C# Language Functional Programming Avoid Null References
Example
C# developers get a lot of null reference exceptions to deal with. F# developers don't because they have the Option type. An Option<> type (some prefer Maybe<> as a name) provides a Some and a None return type. It makes it explicit that a method may be about to return a null record.
For instance, you can't read the following and know if you will have to deal with a null value.
var user = _repository.GetUser(id);
If you do know about the possible null you can introduce some boilerplate code to deal with it.
var username = user != null ? user.Name : string.Empty;
What if we have an Option<> returned instead?
Option<User> maybeUser = _repository.GetUser(id);
The code now makes it explicit that we may have a None record returned and the boilerplate code to check for Some or None is required:
var username = maybeUser.HasValue ? maybeUser.Value.Name : string.Empty;
The following method shows how to return an Option<>
public Option<User> GetUser(int id)
{
var users = new List<User>
{
new User { Id = 1, Name = "Joe Bloggs" },
new User { Id = 2, Name = "John Smith" }
};
var user = users.FirstOrDefault(user => user.Id == id);
return user != null ? new Option<User>(user) : new Option<User>();
}
Here is a minimal implementation of Option<>.
public struct Option<T>
{
private readonly T _value;
public T Value
{
get
{
if (!HasValue)
throw new InvalidOperationException();
return _value;
}
}
public bool HasValue
{
get { return _value != null; }
}
public Option(T value)
{
_value = value;
}
public static implicit operator Option<T>(T value)
{
return new Option<T>(value);
}
}
To demonstrate the above avoidNull.csx can be run with the C# REPL.
As stated, this is a minimal implementation. A search for "Maybe" NuGet packages will turn up a number of good libraries.
