ASP.NET Core | Change Token

A change token in ASP.NET Core is a general-purpose, low-level building block used to track state changes. Let me break it down for you:

1. IChangeToken Interface:

   • The IChangeToken interface propagates notifications that a change has occurred.
   • It resides in the Microsoft.Extensions.Primitives namespace and is implicitly provided to ASP.NET Core apps via the Microsoft.Extensions.Primitives NuGet package.
   • Key properties:
     • ActiveChangeCallbacks: Indicates whether the token proactively raises callbacks. If set to false, a callback is never called, and the app must poll HasChanged for changes.
     • HasChanged: Receives a value that indicates if a change has occurred.
   • Additionally, it includes the RegisterChangeCallback(Action<Object>, Object) method, which registers a callback invoked when the token changes1.

2. ChangeToken Class:

   • The ChangeToken class is static and used to propagate notifications that a change has occurred.
   • It also resides in the Microsoft.Extensions.Primitives namespace and is implicitly provided to ASP.NET Core apps.
   • The ChangeToken.OnChange(Func<IChangeToken>, Action) method registers an action to call whenever the token changes:
     • Func<IChangeToken> produces the token.
     • The Action is called when the token changes.
     • The overload ChangeToken.OnChange<TState>(Func<IChangeToken>, Action<TState>, TState) takes an additional TState parameter passed into the token consumer action.
     • The OnChange method returns an IDisposable, and calling Dispose stops the token from listening for further changes and releases its resources.

3. Example Uses of Change Tokens:

   • Change tokens are used in several areas of ASP.NET Core:
     • File Monitoring: For monitoring changes to files, the IFileProvider’s Watch method creates an IChangeToken for specified files or folders.
     • Cache Evictions: IChangeTokens can be added to cache entries to trigger cache evictions when changes occur.
     • TOptions Changes: The default OptionsMonitor<TOptions> implementation of IOptionsMonitor<TOptions> accepts one or more IOptionsChangeTokenSource<TOptions> instances. Each instance returns an IChangeToken for tracking options changes1.

In summary, change tokens provide a flexible mechanism for tracking changes in various scenarios within ASP.NET Core applications. They allow efficient handling of state changes without unnecessary polling or resource consumption