Discriminated Unions

Sundew.DiscriminatedUnions implement discriminated unions for C#, until a future version of C# provides it out of the box. The idea is that this package can be deleted once unions are supported in C#, without requiring changes to switch expressions and statements.

In addition, the project supports dimensional unions through default interface methods (traits). A dimensional union is a union where cases can be reused in any number of unions, by supporting interface unions through the possibility of implementing multiple interface and default interface members.

How it works

A Roslyn analyzer asserts and report errors in case switch statements or switch expression do not handle all cases. C# 8 and 9 already comes with great pattern matching support for evaluation.

In order that the inheritance hierarchy remain closed (All cases in the same assembly), an analyzer ensures that unions are not derived from in referencing assemblies. Similarly all case classes should be sealed.

Create a union by inheriting from an abstract base (record) class (or interface) marked with the DiscriminatedUnion attribute to build various cases. Either specify the partial keyword to the union for a source generator to implement factory methods or use the codefix PDU0001 to generate them.

Sample

Defining a union

[Sundew.DiscriminatedUnions.DiscriminatedUnion]
public abstract partial record Result
{
    public sealed record Success : Result;

    public sealed record Warning(string Message) : Result;

    public sealed record Error(int Code) : Result;
}

Alternatively, a union can be defined with unnested case classes and interfaces, allowing the possibility of creating dimensional unions (see below).

Evaluation

var message = result switch
{
    Result.Error { Code: > 70 } error => $"High Error code: {error.Code}",
    Result.Error error => $"Error code: {error.Code}",
    Result.Warning { Message: "Tough warning" } => "Not good",
    Result.Warning warning => warning.Message,
    Result.Success => "Great",
};

Dimensional unions

To support dimensional unions, unnested cases help because the cases are no longer defined inside a union. However, for this to work the unions are required to declare a factory method named exactly like the case type and that has the CaseType attribute specifying the actual type. Since version 3, factory methods are generated when the union is declared partial. Alternatively, a code fix (PDU0001) is available to generate the factory methods.

[Sundew.DiscriminatedUnions.DiscriminatedUnion]
public partial interface IExpression;

[Sundew.DiscriminatedUnions.DiscriminatedUnion]
public partial interface IArithmeticExpression : IExpression;

[Sundew.DiscriminatedUnions.DiscriminatedUnion]
public partial interface ICommutativeExpression : IArithmeticExpression;

public sealed record AdditionExpression(IExpression Lhs, IExpression Rhs) : ICommutativeExpression;

public sealed record SubtractionExpression(IExpression Lhs, IExpression Rhs) : IArithmeticExpression;

public sealed record MultiplicationExpression(IExpression Lhs, IExpression Rhs) : ICommutativeExpression;

public sealed record DivisionExpression(IExpression Lhs, IExpression Rhs) : IArithmeticExpression;

public sealed record ValueExpression(int Value) : IExpression;

Evaluating dimensional unions

With dimensional unions it is possible to handle all cases using a sub union. As seen in the example below, handling the ArithmeticExpression covers Addition-, Subtraction-, Multiplication- and DivisionExpression. Typically one would dispatch these to a method handling ArithmeticExpression and where handling all cases would be checked, but it is not required. This makes it convienient to separate handling logic in smaller chucks of code.

public int Evaluate(Expression expression)
{
    return expression switch
        {
            ArithmeticExpression arithmeticExpression => Evaluate(arithmeticExpression),
            ValueExpression valueExpression => valueExpression.Value,
        };
}

public int Evaluate(ArithmeticExpression arithmeticExpression)
{
    return arithmeticExpression switch
        {
            AdditionExpression additionExpression => Evaluate(additionExpression.Lhs) + Evaluate(additionExpression.Rhs),
            SubtractionExpression subtractionExpression => Evaluate(subtractionExpression.Lhs) - Evaluate(subtractionExpression.Rhs),
            MultiplicationExpression multiplicationExpression => Evaluate(multiplicationExpression.Lhs) * Evaluate(multiplicationExpression.Rhs),
            DivisionExpression divisionExpression => Evaluate(divisionExpression.Lhs) / Evaluate(divisionExpression.Rhs),
        };
}

Enum evaluation

As of version 5.1, regular enums can also use the DiscriminatedUnion attribute causing the analyzer to exhaustively check switch statements and expressions.

Generator features

As mentioned a source generator is automatically activated for generating factory methods when the partial keyword is specified. In addition, the DiscriminatedUnion attribute can specify a flags enum (GeneratorFeatures) to control additional code generation.

• Segregate - Generates an extension method for IEnumerable that segregates all items into buckets of the different result.

Supported diagnostics:

Diagnostic Id	Description	Code Fix
SDU0001	Switch does not handled all cases	yes
SDU0002	Switch should not handle default case	yes
SDU0003	Switch has unreachable null case	yes
SDU0004	Class unions must be abstract	yes
SDU0005	Only unions can extended other unions	no
SDU0006	Unions cannot be extended outside their assembly	no
SDU0007	Cases must be declared in the same assembly as their unions	no
SDU0008	Cases should be sealed	yes
SDU0009	Unnested cases should have factory method	PDU0001
SDU0010	Factory method should have correct CaseTypeAttribute	yes
SDU0011	Reported when a case is implemented by throwing NotImplementedException, because CodeCleanup may siliently 'fix' SDU0001.	yes
SDU0012	Reported when a case contains type parameters that are not in the union type parameter list.	yes
PDU0001	Make union partial for code generator	yes
PDU0002	Populate union factory methods	yes
SDU9999	Switch should throw in default case	no
GDU0001	Discriminated union declaration could not be found	no

Issues/Todos

• Switch appears with red squiggly lines in VS: dotnet/roslyn#57041
• Nullability is falsely evaluated when the switch hints null is possible: dotnet/roslyn#57042