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[API Proposal]: Task<T>.CastUp #89778

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#136898
Open
[API Proposal]: Task<T>.CastUp#89778
#136898
Labels
api-approvedAPI was approved in API review, it can be implementedarea-System.Threading.Tasksin-prThere is an active PR which will close this issue when it is merged
Milestone
Future

Description

@alrz
alrz
opened on Aug 1, 2023

Background and motivation

Task does not support covariant type args. This would be similar to ImmutableArray<T>.CastUp.

The current workaround is to simply await the task or use ContinueWith to cast.

API Proposal

public class Task<T>
{
+     public static Task<T> CastUp<TDerived>(Task<TDerived> task) where TDerived : class?, T;
}

public struct ValueTask<T>
{
+     public static ValueTask<T> CastUp<TDerived>(ValueTask<TDerived> task) where TDerived : class?, T;
}

API Usage

Task<string> task1 = e;
Task<object> task2 = Task<object>.CastUp(task);

Alternative Designs

No response

Risks

No response

Activity

  1. alrz
    added
    api-suggestionEarly API idea and discussion, it is NOT ready for implementation
    on Aug 1, 2023
  2. ghost added
    area-System.Threading.Tasks
    on Aug 1, 2023
  3. ghost added
    untriagedNew issue has not been triaged by the area owner
    on Aug 1, 2023

  5. huoyaoyuan commented on Aug 2, 2023

    @huoyaoyuan
    huoyaoyuan
    on Aug 2, 2023
    Member

    A covariant interface may be useful for Task, but for ValueTask a CastUp may help better.

  6. ericstj
    added
    needs-further-triageIssue has been initially triaged, but needs deeper consideration or reconsideration
    and removed
    untriagedNew issue has not been triaged by the area owner
    on Aug 8, 2023
  7. ericstj
    added this to the Future milestone on Aug 8, 2023

  8. ericstj commented on Aug 8, 2023

    @ericstj
    ericstj
    on Aug 8, 2023
    Member

    cc @stephentoub

  9. tannergooding
    added
    api-ready-for-reviewAPI is ready for review, it is NOT ready for implementation
    and removed
    api-suggestionEarly API idea and discussion, it is NOT ready for implementation
    needs-further-triageIssue has been initially triaged, but needs deeper consideration or reconsideration
    on Jul 11, 2026

  10. bartonjs commented on Sep 8, 2026

    @bartonjs
    bartonjs
    on Sep 8, 2026
    Member

    Video

    Looks good as proposed.

    public partial class Task<T>
{
    public static Task<T> CastUp<TDerived>(Task<TDerived> task) where TDerived : class?, T;
}

public partial struct ValueTask<T>
{
    public static ValueTask<T> CastUp<TDerived>(ValueTask<TDerived> task) where TDerived : class?, T;
}
  11. bartonjs
    added
    api-approvedAPI was approved in API review, it can be implemented
    and removed
    api-ready-for-reviewAPI is ready for review, it is NOT ready for implementation
    on Sep 8, 2026
  12. dotnet-policy-service
    added
    in-prThere is an active PR which will close this issue when it is merged
    on Nov 3, 2026

  13. hez2010 commented on Nov 3, 2026

    @hez2010
    hez2010
    on Nov 3, 2026
    Contributor

    We now have another motivating case that can be greatly helped by target-type generic inference. cc: @MadsTorgersen

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