snapshot-testing

Snapshot Testing with Verify

When to Use This Skill

Use snapshot testing when:

• Verifying rendered output (HTML emails, reports, generated code)
• Approving public API surfaces for breaking change detection
• Testing HTTP response bodies and headers
• Validating serialization output
• Catching unintended changes in complex objects

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What is Snapshot Testing?

Snapshot testing captures output and compares it against a human-approved baseline:

1. First run: Test generates a .received. file with actual output
2. Human review: Developer approves it, creating a .verified. file
3. Subsequent runs: Test compares output against .verified. file
4. Changes detected: Test fails, diff tool shows differences for review

This catches unintended changes while allowing intentional changes through explicit approval.

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Installation

Add Verify Package

dotnet add package Verify.Xunit
# or for other test frameworks:
dotnet add package Verify.NUnit
dotnet add package Verify.MSTest

Configure ModuleInitializer

Create a ModuleInitializer.cs in your test project:

using System.Runtime.CompilerServices;

public static class ModuleInitializer
{
    [ModuleInitializer]
    public static void Init()
    {
        // Use source-file-relative paths for verified files
        VerifyBase.UseProjectRelativeDirectory("Snapshots");

        // Configure diff tool (optional - auto-detected)
        // DiffTools.UseOrder(DiffTool.Rider, DiffTool.VisualStudioCode);
    }
}

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Basic Usage

Simple Object Verification

[Fact]
public Task VerifyUserDto()
{
    var user = new UserDto(
        Id: "user-123",
        Name: "John Doe",
        Email: "[email protected]",
        CreatedAt: new DateTime(2025, 1, 15));

    return Verify(user);
}

Creates VerifyUserDto.verified.txt:

{
  Id: user-123,
  Name: John Doe,
  Email: [email protected],
  CreatedAt: 2025-01-15T00:00:00
}

String/HTML Verification