对回调函数的理解

1 public sealed class Timer : MarshalByRefObject, IDisposable
2     {
3         
4         [SecuritySafeCritical]
5         public Timer(TimerCallback callback);
6     }

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace UseDelegate
{
    public partial class FrmOther : Form
    {
        public FrmOther()
        {
            InitializeComponent();
        }

        public void ShowCount(int count)//这个方法并没哟被这个类调用
        {
            label1.Text = count.ToString();
        }
        private void label1_Click(object sender, EventArgs e)
        {

        }
    }
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace UseDelegate
{

    public partial class FrmMain : Form
    {
        private Action<int> ReceiverMethods;//定义一个用于挂接回调函数的委托
        public FrmMain()
        {
            InitializeComponent();
        }

        private FrmOther other = null;
        private int count = 0;
        private void NewForm()
        {
            other = new FrmOther();
            other.Show();
        }
        private void btnNewFrm_Click(object sender, EventArgs e)
        {
            NewForm();
            ReceiverMethods += other.ShowCount;//在这里挂接FrmOther的用于回调的函数
        }

        private void btnAdd_Click(object sender, EventArgs e)
        {
            if (ReceiverMethods != null)
            {
                ReceiverMethods(++count);//在这里执行回调函数
            }
        }
    }
}