> For the complete documentation index, see [llms.txt](https://docs.shouldly.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.shouldly.org/documentation/3to4.md).

# Upgrade 3 to 4

This is a work in progress. Please send a PR with any amendments.

Also see the [4.0 milestone](https://github.com/shouldly/shouldly/milestone/2?closed=1).

## Class constraint added to `ShouldNotBeNull`

In previous versions it was possible to assert that a non-nullable type `ShouldNotBeNull`, even though this logically makes no sense. For exmaple, the following would happily compile, but of course could never cause a test failure:

```
const long value = 1;

value.ShouldNotBeNull();
```

The `class` constraint was added to `ShouldNotBeNull` in v4, which means the above code will no longer compile. This is a good thing because it allows you to find and fix nonsensical tests in your codebase!

## `Func<string> customMessage` removed

All overloads that accepted a `Func<string> customMessage` have been changed to `string customMessage`.

## Diff tool functionality moved to DiffEngine

Diff tool functionality is now provided by [DiffEngine](https://github.com/VerifyTests/DiffEngine).

As such, the following APIs have been removed:

* `Shouldly.Configuration.DiffTool`
* `ShouldlyConfiguration.DiffTools`
* `Shouldly.Configuration.IShouldNotLaunchDiffTool`
* `Shouldly.Configuration.DiffToolConfiguration`
* `Shouldly.Configuration.KnownDiffTools`
* `Shouldly.Configuration.KnownDoNotLaunchStrategies`