using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var cache = new MyCache();

        cache.Add("foo", "bar");

        cache.Add("baz", "qux");

        foreach (var k in new[] { "foo", "baz", "xyz", "abc" })

          Console.WriteLine(k + ": " + (cache[k] ?? "Miss!"));

    }

}

​

public class MyCache 

{

    private IDictionary<string, string> _values = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);

    public void Add(string key, string value) {

        _values.TryAdd(key, value);

    }

    public string this[string key] {  

        get {

            if (_values.TryGetValue(key, out var result))

                return result;

            return null;

        }

   }

}