Updating snapshots

When a snapshot does not match, the test fails and the failure message shows the difference between the expected and the actual value. A missing snapshot (null) is not shown as an empty line: a note at the end of the message says which value is null, so it cannot be confused with an empty string.

InlineSnapshotSettings.SnapshotUpdateStrategy decides whether the library also updates the snapshot in your source code:

With Disallow, copy the actual value from the failure message into the test, or run the tests again with another strategy. Whatever the strategy, 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).

Choose a strategy for a run

Set the INLINESNAPSHOTTESTING_STRATEGY environment variable to the name of a strategy, ignoring case (Overwrite, mergetool, OVERWRITEWITHOUTFAILURE). It sets the default value of SnapshotUpdateStrategy; a strategy set in code takes precedence. A value that is not the name of a strategy is ignored, and the failure message says so.

INLINESNAPSHOTTESTING_STRATEGY=Overwrite dotnet test

Choose a strategy in code

To change the strategy of every test, set it in a module initializer (see Settings):

InlineSnapshotSettings.Default = InlineSnapshotSettings.Default with
{
    SnapshotUpdateStrategy = SnapshotUpdateStrategy.MergeTool,
};

To change it for one assertion:

InlineSnapshot
    .WithSettings(settings => settings.SnapshotUpdateStrategy = SnapshotUpdateStrategy.Disallow)
    .Validate(new { Name = "meziantou" }, "Name: meziantou");

Which one to use

• Disallow never changes your source code during a test run: the right default for shared machines and automation, and for teams that prefer to edit snapshots by hand.
• MergeTool and MergeToolSync suit an interactive session in an IDE: each change opens in your diff tool, and you decide. See Merge tools.
• Overwrite suits a change that updates many snapshots on purpose: run the tests once, review the diff of your source files in your version control tool, and run them again. The first run fails, so you cannot miss that snapshots changed.
• OverwriteWithoutFailure writes new snapshots in a single run. Use it for a run you review afterwards, never as a permanent setting: it hides every regression.

Rewrite every snapshot

ForceUpdateSnapshots rewrites the snapshots even when they match, for instance to apply new string formats or a new indentation to existing snapshots. Combine it with OverwriteWithoutFailure:

InlineSnapshotSettings.Default = InlineSnapshotSettings.Default with
{
    ForceUpdateSnapshots = true, // Override the snapshot even if the value matches the expected value
    SnapshotUpdateStrategy = SnapshotUpdateStrategy.OverwriteWithoutFailure,
};

Migrate from version 3

Since version 4, snapshot updates are disabled by default (Disallow). Version 3 opened a merge tool. To keep the behavior of version 3, set the strategy to MergeTool, as shown above, or to MergeToolSync to wait for the merge tool to close before the test continues.