2011-04-08 Option type implementation in C#

2011-04-08 option-type-implementation-in-csharp

# Title: Option type implementation in C#
# Author: Mikael Lundin
# Link: http://blog.mikaellundin.name/2011/04/08/option-type-implementation-in-csharp.html

FindUserByName.cs

public Option<User> FindUserByName(string name)
{
 var query = from user in Users
    where user.FirstName.Contains(name) || user.Surname.Contains(name)
    select user;
 
 var found = query.FirstOrDefault();
 if (found == null)
  return new None<User>();
 
 return new Some<User>(found);
}

GetIndexOfSubstring.cs

public Option<int> GetIndexOfSubstring(string s, string substring)
{
    var index = s.IndexOf(substring);
    if (index == -1)
        return new None<int>();
 
    return new Some<int>(index);
}

getIndexOfSubstring.fs

let getIndexOfSubstring (s : string) (substring : string) =
    let index = s.IndexOf(substring)
    match index with
    | -1 -> None
    | n -> Some n

Option.cs

// Used as return type from method
public abstract class Option<T>
{
    // Could contain the value if Some, but not if None
    public abstract T Value { get; }
 
    public abstract bool IsSome { get; }
 
    public abstract bool IsNone { get; }
}

OptionExtensions

public static class OptionExtensions
{
    public static Option<T> SomeOrNone<T>(this T reference)
        where T : class
    {
        if (reference == null) return new None<T>();
        return new Some<T>(reference);
    }
}

Some.cs

public sealed class Some : Option
{
 private T value;
 public Some(T value)
 {
  // Setting Some to null, nullifies the purpose of Some/None
  if (value == null)
  {
   throw new System.ArgumentNullException("value", "Some value was null, use None instead");
  }
 
  this.value = value;
 }
 
 public override T Value { get { return value; } }
 
 public override bool IsSome { get { return true; } }
 
 public override bool IsNone { get { return false; } }
}
 
public sealed class None : Option
{
 public override T Value
 {
  get { throw new System.NotSupportedException("There is no value"); }
 }
 
 public override bool IsSome { get { return false; } }
 
 public override bool IsNone { get { return true; } }
}

SubstringBefore.cs

// Get string before substring
private string SubstringBefore(string s, string substring)
{
    var operations = new StringOperations();
    var index = operations.GetIndexOfSubstring(s, substring);
 
    if (index.IsSome)
        return s.Substring(0, index.Value);
 
    return s;
}

Test.cs

[Test]
public void ShouldReturnSomeIndexForExistingSubstring()
{
    /* Test implementation */
}
 
[Test]
public void ShouldReturnNoneWhenSubstringDoesNotExist()
{
    /* Test implementation */
}