Dotnet Verify Email Snapshots

Snapshot Testing Email Templates with Verify

Capture rendered email HTML and compare it against human-approved baselines. Verify generates .received.html files on first run, developers approve them as .verified.html files, and subsequent runs detect unintended template changes through diff comparison. Targets .NET 8+ with xUnit and MJML templates.

Acronyms: HTML (HyperText Markup Language), MJML (Mailjet Markup Language), CI (Continuous Integration), DI (Dependency Injection), GUID (Globally Unique Identifier).

When to Use This Skill

• Testing email template rendering to catch CSS and layout regressions automatically
• Validating MJML templates compile to expected HTML output across template variants
• Reviewing email template changes in code review through visual snapshot diffs
• Ensuring variable substitution renders correctly for each recipient scenario
• Verifying email composer output including subject line and metadata fields
• Catching unintended side effects when modifying shared email template partials

Related Skills

Core Principles

1. One Test Per Template Variant — Each email template variant gets its own snapshot test. Why: isolates regression sources so a failure points directly to the affected template.
2. Scrub Dynamic Values Globally — Replace timestamps, GUIDs, and tokens with stable placeholders in ModuleInitializer. Why: ensures snapshots produce identical results across runs and environments.
3. Human-Approved Baselines — Every .verified.html requires explicit developer review before acceptance. Why: prevents broken rendering from being silently committed as the new baseline.
4. Diff-Based Visual Review — Template changes surface through diff tools and browser preview, not silent assertion failures. Why: visual diffs make unintended CSS and layout changes immediately obvious.
5. Version Control Integration — Commit .verified.html files and ignore .received.html files. Why: baselines are reviewed in PRs and tracked in history alongside template source.

Values: 基礎と型の追求（Verify の「型」—承認ベースライン・スクラブ・差分検出—を守ることで、再現可能なメールテンプレート検証基盤を築く）, ニュートラルな視点（手動目視確認の偏りを排し、スナップショット差分による客観的なレンダリング品質基準を保つ）

Workflow: Email Template Snapshot Testing with Verify

Step 1: Install Verify and Configure Test Fixture

Install the Verify NuGet package and create a shared test fixture for email rendering. Why: centralized fixture configuration ensures consistent rendering across all email tests.

dotnet add package Verify.Xunit

using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;

public class EmailTestFixture : IAsyncLifetime
{
    public IServiceProvider Services { get; private set; } = null!;

    public async Task InitializeAsync()
    {
        var services = new ServiceCollection();

        services.AddSingleton<IConfiguration>(new ConfigurationBuilder()
            .AddInMemoryCollection(new Dictionary<string, string?>
            {
                ["SiteUrl"] = "https://example.com"
            })
            .Build());

        services.AddSingleton<IMjmlTemplateRenderer, MjmlTemplateRenderer>();

        Services = services.BuildServiceProvider();
        await Task.CompletedTask;
    }

    public Task DisposeAsync() => Task.CompletedTask;
}

Values: 基礎と型の追求（テストフィクスチャという初期化の「型」を最初に整えることで、全メールテストの一貫性を保証する）

Step 2: Write Basic Email Snapshot Tests

Return Verify(html, extension: "html") from test methods to generate snapshot files. Why: Verify captures rendered HTML and compares it against the approved baseline automatically.

// ✅ GOOD — descriptive test name becomes the snapshot file name
[Fact]
public async Task UserSignupInvitation_RendersCorrectly()
{
    var renderer = _services.GetRequiredService<IMjmlTemplateRenderer>();

    var variables = new Dictionary<string, string>
    {
        { "PreviewText", "You've been invited to join Acme Corp" },
        { "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");
}

First run creates UserSignupInvitation_RendersCorrectly.received.html. Approve it to create UserSignupInvitation_RendersCorrectly.verified.html.

