static void array_type<type>(type idx,type id)
                    where type : class,ICollection
                {

                    Console.WriteLine(idx[0]);//idx[0];と書きたい。


                    //foreach(var x in idx)
                    //{
                    //    Console.WriteLine(x);
                    //}

                    //for(int i=0; i< idx.Count; i++)
                    //{

                    //}

                }
                static void Main(string[] args)
                {

                    //dtest a = () => { Console.WriteLine("delegate"); };

                    //int[] str = new int[] {1,2,3,4,5,6,7,8,9,10};
                    string[] str = new string[3] { "aaa", "bb", "ccc" };
                    string[] str2 = new string[3] { "aaavvvvvvvvvvv", "bb", "ccc" };


                    array_type<string[]>(str,str2);

static void ArrayTypeA<T>(T idx)
    where T : IList
{
    for (int i = 0; i < idx.Count; ++i)
    {
        Debug.WriteLine($"A. idx[{i}] = " + idx[i]);
    }
}

static void ArrayTypeB<T>(IList<T> idx)
    where T : IEquatable<T>, IComparable<T>
{
    for (int i = 0; i < idx.Count; ++i)
    {
        Debug.WriteLine($"B: idx[{i}] = " + idx[i]);
    }

    Debug.WriteLine("idx[0] equals idx[1]: " + idx[0].Equals(idx[1]));
    Debug.WriteLine("idx[0] compareTo idx[1]:" + idx[0].CompareTo(idx[1]));
}

public void Test()
{
    Debug.WriteLine("List<string>");
    var list = new List<string>() { "A1", "A2" };
    ArrayTypeA(list);
    ArrayTypeB(list);

    Debug.WriteLine("string[]");
    var array = new string[] { "BC", "BC" };
    ArrayTypeA(array);
    ArrayTypeB(array);
}