What’s the .NET collection class that allows an element to be accessed using a unique key?

 Hashtable ht = new Hashtable();

        ht["mykey1"] = "My Value1";

        ht["mykey2"] = "My Value2";

        ht["mykey3"] = "My Value3";

        ht["mykey4"] = "My Value4";



        Response.Write(ht["mykey1"].ToString());