Why do C# out generic type parameters violate covariance?
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29-10-2019 - |
Question
I'm unclear as to why the following code snippet isn't covarient?
public interface IResourceColl<out T> : IEnumerable<T> where T : IResource {
int Count { get; }
T this[int index] { get; }
bool TryGetValue( string SUID, out T obj ); // Error here?
}
Error 1 Invalid variance: The type parameter 'T' must be invariantly valid on 'IResourceColl.TryGetValue(string, out T)'. 'T' is covariant.
My interface only uses the template parameter in output positions. I could easily refactor this code to something like
public interface IResourceColl<out T> : IEnumerable<T> where T : class, IResource {
int Count { get; }
T this[int index] { get; }
T TryGetValue( string SUID ); // return null if not found
}
but I'm trying to understand if my original code actually violates covariance or if this is a compiler or .NET limitation of covariance.
No correct solution
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