Covariance and contravariance on Tasks

Question

Given the followin snippet, i quite dont understand WHY what im going to achieve is not possible:

Interface:

public interface IEntityRepository<out T> : IRepository<IEntity> {

    void RequeryDataBase();

    IEnumerable<T> Search(string pattern);

    Task<IEnumerable<T>> SearchAsync(string pattern);

    SearchContext Context { get; }

    string BaseTableName { get; }
  }

In IRepository<IEntity> are just simple generic CRUD defined.

I get an Error on this line: Task<IEnumerable<T>> SearchAsync(string pattern);

Error:

method return type must be output safe. invalid variance: the type parameter T must be invariantly valid on Task

Please help me understand, why i cant use <out T> with a Task<T>

Answer 1

Task<T> is not covariant. Variance can only be applied to generic interfaces (and delegates, but that's not relevant here).

e.g. Task<IEnumerable<Dog>> cannot be assigned to Task<IEnumerable<Animal>>. Because of this, your interface cannot be marked as covariant either.

You might want to see this related question.