Dictionary with maximun length, keep last N keys on dictionary, using Queue object (C#)

DictWithMaxKeys.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.Threading.Tasks;

namespace DictMaxLength
{
    class DictWithMaxKeys : Dictionary<string, int>, IDictionary
    {
        private Dictionary<string, int> _dict;
        private Queue<string> _queue;
        private int _maxLength;

        public DictWithMaxKeys(int maxLength)
        {
            _dict = new Dictionary<string, int>();
            _queue = new Queue<string>();
            _maxLength = maxLength;
        }

        public int this[string key] {
            get => _dict[key];
            set
            {
                if (!_dict.Keys.Contains(key))
                {
                    if (_queue.Count >= _maxLength)
                    {
                        var lastKey = _queue.Dequeue();
                        _dict.Remove(lastKey);
                    }

                    _queue.Enqueue(key.ToString());
                }

                _dict.Add(key, value);
            }
         }

        public object this[object key] { get => ((IDictionary)_dict)[key]; set => ((IDictionary)_dict)[key] = value; }

        public ICollection Keys => ((IDictionary)_dict).Keys;

        public ICollection Values => ((IDictionary)_dict).Values;

        public bool IsReadOnly => ((IDictionary)_dict).IsReadOnly;

        public bool IsFixedSize => ((IDictionary)_dict).IsFixedSize;

        public int Count => ((IDictionary)_dict).Count;

        public object SyncRoot => ((IDictionary)_dict).SyncRoot;

        public bool IsSynchronized => ((IDictionary)_dict).IsSynchronized;

        public void Add(object key, object value)
        {
            ((IDictionary)_dict).Add(key, value);
        }

        public void Clear()
        {
            ((IDictionary)_dict).Clear();
        }

        public bool Contains(object key)
        {
            return ((IDictionary)_dict).Contains(key);
        }

        public void CopyTo(Array array, int index)
        {
            ((IDictionary)_dict).CopyTo(array, index);
        }

        public IDictionaryEnumerator GetEnumerator()
        {
            return ((IDictionary)_dict).GetEnumerator();
        }

        public void Remove(object key)
        {
            ((IDictionary)_dict).Remove(key);
        }

        public override string ToString()
        {
            // Output dictionary sorted by values

            var pairsList = _dict.ToList();
                
            pairsList.Sort((p1, p2) => p1.Value.CompareTo(p2.Value));

            return string.Join("|", pairsList.Select(x => string.Format("{0}{1}{2}", x.Key, ':', x.Value)));
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return ((IDictionary)_dict).GetEnumerator();
        }
    }
}

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace DictMaxLength
{
    class Program
    {
        static void Main(string[] args)
        {
            var letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

            var d = new DictWithMaxKeys(5);
            Console.WriteLine(d);
        
            for (int i=0; i < letters.Length; i++)
            {
                d[letters[i].ToString()] = i;

                Console.WriteLine(d);
            }
        }
    }
}

Output from Program.cs

a:0
a:0|b:1
a:0|b:1|c:2
a:0|b:1|c:2|d:3
a:0|b:1|c:2|d:3|e:4
b:1|c:2|d:3|e:4|f:5
c:2|d:3|e:4|f:5|g:6
d:3|e:4|f:5|g:6|h:7
e:4|f:5|g:6|h:7|i:8
f:5|g:6|h:7|i:8|j:9
g:6|h:7|i:8|j:9|k:10
...
E:30|F:31|G:32|H:33|I:34
F:31|G:32|H:33|I:34|J:35
G:32|H:33|I:34|J:35|K:36
H:33|I:34|J:35|K:36|L:37
I:34|J:35|K:36|L:37|M:38
J:35|K:36|L:37|M:38|N:39
K:36|L:37|M:38|N:39|O:40
L:37|M:38|N:39|O:40|P:41
M:38|N:39|O:40|P:41|Q:42
N:39|O:40|P:41|Q:42|R:43
O:40|P:41|Q:42|R:43|S:44
P:41|Q:42|R:43|S:44|T:45
Q:42|R:43|S:44|T:45|U:46
R:43|S:44|T:45|U:46|V:47
S:44|T:45|U:46|V:47|W:48
T:45|U:46|V:47|W:48|X:49
U:46|V:47|W:48|X:49|Y:50
V:47|W:48|X:49|Y:50|Z:51