Read-only object cache

This topic describes how to work with read-only object caching, and how this type of caching works behind the scene.

How it works

Most APIs in Episerver CMS return read-only instances of objects, such as DataFactory.GetPage which return a read-only PageData.

To make changes to read-only instances, create a writable clone with the CreateWritableClone method, which has the following advantages:

• Reduce memory consumption. Threads serving web requests get the same instance of an object, which effectively reduces the amount of short-lived objects that are created.
• Improve performance. Returning shared read-only instances offers better performance.
• Cleaner architecture. This simplifies the implementation because you cannot make changes to an instance of an object shared with other threads.

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();
    }

The lifecycle 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. The object is immutable for the remainder of its lifetime.
4. Add the object to the cache.