Now that you know what extended properties are and how they are declared, let’s look at how you use these extended properties with Exchange Web Services methods.
First, we show how to create a message with extended properties by using CreateItem from Chapter 5. We will use this message in our ongoing extended property discussions. Because you are setting the values for these properties, you need to use the ExtendedProperty container element that contains both the metadata (the property in question) and the data that you wish to set. Example 13-9 sets two different properties.
Example 13-9. Creating an item with two extended properties
<CreateItem xmlns=".../messages" xmlns:t=".../types" MessageDisposition="SaveOnly"> <Items> ...