[Fact]
public void Example_Scenario_Failing_Test()
{
var dataParent = new DataParent()
{
Id = "123",
Value = "new",
Children = new List<DataChild>() {
new DataChild()
{
Id = "1234",
Value = "new"
}
}
};
var domainParent = dataParent.ToDomainParent();
var child = domainParent.Children.FirstOrDefault();
child.Value = "updated";
Assert.Equal("updated", child.Value); // True
Assert.Equal("updated", domainParent.Children.ElementAt(0).Value); // False
}We expect the objects to be the same and so both values should have been updated. What the hell is going on? Lets take a look at the ToDomainParent method.
public static DomainParent ToDomainParent(this DataParent dataParent)
{
var children = dataParent.Children.Select(x => new DomainChild
{
Id = x.Id,
Value = x.Value
});
return new DomainParent
{
Id = dataParent.Id,
Value = dataParent.Value,
Children = children
};
}Ok, seems reasonable enough but if instead of assigning Children = children we do this... Children = children.ToList() all our problems go away.
[Fact]
public void Example_Scenario_Passing_Test()
{
var dataParent = new DataParent()
{
Id = "123",
Value = "new",
Children = new List<DataChild>() {
new DataChild()
{
Id = "1234",
Value = "new"
}
}
};
var domainParent = dataParent.MapToDomainParentCorrectly();
var child = domainParent.Children.FirstOrDefault();
child.Value = "updated";
Assert.Equal("updated", child.Value); // True
Assert.Equal("updated", domainParent.Children.ElementAt(0).Value); // True
}Why? Well turns out that if you don't ToList the Select statement you end up with the type SelectListIterator<EFCoreTesting.DataChild, EFCoreTesting.DomainChild> which implements IEnumerable. This means that when we call Children.FirstOrDefault() we re-evaluate the Select statement. Which is why if you call FirstOrDefault without ToList multiple times and compare the objects returned they will not be equal.
[Fact]
public void Call_First_Or_Default_More_Than_Once_Objects_Shoule_Be_The_Same_Failing_Test()
{
var dataParent = new DataParent()
{
Id = "123",
Value = "new",
Children = new List<DataChild>() {
new DataChild()
{
Id = "1234",
Value = "new"
}
}
};
var domainParent = dataParent.ToDomainParent();
var child = domainParent.Children.FirstOrDefault();
var child2 = domainParent.Children.FirstOrDefault();
Assert.True(child.Equals(child2)); // False
}This is a really easy trap to fall into and can cause a number of difficult to diagnose issues. So, is there a nice way to remember that we need to call ToList ? Yes, we if we ensure we don't expose IEnumerable in our domain objects and instead expose ICollection, IReadonlyCollection , IList or IReadonlyList we will get compile time errors if we don't call ToList.
