Manupilation of extended properties with exchange 2007 [[http://www.infinitec.de/post/2007/10/20/Manipulation-of-extended-properties-
with-Exchange-2007-using-the-ExtendedPropertyType-type.aspx|link]]
<code>
 PathToExtendedFieldType propertyId = new PathToExtendedFieldType();
  propertyId.DistinguishedPropertySetId = 
DistinguishedPropertySetType.Appointment;
  propertyId.PropertyId = 0x820e;
  propertyId.PropertyType = MapiPropertyTypeType.SystemTime;
  propertyId.PropertyIdSpecified = true;
  propertyId.DistinguishedPropertySetIdSpecified = true;



 private static ExtendedPropertyType 
CreateExtendedProperty<T>(PathToExtendedFieldType id, IEnumerable<T> values)  
{
     ExtendedPropertyType result = newExtendedPropertyType();

     result.ExtendedFieldURI = id;
     result.Item = (values != null) ? 
newList<T>(values).ConvertAll<string>(delegate(T input) { return 
SerializeValue(input); }).ToArray() : null;

     return result;
 }

 private ExtendedPropertyType 
CreateExtendedProperty<T>(PathToExtendedFieldType id, T value)  {
     ExtendedPropertyType result = newExtendedPropertyType();
     result.ExtendedFieldURI = id;
     result.Item = SerializeValue(value);

     return result;
 }

 private static string SerializeValue<T>(T value)  {
     Type type;
     TypeConverter converter;
     object result;

     result = value;

     if (result == null) return null;

     //Unbox nullable types
     type = Nullable.GetUnderlyingType(typeof(T));
     if (type != null)
     {
         converter = new NullableConverter(typeof(T));
         result = converter.ConvertTo(value, type);
     }
     else type = typeof(T);

     if (type.IsEnum)
     {
         result = Convert.ChangeType(result, Enum.GetUnderlyingType(type));
         return result.ToString();
     }
     elseif (type == typeof(DateTime))
     {
         return XmlConvert.ToString(((DateTime)result).ToUniversalTime(), 
XmlDateTimeSerializationMode.Utc);
     }
     elseif (type == typeof(bool))
     {
         return ((bool)result) ? "1" : "0";
     }
     elseif (type == typeof(byte[]))
     {
         return Convert.ToBase64String((byte[])result);
     }
     else
     {
         return string.Format(CultureInfo.InvariantCulture, "{0}", result);
     }
 }
</code>