Sander Hoogendoorn edited this page Oct 8, 2016 · 3 revisions

During a recent Java project, we created a small library that executes the behaviour of the Scala Try classes (including Success and Failure). I really started to appreciate the style of programming, where we concatenate series of Map() and FlatMap() method, using the power of the Try monad, and thus avoiding abundant try-catch blocks and null checks. Hence, I decided to port this library to C#.

I've only added my first commit to this repository, but will add more updates as the library slowly grows. The unit tests I've added demonstrate the use of the Try classes and methods fairly well already.

Try and succeced

When programming using the Try class, you can actually avoid most try-catch blocks in code, because the monad Try will wrap possible exceptions, and just return whether the actions and functions you've called fail or succeed.

In the (simple) code example below an instance of the class Employee is created, the name of the employee is fetched in the Map() statement where e is really the instance of Employee that was created above, and we check whether it ends with the character s. This will return a Try<bool>, and we use the property Value to get the actual value from the Try.

    var result = Try<Employee>.Invoke(() => repo.Create("Kees"))
        .Map(e => e.Name)
        .Map(s => s.EndsWith("s"))
        .Value;

    Assert.IsTrue(result);

Getting this value will only return a valid value IF all of the statements above have executed succesfully - that is, if we have an instance of the Success class in the end. If not, we have an instance of Failure on our hands and getting the value will throw an exception.

Try and fail

In the next example, some more of the power of Try is exposed. Here one of the statements fails (deliberately).

    var result = Try<Employee>.Invoke(() => repo.Create("Kees"))
        .Map(e => e.WillThrowException());

    Assert.IsTrue(result.IsFailure);

Now, the first Invoke() call return an employee, but the second statement Map() throw an exception. But instead of crashing your programming, the result will be an instance of the Failure class, holding the exception that was thrown. If one of the statements in your code returns an instance of Failure all following statements are being ignored.

Although this example seems trivial, once you get used to programming with Try you will soon realize, it is actually quite powerful in building more robust code, that also becomes much easier to test too.

Recovering from failure

The next thing you might want to do is to recover from failure and continue. To this aim there's the Recover() methods.

    var result = Try<Employee>.Invoke(() => repo.Create("Kees"))
        .FlatMap(e => e.WillThrowException())
        .Recover(ex => repo.Create("Jaap"));

    Assert.IsTrue(result.IsSuccess);
    Assert.AreEqual(result.Value.Name, "Jaap");

In this code example the second statement throws and will return an instance of Failure. What the Recover() method in the next statement will do is help you get back on track. It will create a new employee (with the name Jaap) and allow you to continue. Quite often, recover statements appear at the end of a block of statements, for instance to recover from REST calls that do not return anything useful.

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