public class aaa
    {
        static void Main(string[] args)
        {
            IA ia = new A();
            A[0] = 1;
            Console.Read();
        }
    }
    public interface IA
    {
        private int[] value;
        string this[int index]
        {
            get
            {
                return values[index];
            }
            set
            {
                values[index] = value;
            }
        }
    }
    public class A : IA
    {
        private int[] values;
        string this[int index]
        {
            get
            {
                return values[index];
            }
            set
            {
                values[index] = value;
            }
        }
        public A()
        {
            valuses = new int[10];
        }
    }
}

interface IA
    {
        int this[int i] { get; set; }
    }

    class A : IA
    {
        private int[] value;
        #region IA 成员

        public int this[int i]
        {
            get
            {
                return value[i];
            }
            set
            {
                this.value[i] = value;//你的数组名value,用在这里很不好，和索引器的value同名，所以要用this.指定
            }
        }

        #endregion
    }

IA ia = new A();
A[0] = 1;
Console.Read();

你这个也用得太奇葩了，IA ia = new A();private int[] value;A[0] ==  value[0],你那value是没赋值的，会错误的