Table of Contents

Convention: Everything is Immutable

There are many programming paradigms. One that's become very important to us is immutable programming. This is strongly related to functional APIs

This means than objects inside 2sxc are never modified. Instead, they are copied and modified, and the original object is discarded.


var original = new { Name = "John", Age = 42 };
var modified = SomeService.ChangeAge(original, 43);

var areEqual = original.Age == modified.Age; // false

Use in DataSources

A common place where this affects Razor and WebApis is when using DataSources. Old data sources could do this:

var entityFilter = CreateSource<EntityIdFilter>();
entityFilter.EntityIds = "123,456";
var data = entityFilter.List;

This behavior still works on old data sources to ensure compatibility. But newer DataSources such as AppFiles will not allow this, and will throw an exception if you try to modify the properties. They are immutable. Instead, you must do this:

var appFiles = CreateSource<AppFiles>(options: new { OnlyFolders = true });
// The following would throw an error:
// appFiles.OnlyFolders = true;
var data = appFiles.List;

This new way is the only recommended way of doing this from now on.


As of March 2023, ca. 80% of all objects in 2sxc are immutable, we hope to reach 95% by end of 2023. We cannot reach 100% any time soon, because we must ensure compatibility with existing code.

  1. IEntity is internally 98% immutable - some details left to optimize
  2. DataSources can be immutable. Newer DataSources are immutable, and data sources created using Kit.Data are immutable by default.


  • First introductions in 2sxc 13
  • Spread out more in 2sxc 15, eg. IEntity is now immutable