I created a class to use as a property type in my template. When used as template property:
<#@ property name="TestProp" type="MyPropType"#>
everything is fine. But if I use it as extended property:
<#@ extended name="TestProp" type="MyPropType" owner="Class" #>
it doesn't work. I can see the property in property inspector, but it's not functional.

Example property type:

[TypeConverter(typeof(ExpandableObjectConverter)), DataContract]
public class MyPropType: NotifiedPropertyObjectBase
{

......
[Browsable(true), DefaultValue(true), RefreshProperties(RefreshProperties.Repaint), DisplayName("Write"), Description("Read.")]
public bool Write { get; set; }

....