FakeItEasy ReturnsLazily with out parameter

Question

I'm new to using FakeItEasy and rather stuck at my first attempt. The interface i want to fake has a method like this:

byte[] ReadFileChunk(string path,int offset,int count,out long size);

I would like to see how the arguments are passed on, so I'm using ReturnsLazily. Here is my attempt:

long resSize;
A.CallTo(() => dataAttributesController
     .ReadFileChunk(A<string>.Ignored, A<int>.Ignored, A<int>.Ignored, out resSize))
     .ReturnsLazily((string path, int offset, int count) =>
        {
             return Encoding.UTF8.GetBytes("You requested: " + path + "(" + offset + "," + count + ")");
        })
    .AssignsOutAndRefParameters(123);

This compiles, but when ran it generates this exception:

The faked method has the signature (System.String, System.Int32, System.Int32, System.Int64&), but returns lazily was used with (System.String, System.Int32, System.Int32).

Which is correct, but i can't figure out how to add the out parameter. If I change the ReturnLazily part to this:

.ReturnsLazily((string path, int offset, int count, out long size) =>
   {
      size = 0;
      return Encoding.UTF8.GetBytes("You requested: " + path + "(" + offset + "," + count + ")");
   })

it will not compile, and I don't understand the errors:

error CS1593: Delegate 'System.Func<FakeItEasy.Core.IFakeObjectCall,byte[]>' does not take 4 arguments
error CS1661: Cannot convert lambda expression to delegate type 'System.Func<string,int,int,long,byte[]>' because the parameter types do not match the delegate parameter types
error CS1677: Parameter 4 should not be declared with the 'out' keyword

For a novice like me, this reads like it doesn't like 4 parameters nor understands what to do with 'out'. Can someone please explain me how I should be reading these errors? A working example would be very welcome as well :-)

Thanks a lot!

--- EDIT ---

This seems to work:

A.CallTo(() => dataAttributesController
    .ReadFileChunk(A<string>.Ignored, A<int>.Ignored, A<int>.Ignored, out resSize))
    .ReturnsLazily(x =>
           Encoding.UTF8.GetBytes("You requested: " + x.Arguments.Get<string>(0) + "(" + x.Arguments.Get<int>(1) + "," + x.Arguments.Get<int>(2) + ")"))
    .AssignsOutAndRefParameters((long)123);

A bit less readable then I was hoping for, is this anywhere near the intended use of ReturnsLazily?

Answer 1

Update: the issue I mention below was fixed in FakeItEasy release 1.15, so update to the latest version and you shouldn't have to worry about the out/ref specifiers on the methods when using ReturnsLazily.

Sorry for the delay. I'm glad you found a solution that at least functions for you.

I agree that the syntax you landed on isn't the most readable, but it does appear to be the correct one. The "convenience" versions of ReturnsLazily won't work with out/ref parameters because the lambda can't take the out/ref modifiers.

It doesn't help you today, but I've created FakeItEasy issue 168 to deal with the problem. If you have additional opinions, please drop by and comment or something.

Answer 2

Is that interface under your control?

byte[] ReadFileChunk(string path, int offset, int count, out long size);

if so: Isn't out long size just the same as the size of the returning byte[]? In that case I would just remove the size parameter from the interface method and use the "nice-to-read" ReturnsLazily method as you intended first.