C# – Moq callback with out parameter

ccallbackmoqoutunit testing

I'm trying to use Moq to mock a callback for the following method signature:

ResponseHeader AddIncentives(
        Hs_transRow[] data,
        out ResponseBody responseBody);

I want my callback to use the data which is passed in. However, I'm running into problems which I think are because the second parameter is an out parameter. I can setup and return data without problems, but the callback is an issue.

This is my current setup:

var addIncentiveResponseBody = new ResponseBody();

mockCoinsService
.Setup(service => service.AddIncentives(It.IsAny<Hs_transRow[]>(), out addIncentiveResponseBody))
.Callback((Hs_transRow[] data, ResponseBody body) =>
{
    //I want to use the data variable here
})
.Returns(() => new ResponseHeader
{
     action = ResponseHeaderAction.RESPONSE,
});

When I run this code as part of a unit test, I get the error:

Invalid callback. Setup on method with parameters (Hs_transRow[],ResponseBody&) cannot invoke callback with parameters (Hs_transRow[],ResponseBody).

I can see there is an ampersand difference, which I assume is because that parameter should be an out parameter. However, if I add the out keyword to the callback (and assign it a value within the callback), I get the build time error:

Delegate 'Action' does not take two arguments.

Is Moq unable to handle a callback for a method which has an out parameter, or am I doing something wrong?

[Edit] To clarify beyond any doubt, I am not asking how to set the value of the out parameter. This is already being done by the line:

var addIncentiveResponseBody = new ResponseBody();

Best Answer

This was pointed out in the comments by Jeroen Heier but adding it as an answer should make it more visible. Scott Wegner has written an OutCallback overload which can be used.

This is used as follows:

mockCoinsService
    .Setup(service => service.AddIncentives(It.IsAny<Hs_transRow[]>(), out 
    addIncentiveResponseBody))
    .OutCallback((Hs_transRow[] data, ResponseBody body) =>
    {
        //I can now use the data variable here
    })
    .Returns(() => new ResponseHeader
    {
         action = ResponseHeaderAction.RESPONSE,
    });

Related Solutions

C# – What’s the difference between the ‘ref’ and ‘out’ keywords

ref tells the compiler that the object is initialized before entering the function, while out tells the compiler that the object will be initialized inside the function.

So while ref is two-ways, out is out-only.

A callback function

Developers are often confused by what a callback is because of the name of the damned thing.

A callback function is a function which is:

accessible by another function, and
is invoked after the first function if that first function completes

A nice way of imagining how a callback function works is that it is a function that is "called at the back" of the function it is passed into.

Maybe a better name would be a "call after" function.

This construct is very useful for asynchronous behaviour where we want an activity to take place whenever a previous event completes.

Pseudocode:

// A function which accepts another function as an argument
// (and will automatically invoke that function when it completes - note that there is no explicit call to callbackFunction)
funct printANumber(int number, funct callbackFunction) {
    printout("The number you provided is: " + number);
}

// a function which we will use in a driver function as a callback function
funct printFinishMessage() {
    printout("I have finished printing numbers.");
}

// Driver method
funct event() {
   printANumber(6, printFinishMessage);
}

Result if you called event():

The number you provided is: 6
I have finished printing numbers.

The order of the output here is important. Since callback functions are called afterwards, "I have finished printing numbers" is printed last, not first.

Callbacks are so-called due to their usage with pointer languages. If you don't use one of those, don't labour over the name 'callback'. Just understand that it is just a name to describe a method that's supplied as an argument to another method, such that when the parent method is called (whatever condition, such as a button click, a timer tick etc) and its method body completes, the callback function is then invoked.

Some languages support constructs where multiple callback function arguments are supported, and are called based on how the parent function completes (i.e. one callback is called in the event that the parent function completes successfully, another is called in the event that the parent function throws a specific error, etc).

Best Answer

Related Solutions

C# – What’s the difference between the ‘ref’ and ‘out’ keywords

A callback function

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