Jest Snapshot Dotnet

Simple snapshot testing with inspiration from amazing Jest library.

Installation

You can install it using Nuget.

How it works

If you are unfamiliar with snapshot testing, I recommend you to check Jest documentation.

This library works very similarly. When you run ShouldMatchSnapshot for the first time, it will generate JSON snapshot of the serialization of the object. If you run it second time, it will check if the JSON serialization of the object is the same as the saved JSON snapshot.

It is important to commit snapshots with your code to the git.

Updating snapshots

You can mass update snapshot (useful when you add new property to an object, change default value or remove a property). Simply set environment variable UPDATE to true and run the tests that you want to update. It will update all snapshots that have failed.

If you are using JetBrains Rider, go to Settings -> Build, Execution, Deployment -> Unit Testing -> Test Runner and set it there. Remember to unset the variable again.

Continuous Integration

Snapshots are not created if you are running tests inside Continuous Integration environment (Gitlab CI, Jenkins, Teamcity, Azure/AWS Pipelines etc.). Library detects CI environment by check CI environment variable. If it is set to true, test fails if snapshot is missing.

API

Extensions methods

ShouldMatchSnapshot

public static void ShouldMatchSnapshot(this object actual, string hint = "");

Example

var person = new Person
{
    Age = 13,
    DateOfBirth = new DateTime(2008, 7, 7),
    FirstName = "John",
    LastName = "Bam"
};

person.ShouldMatchSnapshot();

ShouldMatchInlineSnapshot

public static void ShouldMatchInlineSnapshot(this object actual, string inlineSnapshot);

Example

var person = new Person
{
    Age = 13,
    DateOfBirth = new DateTime(2008, 7, 7),
    FirstName = "John",
    LastName = "Bam"
};

person.ShouldMatchInlineSnapshot(@"
    {
        ""FirstName"": ""John"",
        ""LastName"": ""Bam"",
        ""DateOfBirth"": ""2008-07-07T00:00:00"",
        ""Age"": 13,    
    }"
);

ShouldMatchObject

public static void ShouldMatchObject(this object actual, object expected);

Example

var actual = new Person
{
    Age = 13,
    DateOfBirth = new DateTime(2008, 7, 7),
    FirstName = "John",
    LastName = "Bam"
};

var expected = new Person
{
    Age = 13,
    DateOfBirth = new DateTime(2008, 7, 7),
    FirstName = "John",
    LastName = "Bam"
};

actual.ShouldMatchObject(expected);

Methods

If you don't like extension methods, you can use static class JestAssert

ShouldMatchSnapshot

public static void ShouldMatchSnapshot(object actual, string hint = "");

Example

var person = new Person
{
    Age = 13,
    DateOfBirth = new DateTime(2008, 7, 7),
    FirstName = "John",
    LastName = "Bam"
};

JestAssert.ShouldMatchSnapshot(person);

ShouldMatchInlineSnapshot

public static void ShouldMatchInlineSnapshot(object actual, string inlineSnapshot);

Example

var person = new Person
{
    Age = 13,
    DateOfBirth = new DateTime(2008, 7, 7),
    FirstName = "John",
    LastName = "Bam"
};

JestAssert.ShouldMatchInlineSnapshot(person, @"
    {
        ""FirstName"": ""John"",
        ""LastName"": ""Bam"",
        ""DateOfBirth"": ""2008-07-07T00:00:00"",
        ""Age"": 13,    
    }"
);

ShouldMatchObject

public static void ShouldMatchObject(object actual, object expected);

Example

var actual = new Person
{
    Age = 13,
    DateOfBirth = new DateTime(2008, 7, 7),
    FirstName = "John",
    LastName = "Bam"
};

var expected = new Person
{
    Age = 13,
    DateOfBirth = new DateTime(2008, 7, 7),
    FirstName = "John",
    LastName = "Bam"
};

JestAssert.ShouldMatchObject(actual,expected);

Advanced

Excluding properties

If you want to exclude some properties from the diff, you can use SnapshotSettings class to specify your own

• diffing options (use SnapshotSettings.CreateDiffOptions)

Example:

SnapshotSettings.CreateDiffOptions = () => new JsonDiffOptions
{
    PropertyFilter = (s, context) => s != "LastName"
};

or pass JsonDiffOptions as optional argument

var actual = new Person
{
    Age = 13,
    DateOfBirth = new DateTime(2008, 7, 7),
    FirstName = "John",
    LastName = "Bam"
};

var expected = new Person
{
    Age = 13,
    DateOfBirth = new DateTime(2008, 7, 7),
    FirstName = "John",
    LastName = ""
};

// this does not throw an exception and the test completes successfully
// property "LastName" is ignored from the diff
JestAssert.ShouldMatchObject(actual,expected, new JsonDiffOptions
{
    PropertyFilter = (s, context) => s != "LastName"
});

Configuring directory and file extensions

If you need to configure it, you can use SnapshotSettings class to specify your own

• extension instead of .snap (use SnapshotSettings.SnapshotExtension)
• directory instead of __snapshots__ (use SnapshotSettings.SnapshotDirectory)
• function that generates directory, extension and filename (use SnapshotSettings.CreatePath)

Popular use is to change directory of the snapshot files. You can do it like this:

SnapshotSettings.SnapshotDirectory = "__custom__";

var testObject = new Person
{
    Age = 13,
    DateOfBirth = new DateTime(2008, 7, 7),
    FirstName = "John",
    LastName = "Bam"
};

JestAssert.ShouldMatchSnapshot(testObject);

// you can return it back using
SnapshotSettings.SnapshotDirectory = SnapshotSettings.DefaultSnapshotDirectory;

Configuring serialization

For serialization, I am using Json.NET. If you need to configure it, you can use SnapshotSettings class to specify your own

• JsonSerializer (use SnapshotSettings.CreateJsonSerializer)
• JTokenWriter (use SnapshotSettings.CreateJTokenWriter)
• StringWriter (use SnapshotSettings.CreateStringWriter)
• JsonTextWriter (use SnapshotSettings.CreateJsonTextWriter).

Change line endings to LF

Popular use is to change line ending of the .snap files. For example if you want to set line ending to Linux LF, you can do it like this:

SnapshotSettings.CreateStringWriter = () => new StringWriter(CultureInfo.InvariantCulture)
{
    NewLine = "\n"
};

var testObject = new Person
{
    Age = 13,
    DateOfBirth = new DateTime(2008, 7, 7),
    FirstName = "John",
    LastName = "Bam"
};

JestAssert.ShouldMatchSnapshot(testObject);

SnapshotSettings expects you define your own function that returns new configured instance.

Sort properties alphabetically

SnapshotSettings.CreateJsonSerializer = () =>
{
    var serializer = SnapshotSettings.DefaultCreateJsonSerializer();
    serializer.ContractResolver = new AlphabeticalPropertySortContractResolver();

    return serializer;
};

var testObject = new Person
{
    Age = 13,
    DateOfBirth = new DateTime(2008, 7, 7),
    FirstName = "John",
    LastName = "Bam"
};

JestAssert.ShouldMatchSnapshot(testObject);

For Newtonsoft.Json, the resolver can look something like this

public sealed class AlphabeticalPropertySortContractResolver : DefaultContractResolver
{
    protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)
    {
        var properties = base.CreateProperties(type, memberSerialization);

        return properties.OrderBy(p => p.PropertyName).ToList();
    }
}

Caveats

Dynamic objects

You cannot call neither extension nor JestAssert with dynamic object. You need to cast it to object (or real type).

Credits

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