How to use NSubstitute returns on a predicate argument?

Question

New to NSubstitute and having trouble mocking the returns for method calls that take a predicate. For example I have this in the main code

var currReport = this.ClientRepo.Get<Report>(x => x.Uid == reportUid).FirstOrDefault();

I want to do something like this in my test

var parentReport = new Report(){Uid = request.ParentReportUid, Name = "Test"};
this.clientRepository.Get(Arg.Is<Expression<Func<Report, bool>>>(expr => Lambda.Eq(expr, i => i.Uid == request.ParentReportUid))).Returns(new List<Report>() { parentReport }.ToArray());

This is not working. I have confirmed that request.ParentReportUid matches the reportUid in the actual method call. But still it returns a null. If I switch to an Arg.Any then it returns the report, like this

 this.clientRepository.Get(Arg.Any<Expression<Func<Report, bool>>>()).Returns(new List<Report>() { parentReport }.ToArray());

This is the signature of the actual method I am trying to mock.

 T[] Get<T>(System.Linq.Expressions.Expression<Func<T, bool>> predicate = null);

Please advise. Thanks

Answer 1

Try to use NSubstitute class Arg public static T Is<T>(Expression<Predicate<T>> predicate) method. You did not specifed what is type X in your predicate.

I have invested some time and there is a already solution for it. It's a Neleus.LambdaCompare Nuget Package. You can use Lambda.Eq method. I have tried it and it worked fine. In your example it should be something like:

this.Repo.Get<Report>(Arg.Is<Expression<Func<Report, bool>>>(expr => Lambda.Eq(expr, i => i.ParentType == "1AType" && i.OwnerUid == 5))).Returns(reports);

Here is a example that I tried and the test ist green.This example match your signature.

public class ExpresionClass : IExpresionClass
{
    T[] IExpresionClass.Get<T>(Expression<Func<T, bool>> predicate)
    {
        throw new NotImplementedException();
    }
}

public interface IExpresionClass
{
    T[] Get<T>(Expression<Func<T, bool>> predicate = null);
}

public interface ITestClass
{
    Person[] GetPerson();
}

public class Person
    {
        public string ParentType { get; set; }

        public int OwnerUid { get; set; }
    }

public class TestClass : ITestClass
{
    private readonly IExpresionClass expressionClass;

    public TestClass(IExpresionClass expressionClass)
    {
        this.expressionClass = expressionClass;
    }

    public Person[] GetPerson()
    {
        var test = expressionClass.Get<Person>(x => x.ParentType == "1AType" && x.OwnerUid == 10);

        return test;
    }

}

[TestMethod]
    public void DoesLinqMatch()
    {
        var person = new Person();
        person.OwnerUid = 59;
        person.ParentType = "ParentType";

        Person[] personsarray = new Person[] { person };
        var expressionClass = Substitute.For<IExpresionClass>();
        expressionClass.Get(Arg.Is<Expression<Func<Person, bool>>>(expr => Lambda.Eq(expr, i => i.ParentType == "1AType" && i.OwnerUid == 10))).Returns(personsarray);

        var cut = new TestClass(expressionClass);
        var persons = cut.GetPerson();

        persons.First().ParentType.Should().Be("ParentType");
    }