public int this[int param]
{
    get { return array[param]; }
    set { array[param] = value; }
}

using System;

class SampleCollection<T>
{
   // Declare an array to store the data elements.
   private T[] arr = new T[100];

   // Define the indexer to allow client code to use [] notation.
   public T this[int i]
   {
      get { return arr[i]; }
      set { arr[i] = value; }
   }
}

class Program
{
   static void Main()
   {
      var stringCollection = new SampleCollection<string>();
      stringCollection[0] = "Hello, World";
      Console.WriteLine(stringCollection[0]);
   }
}
// The example displays the following output:
//       Hello, World.

public class RedisHelper
{
    private static readonly object _lockObj = new object();
    private static RedisHelper _instance;
    private int dbNum;

    private RedisHelper() { }

    public static RedisHelper Instance 
    {
        get 
        {
            if (_instance == null)
            {
                lock (_lockObj)
                {
                    if (_instance == null)
                    {
                        _instance = new RedisHelper();
                    }
                }
            }
            return _instance;
        }
    }

    public RedisHelper this[int dbid] 
    {
        get
        {
            dbNum = dbid;
            return this;
        }
    }

    public void StringSet(string content) 
    {
        Console.WriteLine($"StringSet to redis db { dbNum }, input{ content }.");
    }
}

RedisHelper.Instance[123].StringSet("測試數據");