C#•2y ago

❔ a question about generics

vague title because I don't know how would I word it

void Welcome<T>(T message) where T : Parent<T> { }

internal abstract record Parent<T>();

internal sealed record Child() : Parent<string>;

void Welcome<T>(T message) where T : Parent<T> { }

internal abstract record Parent<T>();

internal sealed record Child() : Parent<string>;

this might be a weird question, but how could I achieve something like this:

Welcome<Child>("string");

Welcome<Child>("string");

where the provided T type might change the message type, for example, if Child was Parent<int> then Welcome<Child>() would for an integer

15 Replies

Aaron•2y ago

doesnt that already work as written

MODiX•2y ago

Windows10CE#8553

REPL Result: Failure

void Welcome<T>(T message) where T : Parent<T> { }

internal abstract record Parent<T>();

internal sealed record Child() : Parent<string>;
internal sealed record Child2() : Parent<int>;

Welcome<Child>("str");
Welcome<Child2>(123);

void Welcome<T>(T message) where T : Parent<T> { }

internal abstract record Parent<T>();

internal sealed record Child() : Parent<string>;
internal sealed record Child2() : Parent<int>;

Welcome<Child>("str");
Welcome<Child2>(123);

Exception: CompilationErrorException

- Argument 1: cannot convert from 'string' to 'Child'
- Argument 1: cannot convert from 'int' to 'Child2'

- Argument 1: cannot convert from 'string' to 'Child'
- Argument 1: cannot convert from 'int' to 'Child2'

Compile: 644.114ms | Execution: 0.000ms | React with ❌ to remove this embed.

Aaron•2y ago

oh, no i misread the answer is associated types

StarlkOP•2y ago

😅 this is not real code, I'm just prototyping

Aaron•2y ago

which C# does not yet have

StarlkOP•2y ago

sad

thanks for answering :)

Aaron•2y ago

essentially associated types would let you do

public abstract interface IParent
{
    public type MessageType;
}
public sealed class Child1 : IParent
{
    public type MessageType = string;
}
public sealed class Child2 : IParent
{
    public type MessageType = int;
}

static void Welcome<T>(T.MessageType message) where T : IParent

public abstract interface IParent
{
    public type MessageType;
}
public sealed class Child1 : IParent
{
    public type MessageType = string;
}
public sealed class Child2 : IParent
{
    public type MessageType = int;
}

static void Welcome<T>(T.MessageType message) where T : IParent

not the real syntax of course, but shows the idea at least

StarlkOP•2y ago

this looks really nice. I will have to find a walkaround for my issue

Aaron•2y ago

sorry, the lang design people call them existential types https://github.com/dotnet/csharplang/issues/5556

GitHub

[Proposal]: Practical existential types for interfaces · Issue #555...

Practical existential types for interfaces Proposed Prototype: Not Started Implementation: Not Started Specification: Not Started Intro Previously I proposed adding some form of existential types t...

Aaron•2y ago

associated types is the Rust (and I believe Haskell?) name for the same concept

MODiX•2y ago

Retax#0813

REPL Result: Success

void Welcome<T, TParent>(T message) where TParent : Parent<T> { }

internal abstract record Parent<T>();

internal sealed record Child() : Parent<string>;

Welcome<string, Child>("test");

void Welcome<T, TParent>(T message) where TParent : Parent<T> { }

internal abstract record Parent<T>();

internal sealed record Child() : Parent<string>;

Welcome<string, Child>("test");

Compile: 575.137ms | Execution: 83.620ms | React with ❌ to remove this embed.

MODiX•2y ago

Starlk#3366

REPL Result: Failure

void Welcome<T, TParent>(T message) where TParent : Parent<T> { }

internal abstract record Parent<T>();

internal sealed record Child() : Parent<string>;

Welcome<int, Child>("test");

void Welcome<T, TParent>(T message) where TParent : Parent<T> { }

internal abstract record Parent<T>();

internal sealed record Child() : Parent<string>;

Welcome<int, Child>("test");

Exception: CompilationErrorException

- Argument 1: cannot convert from 'string' to 'int'

- Argument 1: cannot convert from 'string' to 'int'

Compile: 645.970ms | Execution: 0.000ms | React with ❌ to remove this embed.

StarlkOP•2y ago

Oooo This is pretty neat Nice idea Thanks! I wish the feature windows10ce mentioned becomes real

Aaron•2y ago

yeah I assumed they didn't want that

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ a question about generics