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.
[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:
[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:
[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);
}
[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
[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:
[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");
}
# Verify - ignore received files (only commit verified)
*.received.*
.gitattributes
# Treat verified files as generated (collapse in PR diffs)
*.verified.txt linguist-generated=true
*.verified.html linguist-generated=true
*.verified.json linguist-generated=true
CI/CD Integration
Fail on Missing Verified Files
[ModuleInitializer]
public static void Init()
{
// In CI, fail instead of launching diff tool
if (Environment.GetEnvironmentVariable("CI") == "true")
{
VerifyDiffPlex.UseDiffPlex(OutputType.Minimal);
DiffRunner.Disabled = true;
}
}
GitHub Actions
- name: Run tests
run: dotnet test
env:
CI: true
- name: Upload snapshots on failure
if: failure()
uses: actions/upload-artifact@v4
with:
name: snapshots
path: |
**/*.received.*
**/*.verified.*
When to Use Snapshot Testing
Scenario
Use Snapshot Testing?
Why
Rendered HTML/emails
Yes
Catches visual regressions
API surfaces
Yes
Prevents accidental breaks
Serialization output
Yes
Validates wire format
Complex object graphs
Yes
Easier than manual assertions
Simple value checks
No
Use regular assertions
Business logic
No
Use explicit assertions
Performance tests
No
Use benchmarks
Best Practices
DO
// Use descriptive test names - they become file names
[Fact]
public Task UserRegistration_WithValidData_ReturnsConfirmation()
// Scrub dynamic values consistently
VerifierSettings.ScrubMembersWithType<Guid>();
// Use extension parameter for non-text content
await Verify(html, extension: "html");
// Keep verified files in source control
git add *.verified.*
DON'T
// Don't verify random/dynamic data without scrubbing
var order = new Order { Id = Guid.NewGuid() }; // Fails every run!
await Verify(order);
// Don't commit .received files
git add *.received.* // Wrong!
// Don't use for simple assertions
await Verify(result.Count); // Just use Assert.Equal(5, result.Count)
Integration with MJML Email Testing
See the aspnetcore/transactional-emails skill for the complete pattern:
