About Read-Only Object Cache

Table of Contents

• Introduction
• Related Changes
• PageReference Changes

Introduction

Most API:s in EPiServer CMS returns read-only instances of objects, for example DataFactory.GetPage return a read-only PageData. To make changes to read-only instances a writable clone has to be created using the CreateWritableClone method.

The implementation has the following advantages:

• Reduce memory consumption. All threads serving web requests will get the same instance of an object, this effectively reduces the amount of short-lived objects being created.
• Improve performance. Returning shared read-only instances offer great performance.
• Cleaner architecture. This simplifies the implementation significantly, it is not possible to make changes to an instance of an object shared with other threads.

All classes that have the read-only support implements the IReadOnly<T> interface which is defined as follows:

public interface IReadOnly
    {
        void MakeReadOnly();
        bool IsReadOnly
        {
            get;
        }
    }

    public interface IReadOnly<T> : IReadOnly
    {
        T CreateWritableClone();
    }

To give some background to what is happening behind the scenes, the life cycle of a typical PageData object is as follows:

1. Create the PageData object as mutable (which means writeable).
2. Populate the PageData object with properties.
3. Call the Core.PageData.MakeReadOnly method to ensure that any contained objects are made read-only. From here on, the object is now immutable for the remainder of its lifetime.
4. Add the object to the cache.