection(props);
        }
    }
    class SimpleDescriptor : PropertyDescriptor
    {
        private readonly Type type;
        public SimpleDescriptor(string name, Type type)
            : base(name, new Attribute[0])
        {
            this.type = type;
        }
        public override Type PropertyType { get { return type;} }
        public override bool SupportsChangeEvents { get { return false; } }
        public override void ResetValue(object component) { SetValue(component, null); }
        public override bool CanResetValue(object component) { return true; }
        public override bool ShouldSerializeValue(object component) { return GetValue(component) != null; }
        public override bool IsReadOnly { get { return false; } }
        public override Type ComponentType { get { return typeof(Foo); } }
        public override object GetValue(object component) { return ((Foo)component).GetExtraValue(Name); }
        public override void SetValue(object component, object value) { ((Foo)component).SetExtraValue(Name, value); }
        public override string Category { get { return "Extra values"; } }
    }
}
[TypeConverter(typeof(FooConverter))]
public class Foo
{
    Dictionary<string, object> extraValues;
    internal object GetExtraValue(string name)
    {
        object value;
        if (extraValues == null || !extraValues.TryGetValue(name, out value)) value = null;
        return value;
    }
    internal void SetExtraValue(string name, object value)
    {
        if (extraValues == null && value != null) extraValues = new Dictionary<string, object>(StringComparer.InvariantCultureIgnoreCase);
        if (value == null) extraValues.Remove(name);
        else extraValues[name] = value;
    }
    public int Id { get; set; }
    public string Name { get; set; }
}