Option and Result concept for .NET

A concept for an alternative to Exceptions and null values. Taking concepts from the realm of functional programming.

Defines the discriminated unions Result<T, TError>och Option<T>, and demonstrates how to produce and consume them in C# using existing features like pattern matching.

Inspired by Beef programming language, a language inspired by C#, and others like Rust.

The syntax is quite verbose, but can be simplified with using static as shown in source code. Read more below.

Result

Either has an Ok value of T, or an Error value of TError.

public abstract record Result<T> : IDisposable
{
    public sealed record Ok(T Value) : Result<T>;

    public sealed record Error() : Result<T>;
}

Used like so:

return new System.Result<string, Exception>.Ok("Test");

return new System.Result<string, Exception>.Error(new InvalidOperationException());

Option

Either has Some value of T, or None.

public abstract record Option<T> : IDisposable
{
    public sealed record Some(T Value) : Option<T>;

    public sealed record None : Option<T>;
}

Used like so:

return new System.Option<string>.Ok("Test");

return new System.Option<string>.None();

// NOTE: Would have been nice if the parentheses of constructors were optional when having no arguments. There is little risk of confusion with fields.

Pattern matching

To query a Result<T, MyError> using pattern matching you do this:

var result = GetResult(3);

switch(result)
{
    case Result<int, MyError>.Ok(int r):
        Console.WriteLine($"Ok: {r}");
        break;
        
    case Result<int, MyError>.Error(MyError error):
        Console.WriteLine("Error");
        break;
}

And for Option<T>:

var someResult = GetSome(3);

switch(someResult)
{
    case Option<int>.Some(int r):
        Console.WriteLine($"Some: {r}");
        break;
        
    case Option<int>.None:
        Console.WriteLine("None");
        break;
}

This also works:

var someResult = GetSome(3);

if(someResult is Option<int>.Some(int r))
{
    // Do something with result
}
else if(someResult is Option<int>.None)
{
    // Do something with None
}

Considerations

Here we explain some of the quirks of these types.

Unit type

Since void cannot be passed as a type parameter, to represent an empty value, consider using a Unit type such as the one found in MediatR.

Why not record structs?

Because we the types are dependant on inheritance.

Using new when creating Ok, Error etc.

Since Ok, Error, Some etc are types, you have to use the new keyword when creating them.

return new System.Result<string, Exception>.Ok("Test");

Having to specify generic type parameters

The C# compiler does not infer the type parameters based on the target type.

So you must specify what type like so everytime you use Result, Option, or any of their derived nested types.

new Result<string, Exception>.Ok("Success");

This can be avoided to certain extent by using static System.Result<int, Exception>. But it doesn't always work.

Import static members and avoid prefixing with Result<int, MyError>

In conjuntion with the previous point you can import the static members to simplify your code:

using static System.Result<string, Exception>;

The name of the type must be fully qualified.

You can then just:

return new Ok("Test");

As noted before, it might not always work depending on the type resolution.

Implicit conversions from Result or Option to value

It would be nice to just me able to:

int result = GetResult(3);

If there was an error it would obviously fail, but at least it is opt in.

Code exists but it messes with pattern matching which used this conversion when it should not.

Perhaps an explicit conversion is prefered here?

int result = (int)GetResult(3);

Again, this could fail due to being an error.

Roslyn Analyzer (Proposal)

It would be nice to have an analyzer that checks whether a result has been fully handled and warns that it has not.