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Best Practices for Getting and Setting Properties

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Keep in mind the following best practices recommendations for getting and setting values for properties:

  • Reference a property directly off the parent object to get and set explicit built-in properties of item objects, for example, MailItem.Subject.
  • Use ItemProperties and ItemProperty to enumerate explicit built-in properties and custom properties, and get and set custom properties for items (except for DocumentItem object).
  • Use UserProperties and UserProperty to enumerate, get and set custom properties for items (except for the DocumentItem object).
  • Use PropertyAccessor to get and set custom properties for the DocumentItem object, built-in item-level properties that are not exposed in the Outlook object model, or properties for the following objects: AddressEntry, AddressList, Attachment, ExchangeDistributionList, ExchangeUser, Folder, Recipient, and Store.
  • To get or set multiple custom properties, use the PropertyAccessor object instead of the UserProperties object for better performance.
  • To create or access custom properties, use the MAPI string namespace for convenience over the MAPI proptag or id namespace. Use the GUID of your add-in as the namespace GUID.
  • When referencing properties by namespaces, be aware that such references are case-sensitive. For example, while n:schemas:contacts:givenName is a valid namespace, urn:schemas:contacts:givenname is not.
  • To get or set multiple properties, use PropertyAccessor.GetProperties and PropertyAccessor.SetProperties, as opposed to repeated PropertyAccessor.GetProperty and PropertyAccessor.SetProperty, for better performance.
  • To have the CustomPropertyChange event fire when the value of an item-level custom property changes, the custom property must be in the item's UserProperties collection. An item-level property added implicitly by SetProperty or SetProperties does not automatically become part of the item's UserProperties collection. An explicit UserProperties.Add is required to include it.
  • To set for the first time a property created by the UserProperties.Add method, use the UserProperty.Value property instead of the SetProperties and SetProperty methods of the PropertyAccessor object.

This section describes the best practices for saving properties on an object:

  • For item objects, calling the item's Save method to save the item to the current folder also saves its properties on the item.
  • For non-item-level objects that don't have a Save method (AddressList, Folder, Recipient, and Store, calling PropertyAccessor.DeleteProperty, PropertyAccessor.DeleteProperties, SetProperty or SetPropertie will implicitly save the properties on the object.

This section describes the best practices for keeping type conversion simple when using the PropertyAccessor to get and set properties. For definitions of MAPI property types such as PT_SYSTIME, see Property Types.

  • Although most Outlook date-time values are stored in Coordinated Universal Time (UTC) format, there is no guarantee that all properties of the MAPI type _SYSTIME will always return UTC. Getting a PT_SYSTIME property will return a VT_DATE value. When setting a PT_SYSTIME property, ensure that you are setting the property as a UTC value rather than a local date-time value. The GetProperty, SetProperty, GetProerties, and SetProperties methods don't perform time zone conversion. Use the helper methods PropertyAccessor.LocalTimeToUTC and PropertyAccessor.UTCToLocalTime to perform explicit time zone conversion.
  • A multi-valued property (Microsoft Visual Basic type _ARRAY) is stored as a two-dimensional array that contains the same number of elements as are there are values in the property. Getting a multi-valued property will return a ARRAY value. When setting a multi-valued property, pass a two-dimensional array (VT_ARRAY) with one element for each value that you want to set for the property.
  • A binary property (MAPI type _BINARY) is stored as an array of bytes rather than a string. Getting a binary property will return a value of type ARRAY. The GetProperty, SetProperty, GetProperties, and SetProperties methods don't perform any conversion between binary array and string. Use the helper methods PropertyAccessor.BinaryToString and PropertyAccessor.StringToBinary to explicitly perform any conversion.
  • Certain MAPI property types, such as _OBJECT, are not supported by the PropertyAccessor. Attempting to get or set such properties will result in a "property operation not supported" error.
  • When getting or setting a property using a reference in the MAPI proptag namespace, make sure that the type specified in the proptag matches the underlying type of the property. Except for the case of a _STRING8 property where you can specify either a type of 001E or 001F in the proptag to get or set the property as a VT_BSTR, getting or setting a property does not involve any type coercion and an error will be returned if there is a type mismatch.
  • When setting a property, it may be less restrictive to use a property reference in the MAPI string namespace than one in the MAPI proptag namespace. Specifying the property in the MAPI string namespace does not strictly require the value to match the underlying type of the property. For example, you can pass a string value like _BSTR to set a date-time property such as PT_SYSTIME, and the type of the property becomes the type of the value, which is STR.

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