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   How to: Access Data in SQL Query Results
   How to: Bind an XPCollection to a LookUp
   How to: Bind an XPCollection to Standard Controls
   How to: Bind an XPCollection to the DataGrid
   How to: Bind an XPCollection to the Grid
   How to: Build Complex Criteria
   How to: Build Simple Criteria
   How to: Change Inheritance Mapping
   How to: Connect to a SQL Server
   How to: Connect to SQL Azure
   How to: Connect XPO to a Database Other than MS SQL or MS Access
   How to: Control Automatic Saving of Objects when Editing in a Grid
   How to: Create an Aggregated Object
   How to: Delay Property Loading
   How to: Filter By DateTime Values
   How to: Filter Persistent Objects by Type
   How to: Handle Persistent Object Initialization
   How to: Implement a Custom Criteria Language Function Operator
   How to: Implement a Full-Text Search
   How to: Implement Custom Functions and Criteria in LINQ to XPO
   How to: Implement Many-to-Many Relationships
   How to: Implement One-to-One Relationships
   How to: Link Classes Located in Different Assemblies
   How to: Make a Class or Property Non-Persistent
   How to: Make XPCollection Create Objects of Specific Type when They are Created via Bound Controls
   How to: Map to Custom Tables (Views) and Columns
   How to: Obtain a Collection of Persistent Objects by a Set of Their IDs
   How to: Obtain a Collection of Persistent Objects for Processing
   How to: Persist a BLOB Field
   How to: Prevent Changes in a Persistent Object from being Automatically Committed
   How to: Restore Deleted Objects
   How to: Use Pageable Collections
   How to: Use Read-Only Persistent Properties
   How to: Use the ContainsOperator for Objects in a Many-to-Many Relationship
   How to: Validate an Object
   How to: Work with Structures
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How to: Use Read-Only Persistent Properties

In this document, you will learn how to define a read-only persistent property of a persistent object. The value of this property is set only once when the object is created. Thereafter, it is never changed. Generally, XPO stores only writable properties, but in certain scenarios, you may need to store the values of read-only properties in the database.

First, a property without a "setter" method in a persistent class must be created. By default, this property is non-persistent and its value can be stored in a private field. Private fields are also non-persistent members of an XPObject. To mark it as persistent, add a PersistentAttribute to the field with the name of the read-only property. This name will be used as the column's name in the database table that corresponds to the persistent object. To avoid data store overhead, associate the public property with the persistent field using the PersistentAliasAttribute, as shown in the following code snippet.

Note

The property is marked with the PersistentAliasAttribute, to be able to use it in filtering, searching, and inclusion in XPBaseCollection.DisplayableProperties.

Another useful example of using read-only persistent properties is saving the object's creation date.

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