Snapshot Testing

What the source does

• FilesystemNone observedNo reads or writes in the source
• NetworkNone observedNo outbound calls in the source
• ShellNone observedNo commands run in the source
• DependenciesPinnedNothing floats
• SecretsNone foundNo keys in source

Five audit layers

Gates applied: no behavioural pass.

CLAUDE.md

Audit 0.4.1 · source sha 871f73ffcae3 · 18 Oct 2026Report an issue or request a re-scan

• PublisherAaronontheweb GitHub userOwner of the repository this skill is read from
• Source folderAaronontheweb/dotnet-skills/skills/snapshot-testingThe exact folder we audited
• LicenceMITAs the repository states it
• Commit pinned for this verdict871f73ffcae3A new audit runs when the source changes
• Repository statusNot archivedLast push 7 days ago
• Sandboxed runNot runSkills are read, not executed; layer L4 is skipped

Its description: Use Verify for snapshot testing in .NET. Approve API surfaces, HTTP responses, rendered emails, and serialized outputs. Detect unintended changes through human-reviewed baseline files.

---
name: snapshot-testing
description: Use Verify for snapshot testing in .NET. Approve API surfaces, HTTP responses, rendered emails, and serialized outputs. Detect unintended changes through human-reviewed baseline files.
user-invocable: false
---

# 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

---

## 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.

---

## Installation

### Add Verify Package

```bash
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:

```csharp
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);
    }
}
```

---

## Basic Usage

### Simple Object Verification

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

### String/HTML Verification

```csharp
[Fact]
public async Task VerifyRenderedEmail()
{
    var html = await _emailRenderer.RenderAsync("Welcome", new { Name = "John" });

    // Use extension parameter for proper file naming
    await Verify(html, extension: "html");
}
```

Creates `VerifyRenderedEmail.verified.html` - viewable in browser.

---

## Email Template Testing

Use Verify to catch unintended changes in rendered email templates:

```csharp
[Fact]
public async Task UserSignupInvitation_RendersCorrectly()
{
    var renderer = _services.GetRequiredService<IMjmlTemplateRenderer>();

    var variables = new Dictionary<string, string>
    {
        { "OrganizationName", "Acme Corporation" },
        { "InviteeName", "John Doe" },
        { "InviterName", "Jane Admin" },
        { "InvitationLink", "https://example.com/invite/abc123" },
        { "ExpirationDate", "December 31, 2025" }
    };

    var html = await renderer.RenderTemplateAsync(
        "UserInvitations/UserSignupInvitation",
        variables);

    await Verify(html, extension: "html");
}
```

**Benefits for email testing:**
- Catches CSS/layout regressions
- Detects broken template variables
- Visual review in diff tool
- Version control tracks email changes

---

## API Surface Approval

Prevent accidental breaking changes to public APIs:

```csharp
[Fact]
public Task ApprovePublicApi()
{
    var assembly = typeof(MyLibrary.PublicClass).Assembly;

    var publicApi = assembly.GetExportedTypes()
        .OrderBy(t => t.FullName)
        .Select(t => new
        {
            Type = t.FullName,
            Members = t.GetMembers(BindingFlags.Public | BindingFlags.Instance | BindingFlags.Static)
                .Where(m => m.DeclaringType == t)
                .OrderBy(m => m.Name)
                .Select(m => m.ToString())
        });

    return Verify(publicApi);
}
```

Or use the dedicated ApiApprover package:

```bash
dotnet add package PublicApiGenerator
dotnet add package Verify.Xunit
```

```csharp
[Fact]
public Task ApproveApi()
{
    var api = typeof(MyPublicClass).Assembly.GeneratePublicApi();
    return Verify(api);
}
```

Creates `.verified.txt` with full API surface - any change requires explicit approval.

---

## HTTP Response Testing

```csharp
[Fact]
public async Task GetUser_ReturnsExpectedResponse()
{
    var client = _factory.CreateClient();

    var response = await client.GetAsync("/api/users/123");

    // Verify status, headers, and body together
    await Verify(new
    {
        StatusCode = response.StatusCode,
        Headers = response.Headers
            .Where(h => h.Key.StartsWith("X-"))  // Custom headers only
            .ToDictionary(h => h.Key, h => h.Value.First()),
        Body = await response.Content.ReadAsStringAsync()
    });
}
```

---

## Scrubbing Dynamic Values

Handle timestamps, GUIDs, and other dynamic content:

```csharp
[Fact]
public Task VerifyOrder()
{
    var order = new Order
    {
        Id = Guid.NewGuid(),  // Different every run
        CreatedAt = DateTime.UtcNow,  // Different every run
        Total = 99.99m
    };

    return Verify(order)
        .ScrubMember("Id")  // Replace with placeholder
        .ScrubMember("CreatedAt");
}
```

Output:
```json
{
  Id: Guid_1,
  CreatedAt: DateTime_1,
  Total: 99.99
}
```

### Global Scrubbing

Configure in `ModuleInitializer`:

```csharp
[ModuleInitializer]
public static void Init()
{
    VerifierSettings.ScrubMembersWithType<DateTime>();
    VerifierSettings.ScrubMembersWithType<DateTimeOffset>();
    VerifierSettings.ScrubMembersWithType<Guid>();

    // Scrub specific patterns
    VerifierSettings.AddScrubber(s =>
        Regex.Replace(s, @"token=[a-zA-Z0-9]+", "token=SCRUBBED"));
}
```

---

## File Organi

Excerpt, first 6,000 characters of SKILL.md, verbatim from the audited commit. It is text from someone else’s repository, quoted so you can judge it before your agent reads it.

git clone https://github.com/Aaronontheweb/dotnet-skills
git -C dotnet-skills checkout 871f73ffcae3

Works in

1. 2026-10-18
   ✓SAFE B 89 / 100

   first audit · source 871f73ffcae3

Questions about Snapshot Testing

• github.com/Aaronontheweb/dotnet-skills/tree/master/skills/snapsh — the audited source at commit 871f73ffcae3 and every finding aboveObserved · 18 Oct 2026

Spotted something wrong? Report a correction · Methodology