Moq：获取表达式谓词的参数值

Question

I'm new to using Moq and I'm trying to get the value passed into a Moq'd method to use in the Returns method.

I was doing the following with success.

        _repositoryMock.Setup(x => x.GetByOrderId(It.IsAny<string>()))
            .Returns((string id) => Task.FromResult(
                new Order
                {
                    Id = id
                }));

Usage in code:

var order = _repository.GetByOrderId("123");

The above worked fine and the id passed into the Returns method is the same ID I passed into the GetByOrderId method in my code.

However, I would like to make my repository more generic so I want to change my GetByOrderId to FindFirstOrDefault that takes an expression predicate instead of an ID.

Usage like this:

var order = _repository.FindFirstOrDefault( o => x.Id == "123");

With unit test changed to this:

_repositoryMock.Setup(moq => moq.FindFirst(It.IsAny<Expression<Func<Order, bool>>>()))
            .Returns((Expression<Func<Order, bool>> expression) => Task.FromResult(
                new Order
                {
                    Id = // ... HOW TO GET ID LIKE THE FIRST SAMPLE?
                }));

So how can I get to that ID? The "123". Is there any way?

Answer 1

I found a work around.

I just set up a list of Order that had all the values I knew would be in my expected result an then I applied the expression to that list to get the item I wanted.

var expected = // INIT MY EXPECTED
List<Order> orders = new List<Order>();

foreach (var expectedItem in expected.Items)
{
    orders.Add(new Order
    {
        Id = expectedItem.Id,
    });
}

Then I setup my Mock like this.

_finicityRepositoryMock.Setup(moq => moq.FindFirst(It.IsAny<Expression<Func<Order, bool>>>()))
            .Returns((Expression<Func<Order, bool>> expression) =>
            {
                var order = orders.AsQueryable().Where(expression).FirstOrDefault();

                if (order == null)
                {
                    return Task.FromResult((Order)null);
                }

                return Task.FromResult(
                    new Order
                    {
                        BorrowerID = order.Id
                    });
            });

Answer 2

You could analyze the Expression .

If you only do x => x.Id == "123" in the predicate expression, the solution could be as simple as:

mock.Setup(x => x.FindFirstOrDefault(It.IsAny<Expression<Func<Order, bool>>>()))
    .Returns<Expression<Func<Order, bool>>>(
        predicate =>
        {
            var id = (string)((ConstantExpression)(((BinaryExpression)predicate.Body).Right)).Value;
            return new Order { Id = id };
        });

If you also use other properties, then you need an ExpressionVisitor which helps you extract values for each property:

class PredicatePropertyValueExpressionVisitor : ExpressionVisitor
{
    public Dictionary<string, object> PropertyValues { get; } = new Dictionary<string, object>();

    protected override Expression VisitBinary(BinaryExpression node)
    {
        if (node.Left is MemberExpression pe && pe.Member is PropertyInfo pi)
        {
            PropertyValues[pi.Name] = (node.Right as ConstantExpression).Value;
        }

        return base.VisitBinary(node);
    }
}

Then mock will be:

mock.Setup(x => x.FindFirstOrDefault(It.IsAny<Expression<Func<Order, bool>>>()))
    .Returns<Expression<Func<Order, bool>>>(
        predicate =>
        {
            var visitor = new PredicatePropertyValueExpressionVisitor();
            visitor.Visit(predicate);
            return new Order { Id = visitor.PropertyValues["Id"].ToString() };
        });