[Proposal]: Union types

[savaparoski]

Union types

• Proposed
• Prototype: Not Started
• Implementation: Not Started
• Specification: Not Started

Summary

In TypeScript we have union types where we can say that variable status can be only one of the following values "On" | "Off" | "Idle"

type status = "On" | "Off" | "Idle"

const whatsTheStatus: status = "On"; // can be only "On" or "Off" or "Idle"

Motivation

Detailed design

Drawbacks

Alternatives

Unresolved questions

Design meetings

[gulshan]

#399

[jnm2]

Moving to a discussion. As mentioned in the issue template:

Hello, and thanks for your interest in contributing to C#! If you haven't been invited by a team member to open an issue, please instead open a discussion marked [draft issue] at https://github.com/dotnet/csharplang/discussions/new and we'll try to give you feedback on how to get to an issue-ready proposal.

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[jnm2]

@savans2 Union types already being discussed for C# are where the possible values are not constants but are rather instances of a given set of types. A union type would look like string | string[] | StringBuilder or IDisposable | IAsyncDisposable. @gulshan already linked #399 and there is a proposal draft at #4628.

The example given uses constant values, though ("On" | "Off" | "Idle"), so looks like it maps to the existing C# concept of an enum. You might be interested in the string-based enum discussion then: #2849