Values: 温故知新（テストの命名規則という「過去の知恵」を、スナップショットファイル名の自動生成という新技術と結びつける）

Step 3: Test Each Template Variant

Create dedicated tests for each email template to catch variant-specific regressions. Why: shared helper methods reduce boilerplate while keeping each variant independently verifiable.

public class EmailTemplateSnapshotTests : IClassFixture<EmailTestFixture>
{
    private readonly IMjmlTemplateRenderer _renderer;

    public EmailTemplateSnapshotTests(EmailTestFixture fixture)
    {
        _renderer = fixture.Services.GetRequiredService<IMjmlTemplateRenderer>();
    }

    [Fact]
    public async Task WelcomeEmail_NewUser() =>
        await VerifyTemplate("Welcome/NewUser", new Dictionary<string, string>
        {
            { "UserName", "John Doe" },
            { "LoginUrl", "https://example.com/login" }
        });

    [Fact]
    public async Task PasswordReset() =>
        await VerifyTemplate("PasswordReset/PasswordReset", new Dictionary<string, string>
        {
            { "UserName", "John Doe" },
            { "ResetLink", "https://example.com/reset/abc123" },
            { "ExpirationMinutes", "30" }
        });

    [Fact]
    public async Task PaymentReceipt() =>
        await VerifyTemplate("Billing/PaymentReceipt", new Dictionary<string, string>
        {
            { "UserName", "John Doe" },
            { "Amount", "$10.00" },
            { "InvoiceNumber", "INV-2025-001" },
            { "Date", "January 15, 2025" }
        });

    private async Task VerifyTemplate(
        string templateName, Dictionary<string, string> variables)
    {
        var html = await _renderer.RenderTemplateAsync(templateName, variables);
        await Verify(html, extension: "html")
            .UseMethodName(templateName.Replace("/", "_"));
    }
}

Values: 成長の複利（一つの VerifyTemplate ヘルパーを習得すれば、全テンプレートバリアントに横展開でき、知識が複利的に増幅する）

Step 4: Scrub Dynamic Values for Deterministic Snapshots

Replace non-deterministic data with stable placeholders. Why: timestamps and GUIDs change every run and cause false failures without scrubbing.

using System.Runtime.CompilerServices;
using System.Text.RegularExpressions;

public static class ModuleInitializer
{
    [ModuleInitializer]
    public static void Init()
    {
        // Store verified files in a dedicated directory
        VerifyBase.UseProjectRelativeDirectory("Snapshots");

        // Scrub dynamic values globally
        VerifierSettings.ScrubMembersWithType<DateTime>();
        VerifierSettings.ScrubMembersWithType<DateTimeOffset>();
        VerifierSettings.ScrubInlineGuids();

        // Scrub URL tokens and generated timestamps
        VerifierSettings.AddScrubber(s =>
            Regex.Replace(s, @"token=[a-zA-Z0-9]+", "token=SCRUBBED"));
    }
}

Per-test scrubbing for email-specific dynamic content:

[Fact]
public async Task EmailWithTimestamp_ScrubsDynamicValues()
{
    var html = await _renderer.RenderTemplateAsync("Welcome", variables);

    await Verify(html, extension: "html")
        .ScrubLinesContaining("Generated at:")
        .ScrubInlineGuids();
}

Values: 余白の設計（動的値をスクラブすることで、環境差異に対する余白を確保し、どの環境でも同一結果を保証する）

Step 5: Verify Composer Output and Configure CI

Test the full email composer including subject and metadata, and configure CI integration. Why: composer-level tests catch subject line and recipient errors; CI configuration prevents diff tool hangs.

