LINQ Expressions in xUnit test

Question

How can I simulate this part of my code in xUnit

    var updateCost =
            await _costOfConsomption.DeferredWhere(x => x.Id == model.CostOfConsomptionId).FirstOrDefaultAsync() ??
            throw new
                NotFoundException();

this the DeferredWhere and FirstOrDefaultAsync must be simulated

Answer 1

We are not simulating the results of LINQ extension methods.

If your goal is to verify that NotFoundException is thrown when you didn't find the cost by id - then you need to encapsulate the search and simulate the output of that method.

Answer 2

FirstOrDefault is extension method, so it is static and cannot be mocked (well, at least easily, with just Moq).

And about mocking DeferredWhere, assuming that _costOfConsomption is of type ICostOfConsomption (interface), you can mock it using Moq library:

var costOfConsomptionMock = new Mock<ICostOfConsomption>();
costOfConsomptionMock
    .Setup(x => x.DeferredWhere(/* here you setup for what arguemnts mock will match */))
    .ReturnsAsync(/* here you setup DeferredWhere to return something or not */);

_costOfConsomption = costOfConsomptionMock.Object;