EntityFramework Mocks

Introduction

This project was built in order to help developers to unit test code that depends on Entity Framework, without the need for any database, be it in-memory or "real".

Personal story: Since there was already 3 occasions that I implemented this bunch of classes in different projects in order to test my data access, I decided to create a nuget that I could reuse in all of those projects and in the next to come.

Nuget package

https://www.nuget.org/packages/CarlosInIt.EntityFramework.Mocks

Install-Package CarlosInIt.EntityFramework.Mocks

Features

This project proposes an implementation of a mock of DbContext. That mock proposes several features:

• Register empty DbSet
• Register a pre-filled DbSet
• Count the number of times SaveChanges or SaveChangesAsync was called
• Define the result that SaveChanges or SaveChangesAsync should return
• Support for async method calls on the DbSet
• Support for Find anf FindAsync method when a find predicate is provided

What is not supported at the moment:

• Entity state tracking
• Include method (for now calling it makes no difference)
• Identity column support
• Advanced scenarios like logical delete
• Simulate database exceptions

Quick start

You can create a DbContext mock by instantiating the Generic DbContextMock class while passing your real DbContext Type as the generic parameter

var dbContextMock = new DbContextMock<FleetContext>();

Then you will need to tell which DbSet you will use

dbContextMock.WithDbSet(c => c.Cars);

If you need the DbSet to be preloaded with data, you can use this overload instead

dbContextMock.WithDbSet(c => c.Cars, new[] { ... });

If you need the DbSet to be able to perform a Find or FindAsync, you need to specify how the id is resolved

dbContextMock.WithDbSet(c => c.Cars, new[] { ... }, (entity, keys)=>entity.Id == (int)keys[0]);

Now you can use the configured DbContext mock

var sut = new CarService(dbContextMock.Object);
// Use the service now...

If you need to check if SaveChanges or SaveChangesAsync was called (which you do if you are saving data to the database)

Assert.Equal(1, dbContextMock.SaveChangesCalls);
// or in case of async
Assert.Equal(1, dbContextMock.SaveChangesAsyncCalls);

You can also specify what SaveChanges and SaveChangesAsync should return

dbContextMock.WithCallToSaveChanges(numberYouWantToBeReturned);
// or in case of async
dbContextMock.WithCallToSaveChangesAsync(numberYouWantToBeReturned);

Here goes a full example of an update

public void SaveCarTest()
{
    // Arrange
    var existingCars = new[]
    {
        new Car { Id = 7, Brand = "VW", Model = "Polo", PlateNumber="AZERTY7" },
        new Car { Id = 8, Brand = "VW", Model = "Golf", PlateNumber="AZERTY8" },
        new Car { Id = 9, Brand = "VW", Model = "Passat", PlateNumber="AZERTY9" },
        new Car { Id = 10, Brand = "VW", Model = "Jetta", PlateNumber="AZERTY10" }
    };
    var dbContextMock = new DbContextMock<FleetContext>();
    dbContextMock.WithDbSet(c => c.Cars, existingCars, (car, keys) => car.Id == (int)keys[0]);
    var expectedCar = existingCars[new Random().Next(0, existingCars.Length - 1)];
    var updatedCar = new Car { Id = expectedCar.Id, Brand = "Skoda", Model = "Superb" };
    dbContextMock.WithCallToSaveChanges(1);

    // Act
    var sut = new CarService(dbContextMock.Object);
    sut.SaveCar(expectedCar.Id, updatedCar);

    // Assert
    Assert.Equal(updatedCar.Brand, expectedCar.Brand);
    Assert.Equal(updatedCar.Model, expectedCar.Model);
    Assert.Equal(1, dbContextMock.SaveChangesCalls);
}

More examples

A complete example of a service (depending on DbContext) being tested can be found in this repository: https://github.com/carlosinit/EntityFramework.Mocks/tree/master/CarlosInIt.EntityFramework.Mocks.Examples

Note: This examples use FluentAssertions and XUnit

What's next?

All the unsupported features described above

Remarks

• The implementation of this mock is based on the following Microsoft documentation: https://msdn.microsoft.com/en-us/library/dn314431(v=vs.113).aspx
• The package has a dependency to Castle.Core in order to allow more specific method call interception in the future.