Sample test code using EF Core In-Memory provider.

EFCoreInMemoryTest.cs

[Fact]
public void GetClients_WhenCalledWithInMemoryDataContext_ReturnsExpectedResult()
{
    var inMemoryDataContextOptions = new DbContextOptionsBuilder<EventDataContext>()
        .UseInMemoryDatabase(databaseName: "Test_With_In_Memory_Database")
        .Options;

    // NOTE: Because we will need to assert against known data, 
    // we need to seed the in-memory test database
    // with the same context options as the unit test      
    CreateTestClient(inMemoryDataContextOptions);
    var eventDataContext = new EventDataContext(inMemoryDataContextOptions);
    var controller = new ClientsController(eventDataContext);

    // NOTE: As a side note, in a real world test you will ideally have a specific
    // Assert per test, but for sample purposes multiple asserts are used here.
    OkObjectResult okResult = Assert.IsType<OkObjectResult>(controller.GetClients());
    List<Client> returnSession = Assert.IsType<List<Client>>(okResult.Value);
    Assert.True(returnSession.FirstOrDefault().FirstName == "first");
}