c# - Moq.Mock<T> - how to set up a method that takes an expression

Question

I am Mocking my repository interface and am not sure how to set up a method that takes an expression and returns an object? I am using Moq and NUnit.

Interface:

public interface IReadOnlyRepository : IDisposable
{
    IQueryable<T> All<T>() where T : class;
    T Single<T>(Expression<Func<T, bool>> expression) where T : class;
}

Test with IQueryable is already set up, but don't know how to set up the T Single:

private Moq.Mock<IReadOnlyRepository> _mockRepos;
private AdminController _controller;
[SetUp]
public void SetUp()
{
    var allPages = new List<Page>();
    for (var i = 0; i < 10; i++)
    {
        allPages.Add(new Page { Id = i, Title = "Page Title " + i, Slug = "Page-Title-" + i, Content = "Page " + i + " on page content." });
    }
    _mockRepos = new Moq.Mock<IReadOnlyRepository>();
    _mockRepos.Setup(x => x.All<Page>()).Returns(allPages.AsQueryable());
    //Not sure what to do here???
    _mockRepos.Setup(x => x.Single<Page>()
    //----
    _controller = new AdminController(_mockRepos.Object);
}

Answer 1

You can set it up like this:

_mockRepos.Setup(x => x.Single<Page>(It.IsAny<Expression<Func<Page, bool>>>()))//.Returns etc...;

However you are coming up against one of Moq's shortcomings. You would want to put an actual expression there instead of using It.IsAny, but Moq doesn't support setting up methods that take expressions with specific expressions (it's a difficult feature to implement). The difficulty comes from having to figure out whether two expressions are equivalent.

So in your test you can pass in any Expression<Func<Page,bool>> and it will pass back whatever you have setup the mock to return. The value of the test is a little diluted.