Cannot set Document properties from C#

Using X8 and Visual Studio 2015 Community Edition.

Sample code below gives a compile error of - "Error    CS1955    Non-invocable member 'IVGDocument.Properties' cannot be used like a method."

private void corelApp_WindowActivate(Corel.Interop.VGCore.Document doc, Corel.Interop.VGCore.Window window)
{                        
    if (!doc.Properties.Exists("Property Name", 1))
    {                    
        doc.Properties("Property Name", 1) = "Value";
    }

}

Any ideas pls.

Regards

Steve