Unable to use extension everything to write a generic Cast that works with LINQ query comprehensions

@jcouv

The LINQ specification expands:

from T t in source

to use source.Cast<T>().

For Enumerable.Cast<T>, this works because the source is a non-generic IEnumerable, and thus there's only the one generic type parameter for the result.

For AsyncEnumerable.Cast, this falls down. Such a Cast extension method today needs to be generic on both the TSource and TResult, e.g.

public static IAsyncEnumerable<TResult> Cast<TSource, TResult>(this IAsyncEnumerable<TSource> source)

and the C# compiler refuses to bind the from cast syntax when the arity of Cast doesn't match.

I was expecting extension everything would fix this, e.g.

extension<TSource>(IAsyncEnumerable<TSource> source)
{
    public IAsyncEnumerable<TResult> Cast<TResult>() { ... }
}

but unfortunately it doesn't. While I can write the above, it doesn't provide any expressive power that didn't already exist with extension methods today, because the resulting method still has a generic arity of 2, and I still need to provide both type arguments in order to use it, e.g.

IAsyncEnumerable<Base> source = ...;
var result = source.Cast<Base, Derived>();

and the C# compiler refuses to use it to satisfy Cast.

IAsyncEnumerable<Derived> results1 = from x in source // OK
                                     select (Derived)x;

IAsyncEnumerable<Derived> results2 = from Derived x in source // Error!!
                                     select x;

cc: @jcouv, @MadsTorgersen

@stephentoub

@stephentoub would breaking the method down into 2 components help here?

public static AsyncEnumerableCaster<TResult> Cast<TSource>(this IAsyncEnumerable<TSource> source)

public class AsyncEnumerableCaster
{
    public IAsyncEnumerable<TResult> To<TResult>();
}

Obviously it's not ideal since the call pattern then becomes different than the original IEnumerable one.

I've used this trick many times to overcome the lack of partial generic type inference in C#. A common example for us:

public TTarget Map<TSource, TTarget>(TSource source);

Becomes:

public Mapping<TSource> Map<TSource>(TSource source);

public class Mapping<TSource>
{
    TTarget To<TTarget>();
}

How does that help the LINQ query comprehension?

How does that help the LINQ query comprehension?

Well it was more of a question as to whether it could help or not. If the issue was the lack of partial inference, and an additional mapping could be added for Cast().To<T>() instead of just Cast<T>() in the pattern recognized by from T t in source that could work.

Otherwise just ignore the idea 😝

Thanks, but if the pattern for from T t in source is going to be modified, it could also be modified to recognize the two-generic-parameter signature.

Maybe introducing discards for generic arguments could solve this?

extension(IEnumerable<_>)
{
    // 1-phase lookup, for compat, generic argument in the method declaration
    public static IEnumerable<T> M<T>(out T t) => ...;
}

extension<T>(IEnumerable<T>)
{
    // 2-phase lookup, generic argument in the extension declaration
    public static IEnumerable<T> M(out T t) => ...;
}

I don't see TSource in the official implementation: AsyncEnumerable.Cast(IAsyncEnumerable)
Wouldn't this extension method work for you?

I don't see TSource in the official implementation: AsyncEnumerable.Cast(IAsyncEnumerable) Wouldn't this extension method work for you?

I wrote that extension, as a compromise. That shape doesn't work with sources that are of value types, unconstrained generics, etc.

I wrote that extension, as a compromise. That shape doesn't work with sources that are of value types, unconstrained generics, etc.

I see. Please make your concerns heard in dotnet/csharplang#8696 where my alternate proposal would actually address this issue.

I don't see TSource in the official implementation: AsyncEnumerable.Cast(IAsyncEnumerable) Wouldn't this extension method work for you?

I wrote that extension, as a compromise. That shape doesn't work with sources that are of value types, unconstrained generics, etc.

But doesn't suffer Enumerable.Cast<T> from a very similar problem?

But doesn't suffer Enumerable.Cast from a very similar problem?

No, it operates over the non-generic IEnumerable, and code like:
https://sharplab.io/#v2:EYLgxg9gTgpgtADwGwBYA+ABADAAgwRhQG4BYAKGz3wDoAZASwDsBHUs8ggZgB4mAXAHw4YOALw4A2vgA0OAEyzOAXTbkAbgEMoOWAGcArgBs+YnADMoEALY5+t242HkcL129e6YhmGBP0iQA===
works fine.

But what, if one writes:

IEnumerable<int> e = [1, 2, 3];

var result = from long i in e
             select i;

That would not work - as with any value type - because of the boxing performed under the hood...

And isn't the idea of having a Cast<T> operator exactly that - converting the element to the desired type (note, that (long)i is possible). With reference types one can cast to arbitrary target types (as long as there is a conversion).

BTW: how would a fully generic version of Cast<T,TResult> look like - I seem to miss something with the generics...

From offline and LDM discussion, the is by-design. The extensions everything feature does not make this better or worse. We would likely need a proposal for LINQ to solve this. I'll move to csharplang

From offline and LDM discussion, the is by-design. The extensions everything feature does not make this better or worse. We would likely need a proposal for LINQ to solve this. I'll move to csharplang

Please consider adding support for generic containing classes for extensions.

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Unable to use extension everything to write a generic Cast that works with LINQ query comprehensions #78415

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Unable to use extension everything to write a generic Cast that works with LINQ query comprehensions #78415

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julealgon commented on May 2, 2025

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