Value assigned to an out parameter being read by Moq. How?

Question

This code is based on this answer here: https://stackoverflow.com/a/3135677/360211

public interface IService
{
    void DoSomething(out string a);
}

[Test]
public void Test()
{
    var mock = new Mock<IService>();
    string expectedValue = "value";
    mock.Setup(s => s.DoSomething(out expectedValue));

    string actualValue;
    mock.Object.DoSomething(out actualValue);
    Assert.AreEqual("value", actualValue);
}

The assignment string expectedValue = "value"; is not required as far as I can see and as far as resharper is concerned. But remove it and the Assert fails!

So how is Moq reading the value from an out parameter?

The answer you linked says "I'm guessing that Moq looks at the value of 'expectedValue' when you call Setup and remembers it." That is your out method will just return the expectedValue you gave it. If you don't give it a value it doesn't return that value. — Chris, Dec 15 '15 at 15:13
@Chris yes another value makes it fail. I know what it does, the question is how it does it. — weston, Dec 15 '15 at 15:19
What do you get when you write the contents of `expectedValue` and/or `actualValue` after running it uninitialized. — gmiley, Dec 15 '15 at 15:51
@gmiley Sorry, uninitialized, they are both `null`, well "actual" is, I can't inspect "expected". — weston, Dec 15 '15 at 15:52
So regardless of what you pass in, the outcome is always "value"? — gmiley, Dec 15 '15 at 15:55
@weston: Ah, ok. I was a little confused. When you said "how is it reading the value of the out parameter" I assumed you meant in the Assert line. I'd probably have chosen to phrase the question as "How is moq setting the out parameter" or something similar. I now realise how interesting a question it is. :) — Chris, Dec 15 '15 at 17:09
@Chris yes it's both reading the value from the out parameter and writing it to the actual variable. But the reading is the only unusual feature, so that's why I worded that way. — weston, Dec 15 '15 at 18:55

Borophyll · Answer 1 · 2015-12-15T15:30:12.920

The important line is this:

mock.Setup(s => s.DoSomething(out expectedValue));

This setup means that any method that invokes DoSomething will always put whatever the value is for expectedValue into the actual out variable when the method executes.

Perhaps consider a more useful scenario that isn't directly testing the mock.Object, but is testing a class that has an IService injected.

public class Foo
{
    private IService service;
    public Foo(IService service)
    {
        this.service = service;
    }

    public string GetData()
    {
        string outData;
        service.DoSomething(out outData);

        return outData != "" ? outData : "There was no data";
    }
}

[Test]
public void FooTest_ServiceReturnsEmptyString()
{
    var mock = new Mock<IService>();
    // ReSharper disable once RedundantAssignment
    string expectedValue = "";
    mock.Setup(s => s.DoSomething(out expectedValue));

    var fooObj = new Foo(mock.Object);
    var result = fooObj.GetData();

    Assert.AreEqual("There was no data", result);
}

[Test]
public void FooTest_ServiceReturnsValue()
{
    var mock = new Mock<IService>();
    string expectedValue = "data";
    mock.Setup(s => s.DoSomething(out expectedValue));

    var fooObj = new Foo(mock.Object);
    var result = fooObj.GetData();

    Assert.AreEqual(expectedValue, result);
}

In the two different tests, you can change what the mock returns, and you are now able to test how your class behaves given different data from your service.

thanks, question is not about what's useful to test and what's not, just how moq is doing something that should be illegal. The example is a minimal example of the issue. — weston, Dec 15 '15 at 15:20
Your `FooTest_ServiceReturnsValue` would have the same issue. A warning that value assigned to `expectedValue` is not used, but you cannot remove it without breaking the test. — weston, Dec 15 '15 at 15:26
@weston see my edits.. With the second test, resharper will not flag the variable like it can be safely removed if you include it in the `Assert`. For the first test, where you specifically need to set the value to empty string in order to make the test pass.. the only thing you can really do is add a comment to disable the resharper warning. — Borophyll, Dec 15 '15 at 15:32

gmiley · Answer 2 · 2015-12-15T15:28:20.053

0

An out parameter is not required to be initialized, however it must be assigned to by the time the function returns.

I don't know what is going on inside mock.Object.DoSomething(out actualValue); however, it seems weird that it is defined using an out parameter. Judging by the behavior you are reporting, I would have though it should be a ref instead. A ref parameter must be initialized before being passed into the function as a parameter.

edited Dec 15 '15 at 15:28

answered Dec 15 '15 at 14:59

gmiley

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"however it can be read inside the function call" No, this doesn't compile `"CS0269 Use of unassigned out parameter 'testVar'` – weston Dec 15 '15 at 15:18

score 0 · Answer 3 · answered Dec 15 '15 at 18:16

I think the reason is that what it is passing is just an expression of type Expression<Action<IService>> and the arguments in the expression are captured (so if you changed the value of expectedValue the value in the expression changes too). Of note is that the expression is identical if you make it with a normal parameter instead of an out parameter.

I think probably it is just not doing anything to that parameter at all. When you setup the method you put a bunch of metadata in the Mock object telling it what you've done and when you call the method on your mock object it uses that metadata to work out what the return is or what else it might need to do.

Of note is that it probably isn't calling anything and passing it your parameter with the out modifier so the result is basically the same as if you didn't have that line, that is Mock just doesn't even try to set a value of that variable. This means that basically you set the value of expectedValue when you declare it and then nothing changes it and then you have an assertion based on it.

I'm not in a position to do any testing to prove this theory and I am not confident enough in my understanding of expressions or Mock to say this is definitely what is going on but from what I've seen of the code I think its at least close to this. My uncertainty was such that I was going to post this as a comment but it is a wee bit too long. ;-)

Having said all of that even if Mock is doing something with it the reason it can read your variable despite being an out parameter is that it is part of the expression. You should be able to see this by using the following code, setting an appropriate breakpoint and inspecting:

string expectedValue = "FNORDFNORD"; 
Expression<Action<IService>> expression = s => s.DoSomething(out expectedValue);

Value assigned to an out parameter being read by Moq. How?

3 Answers3