Question

Writing code with implicit cast generates CS0266.

Dictionary<Int32, string[]> d1 = new Dictionary<int, string[]>();
IDictionary<int, IEnumerable<string>> ide1 = d1; // CS0266

The hint that an explicit cast exists suggests an explicit cast would fix the issue.

IDictionary<int, IEnumerable<string>> ide1 = (IDictionary<int, IEnumerable<string>>)d1; // No error, but throws an exception

Is there a way to make this cast ? Would it work for string[][] and IEnumerable<IEnumerable<string>> ?

namespace Quickie
{
    class QuickieArray
    {
        private static void TestCastDictionaryWithArrayValue()
        {
            Console.WriteLine();
            Console.WriteLine("===TestCastDictionaryWithArrayValue===");
            Dictionary<Int32, string[]> d1 = new Dictionary<int, string[]>();
            d1[1] = new string[]{"foo"};
            IDictionary<int, string[]> id1 = d1;
            Console.WriteLine("id1 is null: {0}", id1 == null);
            IDictionary<int, IEnumerable<string>> ide1 = id1 as IDictionary<int, IEnumerable<string>>;
            IDictionary<int, IEnumerator<string>> iden1 = id1 as IDictionary<int, IEnumerator<string>>;
            Console.WriteLine("ide1 is null: {0}", ide1 == null);
            Console.WriteLine("iden1 is null: {0}", iden1 == null);

            Console.WriteLine();
        }
        internal static void Test()
        {
            Console.WriteLine();
            Console.WriteLine("=QuickieArray=");
            TestCastDictionaryWithArrayValue();
        }
    }
}

Output:

===TestCastDictionaryWithArrayValue===
id1 is null: False
ide1 is null: True
iden1 is null: True

Answer 1

As @adam-maras and @d-stanley stated, this is a covariance problem. Therefore, wherever possible use interfaces. For example, use IDictionary<Int32, IEnumerable<string>> as the less-derived type holding the data instantiated with a more-derived type:

using System;
using System.Collections.Generic;

namespace Quickie
{
    class QuickieArray
    {
        private static void TestCastIDictionaryWithArrayValue()
        {
            Console.WriteLine();
            Console.WriteLine("===TestCastIDictionaryWithArrayValue===");
            IDictionary<int, IEnumerable<string>> d1 = new Dictionary<int, IEnumerable<string>>();
            d1[1] = new string[] { "foo" };
            IDictionary<int, IEnumerable<string>> id1 = d1;
            Console.WriteLine("id1 is null: {0}", id1 == null);
            IDictionary<int, IEnumerable<string>> ide1 = id1 as IDictionary<int, IEnumerable<string>>;
            IDictionary<int, IEnumerator<string>> iden1 = id1 as IDictionary<int, IEnumerator<string>>;
            Console.WriteLine("ide1 is null: {0}", ide1 == null);
            Console.WriteLine("iden1 is null: {0}", iden1 == null);

            Console.WriteLine();
        }

        internal static void Test()
        {
            Console.WriteLine();
            TestCastIDictionaryWithArrayValue();
        }
    }
}

And as far as an explicit cast existing ? No it doesn’t.