Question

The mocking library Moq has a Setup() method with the signature:

public ISetup<T, TResult> Setup<TResult>(Expression<Func<T, TResult>> expression)

So I can do something like this just fine (using the library’s recursive mocking):

Mock<Thing> _thing = new Mock<Thing>();
_thing.Setup((Thing t) => t.PropA.SubPropB).Returns(string.Empty);

But this fails:

Expression<Func<Thing, object>> test = (Thing t) => t.PropA.SubPropB;
_thing.Setup(test).Returns(string.Empty);

with the error:

Expression is not a method invocation: t => (Object)t.PropA.SubPropB

What’s the difference between the inlined lambda and the one assigned to a variable first? Aren’t both expression trees and not yet compiled (Moq parses the tree)?

Edit – Looks like the issue is with the Func<Thing, object> typing. Why is, e.g., string acceptable, but object is not?

Answer 1

Why is, e.g., string acceptable, but object is not?

because function declaration in moq is not covariant by generic types. Try to Setup moq in next way

 _thing.Setup(test).Returns((object)string.Empty);

because you have next signature Expression<Func<Thing, object>> and it’s not covariant with respect to object

Or change your signature to string like this (assume t.PropA.SubPropB returns string):

Expression<Func<Thing, string>> test = (Thing t) => t.PropA.SubPropB;
                        //^here should be string

Real-case
I have created a test project with Expression as local variable, it all works fine with string and object. Please check my configuration if I missed something. Moq – 4.0.10827v

[TestFixture]
public class Class1
{
    [Test]
    public void TestMethod()
    {
        Mock<Thing> _thing = new Mock<Thing>();

        Expression<Func<Thing, string>> setup = t => t.PropA.SubPropB;
//                               ^ works with string and object
        _thing.Setup(setup).Returns(string.Empty);

        Assert.IsEmpty(_thing.Object.PropA.SubPropB);

    }
}

public class Thing
{
    public virtual Thingy PropA { get; set; }
}

public class Thingy
{
    public virtual string SubPropB { get; set; }
}