How to mock Automapper ProjectTo<> method in Unit Tests?

Question

I am using the ProjectTo<T> method to get data from the DB.

names = _mapper.ProjectTo<ItemNameDto>(query).ToList();

In the unit tests I would like to mock this method to return a specific collection.

Based on the second answer to this question - I figured out I need to pass all parameters to the setup.

When I pass null to the second parameter - the setup is ok, but when I pass null to the third one - the setup does not return the collection I want.

What value should I pass to the Expression parameter in this particular case? I really wouldn't like to leave it with It.IsAny<> because it seems too broad to me. I would like to write a setup which reflects exactly my use case.

_mapperMock
    .Setup(c => c.ProjectTo(
        It.Is<IQueryable<Item>>(x => x.HaveTheSameElements(filteredItems)),
        It.IsAny<object>(),
        It.IsAny<Expression<Func<ItemNameDto, object>>[]>()))
    .Returns(filteredItemNameDtos.AsQueryable());

This is the method signature I would like to setup.

IQueryable<TDestination> ProjectTo<TDestination>(IQueryable source, object parameters = null, params Expression<Func<TDestination, object>>[] membersToExpand);

Answer 1

To mock a call to ProjectTo where no membersToExpand are provided, you should use It.Is :

_mapperMock
    .Setup(c => c.ProjectTo(
        It.Is<IQueryable<Item>>(x => x.HaveTheSameElements(filteredItems)),
        It.IsAny<object>(),
        It.Is<Expression<Func<ItemNameDto, object>>[]>(x => x.Length == 0)))
    .Returns(filteredItemNameDtos.AsQueryable());

x => x.Length == 0 indicates an empty array, which the framework instantiates when no params have been passed.