OneOf.TypeExtensions

An incremental source generator that creates extension methods for OneOf<> types for more readable and robust code.

Purpose

The reason for this package is that methods and fields of OneOf objects do not give any indication of what type you are referring to. This makes reading code, reviewing PRs, and altering signatures especially hard. Take for example,

var oneOf = SomeFunction();
if (oneOf.IsT1)
{
    // do stuff
}

If you saw this code in a PR, you would have no idea what IsT1 is checking for. Furthermore, imagine if the return type of SomeFunction() changed from OneOf<int, string> to OneOf<char, int, string>. IsT1 would change from checking if the OneOf is a string to checking if it's an int, and you would have no idea. This package solves that by having typed extension methods. oneOf.IsString()

Important

Source generated files can have very long names and may exceed the default windows file path length limit of 256 if you have not increased it on your computer. See this article for instructions on increasing the limit on Windows.

Installation

Via nuget NuGet\Install-Package OneOf.TypeExtensions

Analyzer with code fix

There is also an analyzer with code fix that will let you quickly replace all usages of IsT_, AsT_, MapT_, and TryPickT_, with the source generated extension methods. I personally won't be keeping this package as part of my projects because I don't want to bog down my IDE, but you may find it useful to quickly make changes to an entire project/solution.

Install: NuGet\Install-Package OneOf.TypeExtensions.Analyzer

Example Generated Code

For the type OneOf<int, string>, these extension methods will be generated.

OneOfExtensions_String_Int.g.cs

// <auto-generated>

using OneOf;

public static partial class OneOfTypeExtensions
{
    public static bool IsString(this OneOf<string, int> oneOf)
    {
        return oneOf.IsT0;
    }
    public static string AsString(this OneOf<string, int> oneOf)
    {
        return oneOf.AsT0;
    }
    public static OneOf<TResult, int> MapString<TResult>(this OneOf<string, int> oneOf, Func<string, TResult> mapFunc)
    {
        return oneOf.MapT0(mapFunc);
    }
    public static bool TryPickString(this OneOf<string, int> oneOf, out string value, out int remainder)
    {
        return oneOf.TryPickT0(out value, out remainder);
    }
    public static bool IsInt(this OneOf<string, int> oneOf)
    {
        return oneOf.IsT1;
    }
    public static int AsInt(this OneOf<string, int> oneOf)
    {
        return oneOf.AsT1;
    }
    public static OneOf<string, TResult> MapInt<TResult>(this OneOf<string, int> oneOf, Func<int, TResult> mapFunc)
    {
        return oneOf.MapT1(mapFunc);
    }
    public static bool TryPickInt(this OneOf<string, int> oneOf, out int value, out string remainder)
    {
        return oneOf.TryPickT1(out value, out remainder);
    }
}

Supported Types

The generator should support practically any type, if you find a type or combination of types that causes an error, feel free to create an issue or submit a PR.

Special consideration was given to support these types

• Nullable value types int?
• Nullable annotated reference types string?
• Value Tuples (int?, string?)
• Value Tuples with named fields (int? Id, string? Name)
• Generics List<int?>
• Multiple Generics Dictionary<string?, int?>
• Nested generics List<List<int?>>
• Nested types Class1.Class2
• Any combination of the above

Breaking changes

This project is still in beta and therefore breaking changes may occur. The only thing I forsee changing is how generated functions will be named when there are generics. Currently, OneOf<int, Dictionary<string, int>> will create extension methods like so oneOf.IsDictionaryOfString_Int. This looks fine but it can get a little ugly when there are multiple nested generics.

Also, generated code currently has no namespace which is fine with me but may be changed based on feedback.

Performance

I have tried to make the generator somewhat performant, but I am by no means an expert at incremental source generators. Feel free to submit a PR improving it's efficiency.

Roadmap

The project is still very much at it's infancy. I have a few ideas for future features.

• Control what extension methods get generated through csproj properties
• Improve efficiency of generator
• Better extension method names with nested generics
• Keep hint names shorter and more understandable while ensuring they are unique