Question

I have a struct that implements some interface. This works fine until I have an array of the struct implementation and try to implicitly cast that array to another array of the interface type. (See the below code example)

using System.Collections.Generic;

namespace MainNS
{
    public interface IStructInterface
    {
        string Name { get; }
    }

    public struct StructImplementation : IStructInterface
    {
        public string Name
        {
            get { return "Test"; }
        }
    }

    public class MainClass
    {
        public static void Main()
        {
            StructImplementation[] structCollection = new StructImplementation[1]
            {
                new StructImplementation()
            };

            // Perform an implicit cast
            IEnumerable<IStructInterface> castCollection = structCollection;    // Invalid implicit cast
        }
    }
}

When compiling the above code, I get the error:

error CS0029: Cannot implicitly convert type ‘MainNS.StructImplementation[]’ to ‘MainNS.IStructInterface[]’

If I change StructImplementation to a class I have no problems, so I’m assuming what I’m trying to do is either not valid; or I’m being blind and missing something obvious.

Any advice or explanation for this would be appreciated.

EDIT

In case anyone else has this issue and using a different approach is less than ideal (as was the case in my situation), I worked around my issue using the LINQ method Cast<T>(). So in the example above, I would perform the cast using something like:

IEnumerable<IStructInterface> castCollection = structCollection.Cast<IStructInterface>();

There is a good article on MSDN about Variance in Generic Types, which I found very useful.

Answer 1

Array variance ony allows for the reference preserving case, and so only works for classes. It is essentially treating the original data as a reference to a different type. This is simply not possible with structs.