[Fact]
public async Task SignupInvitation_ComposesCorrectEmail()
{
    var composer = _services.GetRequiredService<IUserEmailComposer>();

    var email = await composer.ComposeSignupInvitationAsync(
        recipientEmail: new EmailAddress("john@example.com"),
        recipientName: new PersonName("John Doe"),
        inviterName: new PersonName("Jane Admin"),
        organizationName: new OrganizationName("Acme Corp"),
        invitationUrl: new AbsoluteUri("https://example.com/invite/abc123"),
        expiresAt: new DateTimeOffset(2025, 12, 31, 0, 0, 0, TimeSpan.Zero));

    await Verify(new { email.To, email.Subject, email.HtmlBody });
}

CI configuration — disable diff tool and fail on missing baselines:

[ModuleInitializer]
public static void Init()
{
    if (Environment.GetEnvironmentVariable("CI") == "true")
    {
        VerifyDiffPlex.UseDiffPlex(OutputType.Minimal);
        DiffRunner.Disabled = true;
    }

    // Fail if no baseline exists
    VerifierSettings.ThrowOnMissingVerifiedFile();
}

Git configuration for snapshot files:

*.received.*

*.verified.html linguist-generated=true
*.verified.html diff=html

Values: 継続は力（CI パイプラインでのスナップショット検証を自動化し、継続的にリグレッションを検出する仕組みを構築する）

Good Practices

• ✅ Use descriptive test names — they become snapshot file names for easy identification
• ✅ Create one test per email template variant to isolate regression sources
• ✅ Use the extension: "html" parameter for rendered email output
• ✅ Scrub all dynamic values globally via ModuleInitializer to avoid per-test repetition
• ✅ Commit .verified.html files to source control and review changes in pull requests
• ✅ Use linguist-generated=true in .gitattributes to collapse snapshot diffs in PRs
• ✅ Disable diff tool launching in CI with DiffRunner.Disabled = true
• ✅ Open .received.html in a browser to preview actual email rendering

Common Pitfalls

1. Unscrubbed Dynamic Values — GUIDs and timestamps cause email snapshots to fail on every run. Fix: use ScrubInlineGuids() and ScrubMembersWithType<DateTime>() in ModuleInitializer.
2. Committing Received Files — Adding *.received.html to source control creates noise and merge conflicts. Fix: add *.received.* to .gitignore immediately.
3. Diff Tool Launches in CI — Verify tries to open a diff tool, causing CI to hang indefinitely. Fix: set DiffRunner.Disabled = true when CI environment variable is detected.
4. Generic Test Names — Names like Test1 produce unidentifiable snapshot files. Fix: use descriptive names like WelcomeEmail_NewUser_RendersCorrectly.
5. Missing Template Variant Coverage — Testing only one variant misses regressions in others. Fix: create separate tests for each template variant using a shared VerifyTemplate helper method.

Anti-Patterns

❌ Auto-Accepting Without Review → ✅ Review Diffs Before Accepting

// ❌ BAD — blindly accepting all changes without review
// verify accept --all
// This may commit broken rendering as the new baseline

// ✅ GOOD — review each change in diff tool or browser
// verify review
// Open .received.html in browser to verify rendering

Why: auto-accepting bypasses the human review that is the foundation of snapshot testing quality.

❌ Testing Only Happy Path → ✅ Cover Each Template Variant

// ❌ BAD — only testing one email template
[Fact] public Task WelcomeEmail_Renders() => VerifyTemplate("Welcome");

// ✅ GOOD — testing each variant independently
[Fact] public Task WelcomeEmail_NewUser() => VerifyTemplate("Welcome/NewUser");
[Fact] public Task WelcomeEmail_InvitedUser() => VerifyTemplate("Welcome/InvitedUser");
[Fact] public Task PasswordReset() => VerifyTemplate("PasswordReset");

Why: each template variant can regress independently; full coverage catches all rendering issues.

❌ Monolithic Email Snapshots → ✅ Focused Projections

// ❌ BAD — captures entire composer result with irrelevant metadata
await Verify(emailResult);

// ✅ GOOD — focuses on the fields that matter for email verification
await Verify(new { emailResult.Subject, emailResult.HtmlBody });

Why: monolithic snapshots break on any field change, making it hard to identify the actual rendering regression.

Quick Reference

Email Snapshot Testing Decision Table

Verify API for Email Testing

Review Workflow

Resources

• Verify GitHub — Main repository and documentation
• Verify.Xunit — xUnit integration package
• DiffEngine — Diff tool configuration
• Verify.DiffPlex — Inline diff for CI environments