为什么提示数组a不包含Max方法，找不到错误原因

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

namespace 类方法的数组性参数
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Maxer m = new Maxer();
            int[] a = new int[] { 4, 7, 5, 6, 8, 9 };
            int max = a.Max(a);
           // int max=a.Maxp(5,4,6,2,8,99,4,66,3,5);
            this.textBox1.Text = max.ToString();
           // this.textBox2.Text = maxp.ToString();

        }
        class Maxer
        {
            public int Max(int[] numbers)
            {
                int k = 0;

                for (int i = 0; i < numbers.Length; i++)
                {
                    if (numbers[k] < numbers[i])
                  

                        k = i;
                    }
                        return numbers[k];
                    }
                }
            
           /*  public int Maxp(int[] numbers)
            {
                int k = 0;

                for (int i = 0; i < numbers.Length; i++)
                {
                    if (numbers[k] < numbers[i])
                    

                        k = i;
                    }
                    return numbers[k];


                  }*/


            }
        
    }
}
}