Using Expression Trees as an argument constraint - fakeiteasy

Question

Can I use an Expression Tree as an argument constraint in a FakeIteasy CallTo assertion?
Given a method on an interface with the following signature:
interface IRepository<TEntity>
{
TEntity Single(Expression<Func<TEntity, bool>> predicate);
Being called in code like so:
Flight flight = repository.Single(f => f.ID == id);
I have in mind a unit test doing something like this:
Expression<Func<Flight, bool>> myExpression = flight => flight.ID == 1;
A.CallTo(() => repository.Single(
A<Expression<Func<Flight, bool>>>.That.Matches(myExpression)))
.Returns(new Flight());
However this produces a warning: Try specifying type arguments explicitly.
I am currently having to use the Ignored property which is not ideal.

Answer 1

The "Matches"-method takes a lambda but you're trying to pass it the expression. What are you trying to say with the "Matches"-call? Are you matching on equality? In that case you'd just write:
A.CallTo(() => repository.Single(myExpression)).Returns(new Flight());
If you want to constrain the expression on something else you'd have to pass a predicate of the type: Func<Expression<Func<Flight, bool>>, bool> to the "Matches"-method.

Answer 2

Thanks Patrik,
Examining the expression was exactly what I needed to do, i.e. parse the expression (f => f.ID == id) and execute the Right side of the == to get its runtime value.
In code this looks like this:
A.CallTo(() => flightRepository.Single(A<Expression<Func<Flight, bool>>>.That
.Matches(exp => Expression.Lambda<Func<int>>(((BinaryExpression)exp.Body).Right).Compile().Invoke() == 1)))
.Returns(new Flight());
However I can't help thinking that there must be a more elegant way to achieve the same end. I'll leave that for another day though.
Thanks again,
Michael McDowell

Answer 3

I had the same problem while attempting to assert an expression as an argument but I was using Moq. The solution should work for you though as well...
I give most of the credit to this answer to a similar question:
Moq Expect On IRepository Passing Expression
It basically says you can do a ToString() on the expressions and compare them. It is kind of hacky but it only has one downside; the variables names in the lambda expression must match.
Here is an example...
[Test]
public void TestWhichComparesExpressions()
{
// setup
_mockRepository.Setup(x => x.GetByFilter(MatchQuery())).Returns(new List<Record>());
// execute
var records = _service.GetRecordsByFilter();
// assert
Assert.IsNotNull(records);
Assert.AreEqual(0, records.Count());
}
private static Expression<Func<DomainRecord, bool>> MatchQuery()
{
return MatchExpression(ServiceClass.QueryForTheRecords); // constant
}
// https://stackoverflow.com/questions/288413/moq-expect-on-irepository-passing-expression/1120836#1120836
private static Expression<Func<DomainRecord, bool>> MatchExpression(Expression<Func<DomainRecord, bool>> expression)
{
return It.Is<Expression<Func<DomainRecord, bool>>>(e => e.ToString() == expression.ToString());
}
I decided to put the expression into a constant on the class which used it which guaranteed it would be the same in the test if someone changed the lambda expressions's variable names.