verify-email-snapshots

name	verify-email-snapshots
description	Snapshot test email templates using Verify to catch regressions. Validates rendered HTML output matches approved baseline. Works with MJML templates and any email renderer. Use when this capability is needed.
metadata	{"author":"aaronontheweb"}

Snapshot Testing Email Templates with Verify

When to Use This Skill

Use this skill when:

• Testing email template rendering for regressions
• Validating MJML templates compile to expected HTML
• Reviewing email changes in code review (diffs are visual)
• Ensuring variable substitution works correctly

Related skills:

• aspnetcore/mjml-email-templates - MJML template authoring
• aspire/mailpit-integration - Test email delivery locally
• testing/snapshot-testing - General Verify patterns

---

Why Snapshot Test Emails?

Email templates are:

1. Visual - Small changes can break rendering across clients
2. Hard to unit test - Output is complex HTML, not simple values
3. Prone to regression - Template changes can have unintended effects

Snapshot testing captures the rendered HTML and fails when it changes unexpectedly.

---

Installation

dotnet add package Verify.Xunit  # or Verify.NUnit, Verify.MSTest

---

Basic Email Snapshot Test

[Fact]
public async Task UserSignupInvitation_RendersCorrectly()
{
    // Arrange
    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" }
    };

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

    // Assert
    await Verify(html, extension: "html");
}

This creates UserSignupInvitation_RendersCorrectly.verified.html on first run.

---

Reviewing Email Changes

When a template changes, the test fails with a diff. Review options:

1. Visual Diff Tool

# Configure diff tool (one-time)
dotnet tool install -g verify.tool
verify accept  # Accept all pending changes
verify review  # Open diff tool

2. Browser Preview

Open the .received.html file in a browser to see the actual rendering.

3. IDE Integration

Most IDEs show inline diffs for .verified.html vs .received.html files.

---

Test Each Template Variant

Create tests for each email template to catch regressions:

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 WelcomeEmail_InvitedUser() =>
        await VerifyTemplate("Welcome/InvitedUser", new Dictionary<string, string>
        {
            { "UserName", "John Doe" },
            { "InviterName", "Jane Admin" },
            { "OrganizationName", "Acme Corp" }
        });

    [Fact]
    public async Task PasswordReset() =>
        await VerifyTemplate("PasswordReset/PasswordReset",  Dictionary<, >
        {
            { ,  },
            { ,  },
            { ,  }
        });

    []
     =>
         VerifyTemplate(,  Dictionary<, >
        {
            { ,  },
            { ,  },
            { ,  },
            { ,  }
        });

    
    {
         html =  _renderer.RenderTemplateAsync(templateName, variables);
         Verify(html, extension: )
            .UseMethodName(templateName.Replace(, ));
    }
}

---

Scrubbing Dynamic Values

Some values change between test runs. Scrub them:

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

    await Verify(html, extension: "html")
        .ScrubLinesContaining("Generated at:")
        .ScrubInlineGuids();  // Scrubs GUIDs in URLs
}

Common Scrubbers

// Scrub dates
.ScrubLinesContaining("Date:")
.AddScrubber(s => Regex.Replace(s, @"\d{4}-\d{2}-\d{2}", "SCRUBBED-DATE"))

// Scrub URLs with tokens
.AddScrubber(s => Regex.Replace(s, @"token=[a-zA-Z0-9]+", "token=SCRUBBED"))

// Scrub GUIDs
.ScrubInlineGuids()

---

Test Fixture for Email Tests

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;
}

---

Composer Snapshot Tests

Test the full composer output including subject and metadata:

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

    var email = await composer.ComposeSignupInvitationAsync(
        recipientEmail: new EmailAddress("[e-mail]"),
        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));

    // Verify the full email object (subject, to, body)
    await Verify(new
    {
        email.To,
        email.Subject,
        HtmlBody = email.HtmlBody  // Will be stored as .html extension
    });
}

---

CI Integration

Fail on Missing Baseline

In CI, fail if no .verified.html file exists (prevents accidental acceptance):

// In test setup or ModuleInitializer
VerifierSettings.ThrowOnMissingVerifiedFile();

Git Configuration

Add to .gitattributes to improve diff handling:

*.verified.html linguist-language=HTML
*.verified.html diff=html

---

Best Practices

DO

// DO: Test each template variant
[Fact] Task WelcomeEmail_NewUser_RendersCorrectly()
[Fact] Task WelcomeEmail_InvitedUser_RendersCorrectly()

// DO: Use descriptive test names
[Fact] Task PaymentReceipt_WithRefund_ShowsRefundAmount()

// DO: Scrub dynamic values consistently
.ScrubLinesContaining("Generated at:")

// DO: Review diffs carefully before accepting
verify review

DON'T

// DON'T: Skip email testing
// DON'T: Auto-accept changes without review
verify accept --all  // Dangerous!

// DON'T: Test only happy path
// DON'T: Ignore snapshot test failures

---

Workflow

1. Create template - Write MJML template
2. Write test - Add snapshot test with sample variables
3. Run test - First run creates .verified.html
4. Review - Open in browser, verify rendering
5. Commit - Include .verified.html in source control
6. Iterate - Changes fail test, review diff, accept if correct

---

Resources

• Verify: https://github.com/VerifyTests/Verify
• Verify.Xunit: https://github.com/VerifyTests/Verify#xunit
• Diff Tools: https://github.com/VerifyTests/DiffEngine

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