Snapshot Testing Demo with Verify and NUnit

A minimal .NET 9 project demonstrating snapshot testing using Verify and NUnit.

What is Snapshot Testing?

Snapshot testing captures the output of your code and saves it as a verified snapshot file. Future test runs compare new output against these snapshots, making it easy to detect unexpected changes. It's particularly useful for testing complex object graphs where writing manual assertions would be tedious and error-prone.

Project Structure

snapshot-testing/
├── SnapshotDemo/                 # Production code
│   ├── Person.cs                 # Person and Address records
│   ├── ValidationResult.cs       # ValidationResult and ValidationMetadata records
│   └── PersonProcessor.cs        # System under test
└── SnapshotDemo.Tests/
    └── PersonProcessor/
        └── ValidatePersonTests/
            ├── Tests.cs          # Test cases
            ├── Tests.Basic.verified.txt
            ├── Tests.ExplicitScrubbing.verified.txt
            └── Tests.TestThatIncludesInput.verified.txt

Key Concepts Demonstrated

1. Basic Snapshot Testing

The Basic test shows the simplest use case - verify the entire output structure with minimal configuration.

2. Implicit Scrubbing

Verify automatically scrubs (replaces with placeholders) certain types like Guid and DateTime to make tests deterministic. These appear as Guid_1, DateTime_1, etc. in snapshots.

3. Explicit Scrubbing

The ExplicitScrubbing test demonstrates how to scrub specific properties (like Status) that aren't automatically scrubbed.

4. Input/Output Verification

The TestThatIncludesInput test shows a powerful pattern: by including both input and output in the snapshot with Verify(new { input, output }), you can:

• Make snapshots more readable by showing the transformation
• Implicitly assert that certain values (like PersonId) are preserved from input to output

When the same Guid appears in both input and output, Verify assigns it the same placeholder (e.g., Guid_2), effectively asserting equality without explicit code.

5. Nested Objects

Both input (Person with nested Address) and output (ValidationResult with nested ValidationMetadata) demonstrate that snapshot testing handles complex object graphs effortlessly.

Running the Tests

dotnet test

The first time you run a new test, Verify creates .received.txt files. Review these, and if correct, copy them to .verified.txt to approve the snapshot.

File Organization

• .verified.txt files are committed to Git and represent the approved baseline
• .received.txt files are generated during test runs and ignored by Git (via .gitignore)
• Snapshots are co-located with test files for easy discovery

Technologies

• .NET 9
• NUnit 4.4
• Verify.NUnit 31.0

Learning Resources

• Verify Documentation
• Verify Getting Started Wizard