Question

I have a class named “CollectionPager” which has a collection inside that of type List. I have a method “RetrieveList” which takes a “CollectionPager” as input and populates “List” in side that. This method doesn’t return any value.

I need to mock a function which calls “RetrieveList”. However, since it doesn’t return any value, whatever input was fed into Mock is not taken into consideration and this collection(List) always has a count of 0.

Any possible ways to resolve this?

Answer 1

I guess you need the Callback (see also Moq quickstart) method to setup some logic when mocking a void function.

Here is sample test which demonstrates the usage:

var mock = new Mock<IRetrieveListService>();
mock.Setup(m => m.RetrieveList(It.IsAny<CollectionPager>()))
    .Callback<CollectionPager>(p =>
                                    {
                                        p.List.Add("testItem1");
                                        p.List.Add("testItem2");
                                    });
var sut = new OtherService(mock.Object);
sut.SomeMethodToTest();

Assuming your classes looks like something like these:

public class CollectionPager
{
    public CollectionPager()
    {
        List = new List<string>();
    }

    public List<string> List { get; private set; }
}

public interface IRetrieveListService
{
    void RetrieveList(CollectionPager pager);
}

public class RetrieveListService : IRetrieveListService
{
    public void RetrieveList(CollectionPager pager)
    {
        pager.List.Add("item1");
        pager.List.Add("item2");
    }
}

public class OtherService
{
    private readonly IRetrieveListService retrieveListService;

    public OtherService(IRetrieveListService retrieveListService)
    {
        this.retrieveListService = retrieveListService;
    }

    public void SomeMethodToTest()
    {
        var collectionPager = new CollectionPager();
        retrieveListService.RetrieveList(collectionPager);
        // in your test collectionPager.Item contains: testItem1, testItem2
    }
}