Bring object through interface in order to mock it - unit-testing

Question

I am at a somewhat of a crossroads with the following issue. I have, let's say, the following class
public class Foo
{
internal Bar bar { get; }
internal Foo(Bar bar)
{
this.bar = bar;
}
}
and I am trying to a Unit test of it using XUnit and Moq. The initial test I created was
[Theory]
[AutoMockData]
internal void CtorShouldCreateInstance(Bar bar)
{
var sut = new Foo(bar);
Assert.NotNull(sut);
}
which was a valid unit test, but by passing Bar as a parameter it bring all it's dependencies making it as a concrete type. The suggestion from my colleague was to bring the Bar object through an Interface and Mock the interface but I am not sure how to do it.
I've thought of making the interface, a method to CreateBar with its required parameters, inherit the interface to the Foo class, implement the method and then add it to the unit test, but I've wanted to get a clarification or approval as I am not sure this is the correct way.
Thanks in advance for all the help!

Answer 1

You have to derive Bar from IBar, so then you can Mock it easily with Mocking framework like FakeItEasy, here is an example :
public interface IBar
{
string SayHello();
}
public class Bar : IBar
{
public string SayHello()
{
// A complex work to retrieve data from external resource
return "Hello";
}
}
public class Foo
{
private readonly IBar _bar;
public Foo(IBar bar)
{
_bar = bar;
}
public string Greeting()
{
return _bar.SayHello();
}
}
Now you can mock IBar to return your desired result :
[TestFixture]
public class FooTests
{
[Test]
public void Bar_ShouldReturn_HiInsteadOfHello()
{
IBar fakeBar = A.Fake<IBar>();
A.CallTo(() => fakeBar.SayHello()).Returns("Hi");
Foo sut = new Foo(fakeBar);
Assert.AreEqual(sut.Greeting(), "Hi");
}
}
* Side note: You can also Mock your method directly without deriving that from an interface by making that virtual, but as a best practice it's better to use interfaces instead :
public class Bar
{
public virtual string SayHello()
{
// A complex work to retrieve data from external resource
return "Hello";
}
}
public class Foo
{
private readonly Bar _bar;
public Foo(Bar bar)
{
_bar = bar;
}
public string Greeting()
{
return _bar.SayHello();
}
}
[TestFixture]
public class FooTests
{
[Test]
public void Bar_ShouldReturn_HiInsteadOfHello()
{
Bar fakeBar = A.Fake<Bar>();
A.CallTo(() => fakeBar.SayHello()).Returns("Hi");
Foo sut = new Foo(fakeBar);
Assert.AreEqual(sut.Greeting(), "Hi");
}
}

Answer 2

Apparently all I had to do was to pass the IBar interface as the parameter instead of the normal Bar class, because AutoMockData took care of everything.
So yeah the answer was to make the Foo class
public class Foo
{
internal IBar bar { get; }
internal Foo(Bar bar)
{
this.bar = bar;
}
}
and the test becomes
[Theory]
[AutoMockData]
internal void CtorShouldCreateInstance(IBar bar)
{
var sut = new Foo(bar);
Assert.NotNull(sut);
}
Thanks everyone for all the help!