This is a short and sweet article showing how the Option class can bring functional programming concepts to C# codebases and prevent null reference exceptions.

This article will be using the extremely popular language-ext library as the most popular .NET functional programming library built for object oriented languages (aside from F#).

Installing the Language-Ext Library

In order to work with the language-ext library, you will need to add a NuGet reference to the package. This can be done via NuGet Package Manager in Visual Studio or by running Install-Package LanguageExt.Core from the package manager console.

What is an Option?

Options are a reference type that either contain some value or have none. As an example, look at the following method that searches a dictionary and returns a value from it:

public Option<ResumeKeyword> GetKeyword(
   IDictionary<string, ResumeKeyword> keywords, 
   string word) { 

  // Ensure we're searching for a lower-cased non-null word
  string key = word ?? word.ToLowerInvariant() : string.Empty;

  // If the keyword is present, return it
  if (keywords.ContainsKey(key)) {
    return keywords[key]; // Implicitly converted to Some<ResumeKeyword>
  }

  // No keyword found
  return Option<ResumeKeyword>.None;
}

In this method, since the return type is Option<ResumeKeyword>, the return type will either be of Some<ResumeKeyword> or None. The result will never explicitly be null so callers will never have to worry about null values.

The compiler is able to convert a standard value to a Some value, but the syntax of the None return type is less than ideal as we can start to see the added complexity of working with Option.

Thankfully, the return type can be simplified if we add the following using at the top of the file: using static LanguageExt.Option<ResumeKeyword>;

This allows us to simply return None; instead of doing return Option<ResumeKeyword>.None; which reads significantly better.

Working with Options

Now that you have an Option, you want to be able to work with it. This is where the appeal of Option starts to come into play. Because we're working with an Option the compiler doesn't let us make potentially buggy code like the following:

// Give a bump for various words in the title
foreach (var word in job.Title.Split())
{
    var keyword = FindKeyword(keywordBonuses, key);
    jobScore += keyword.Value; // Does not compile
}

This code does not compile because keyword is an Option<ResumeKeyword> instead of a ResumeKeyword and so it cannot access the members of ResumeKeyword directly. This inconvenience is actually a good thing because we know that FindKeyword can sometimes not find any keyword match, which would result in a NullReferenceException in a standard application if we forgot to check for null.

Instead we write the following:

// Give a bump for various words in the title
foreach (var word in job.Title.Split())
{
    var keyword = FindKeyword(keywordBonuses, key);
    jobScore += keyword.Some(k => k.Value)
                       .None(0);
}

Here we're saying that we want to increase the score by the Value of the ResumeKeyword if Some keyword is present and, if None is present, score the word as 0 or filler.

The compiler's type checking will enforce these rules and make sure we arrive at the correct types, forcing us to think about whether or not the value is present and act accordingly.

For scenarios where you only want to do something if a value is present or absent and you don't need the actual value, you can check the IsSome or IsNone properties. For example:

if (keyword.IsNone) {
   Console.WriteLine($"No value defined for keyword '{word}');
}

What about nullable types?

.NET has had the concept of nullable types for awhile via Nullable<T> (often coded as T? myVar).

A nullable type is a way of representing a value type as a nullable object. This is different from an Option because:

1. Options work with both reference and value types
2. While Nullable<T> offers HasValue and Value members, Option<T> offers convenience methods to react in specific ways to Some or None conditions as we saw above.

Summary

Language-Ext's Option class forces you to make intelligent decisions and work with potentially missing values in a safe way.

The downside of this is that it adds complexity to both reading and writing your code, but in quality critical aspects of your codebase or areas where null values are frequently possible, it might make sense to incorporate Option<T> into your code.

Also, bear in mind that Option<T> is only a slice of the capabilities offered by Language-Ext. Check out the full library to read more or follow me for notifications of future posts.

If you're curious about more ways of eliminating entire classes of defect, check out my article on making defects impossible.

Making Defects Impossible

Matt Eland ・ Aug 31 '19

#testing #dotnet #javascript #codequality