Meziantou.Framework

Introduction

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Meziantou.Framework.InlineSnapshotTesting replaces long lists of assertions with a single call whose expected value is written in the test itself. InlineSnapshot.Validate serializes the value under test to readable text and compares it with the string you pass. When they differ, the test fails and shows the difference: either the change is a bug, or the snapshot must be updated, and the library can rewrite the string in your source file for you.

How it works

  1. Write InlineSnapshot.Validate(value) without an expected value, and run the test.
  2. The value is serialized, for instance FirstName: John and LastName: Doe on two lines. It does not match the missing expected value, so the test fails and shows the difference.
  3. Depending on the update strategy, the library writes the new snapshot in the source file, as a second argument of the call, or opens a merge tool to let you accept it. By default, it changes nothing and you copy the value from the failure message.
  4. From now on, the test compares the serialized value with that string, and fails when they differ.

The snapshot is part of the test: you read the expected result next to the code that produces it, and a code review shows both.

What you can do with it

  • Validate any object as readable text, or as JSON. See Serializers.
  • Make snapshots deterministic by scrubbing GUIDs, dates, user names and other unstable values. See Deterministic snapshots.
  • Control how snapshots are written in your code: quoted, verbatim or raw strings, indentation and line endings. See String formats and source files.
  • Write your own assertion methods that update the snapshot of their caller. See Helper methods.
  • Keep CI strict: snapshots are never updated on a build server, in a continuous testing runner or when the tests are run by an LLM agent. See Continuous integration.

All the types are in the Meziantou.Framework.InlineSnapshotTesting namespace, and the serializers in Meziantou.Framework.InlineSnapshotTesting.Serialization.

Inline snapshots or file snapshots

Meziantou.Framework.SnapshotTesting works the same way, but stores each snapshot in a file next to the test. Inline snapshots suit short text values you want to read in the test; file snapshots suit large values, binary files such as images and PDF documents, and generated code. See File snapshots or inline snapshots.

Next steps

  • Getting started: install the package and write your first inline snapshot test.
  • Settings: configure every test in one place, or a single assertion.
  • Updating snapshots: the update strategies and how the source file is rewritten.