File snapshots or inline snapshots

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Two packages implement snapshot testing. They serialize values the same way, but store the expected value in different places:

Meziantou.Framework.SnapshotTesting (this section) stores it in a file next to the test.
Meziantou.Framework.InlineSnapshotTesting stores it in the source code of the test, as a string argument of the call, and rewrites that string when the snapshot is updated.

var user = new { Name = "John", Age = 42 };

// Meziantou.Framework.SnapshotTesting: the expected value is a file next to the test
Snapshot.Validate(user); // __snapshots__/SnapshotTests_DocumentationExample_FileAndInlineSnapshots.verified.txt

// Meziantou.Framework.InlineSnapshotTesting: the expected value is in the source code of the test
InlineSnapshot.Validate(user, """
    Name: John
    Age: 42
    """);

Differences

	File snapshots	Inline snapshots
Assertion	`Snapshot.Validate(value)`	`InlineSnapshot.Validate(value, "expected")`
Expected value	A `.verified.*` file in `__snapshots__`	A string literal in the test
Formats	Text, and any binary format: images, PDF documents, generated `.cs` files, ...	Text only
Several files per assertion	Yes, such as the pages of a PDF document	No
Serialization	Several serializers chosen by the type of the value; extensions for images, PDF and Roslyn	One serializer: human-readable, JSON or your own
Comparison	A comparer per format: text, bytes, pixels, ...	One comparer for the text
Snapshot name	Built from the test; must be unique	None: the snapshot is where the call is
Failed assertion	Writes an `.actual` file	Shows the difference in the failure message
Approval	Approval tool, update strategy or merge tool	Update strategy or merge tool, which edit the source file
Settings	SnapshotSettings	`InlineSnapshotSettings`
Environment variables	`SNAPSHOTTESTING_STRATEGY`, `SNAPSHOTTESTING_AUTODETECT_CONTINUOUS_ENVIRONMENT`	`INLINESNAPSHOTTESTING_STRATEGY`, `INLINESNAPSHOTTESTING_AUTODETECT_CONTINUOUS_ENVIRONMENT`

Both packages share the same update strategies (Disallow by default, Overwrite, OverwriteWithoutFailure, MergeTool, MergeToolSync), the same merge tool detection, the same line scrubbers, and never update snapshots on a build server, in a continuous testing runner or in an LLM agent.

Which one to choose

Choose inline snapshots when the serialized value is short: a few lines that you want to read next to the code that produces them, without opening another file. The test documents the expected result by itself, and a code review shows the code and its result together.

Choose file snapshots when:

the value is large, such as an API response, a generated file or a long report: a file keeps the test readable;
the value is binary or has a format of its own: an image, a PDF document, an SVG, JSON or C# you want to open with the right editor;
an assertion produces several files, such as the output of a source generator;
you want to review new snapshots before they are written, with the approval tool, rather than have the test run edit your source code.

You can use both packages in the same project, even in the same test.

Use both packages in the same file

Some types have the same name in both packages: SnapshotUpdateStrategy, MergeTool, Scrubber, and the scrub extension methods such as ScrubGuid. When a file imports both namespaces, these names are ambiguous. Import the namespace you use the most, and give the type you need from the other one an alias, such as using InlineSnapshot = Meziantou.Framework.InlineSnapshotTesting.InlineSnapshot;.