verify-email-snapshots

Aaronontheweb/dotnet-skillsDeveloper ToolsAudit passedSnapshot b73a4d2647b2

Installs: 0

Summary

Snapshot test email templates using Verify to catch regressions. Validates rendered HTML output matches approved baseline. Works with MJML templates and any email renderer.

SKILL.md

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:

Visual - Small changes can break rendering across clients
Hard to unit test - Output is complex HTML, not simple values
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");
}

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

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", 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("/", "_"));
    }
}

[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
}

// 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()

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;
}

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

    var email = await composer.ComposeSignupInvitationAsync(
        recipientEmail: new EmailAddress("[email protected]"),
        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
    });
}

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

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

// 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: 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

verify-email-snapshots

Summary

SKILL.md

Snapshot Testing Email Templates with Verify

When to Use This Skill

Why Snapshot Test Emails?

Installation

Basic Email Snapshot Test

Reviewing Email Changes

1. Visual Diff Tool

2. Browser Preview

3. IDE Integration

Test Each Template Variant

Scrubbing Dynamic Values

Common Scrubbers

Test Fixture for Email Tests

Composer Snapshot Tests

CI Integration

Fail on Missing Baseline

Git Configuration

Best Practices

DO

DON'T

Workflow

Resources

Related skills