E gọi hàm nhập danh sách nhân viên mà nó cứ bị lỗi

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace bai_28
{
class CNhanVien
{
    protected string HoTen, PhongBan;
    protected int CMND;
    //Rang buoc
    private bool KiemTraChungMinh(int a)
    {
        return (a > 0);
    }
    //Khoi tao
    public CNhanVien()
    {
        HoTen = "Pham Thanh Trung";
        PhongBan = "IT";
        CMND = 17110394;
    }
    public CNhanVien(string t, string p, int c)
    {
        if (KiemTraChungMinh(c))
        {
            HoTen = t; PhongBan = p; CMND = c;
        }
        else
        {
            HoTen = "Pham Thanh Trung";
            PhongBan = "IT";
            CMND = 17110394;
        }
    }
    public CNhanVien(CNhanVien x)
    {
        HoTen = x.HoTen;
        PhongBan = x.PhongBan;
        CMND = x.CMND;
    }
    //Cap nhat va truy xuat thong tin
    public string Ten
    {
        get { return HoTen; }
        set { HoTen = value; }
    }
    public string Ban
    {
        get { return PhongBan; }
        set { PhongBan = value; }
    }
    public int MaSoCMND
    {
        get { return CMND; }
        set { CMND = value; }
    }
    //Xu ly thong tin
    public virtual void Nhap()
    {
        Console.Write("Nhap ho va ten: "); HoTen = Console.ReadLine();
        Console.Write("Nhap phong ban: "); PhongBan = Console.ReadLine();
        Console.Write("Nhap CMND: "); CMND = int.Parse(Console.ReadLine());
    }
    public virtual void Xuat()
    {
        Console.WriteLine("Ho va ten: "+HoTen);
        Console.WriteLine("Phong ban: "+PhongBan);
        Console.WriteLine("CMND: "+CMND);
    }
}
class CBienChe : CNhanVien
{
    private int BacLuong;
    //Rang buoc
    private bool KiemTra(int b)
    {
        return (b > 0);
    }
    //cap nhat va truy cuat thong tin
    public int Luong
    {
        get { return BacLuong; }
        set
        {
            if (KiemTra(value))
                BacLuong = value;
        }
    }
    //Xu ly thong tin
    public override void Nhap()
    {
        base.Nhap();
        Console.Write("Nhap bac luong: ");
        do
        {
            BacLuong = int.Parse(Console.ReadLine());
            if (KiemTra(BacLuong) == false)
                Console.Write("Nhap lai bac luong: ");
        } while (KiemTra(BacLuong) == false);
    }
    public override void Xuat()
    {
        base.Xuat();
        Console.WriteLine("Luong cua nhan vien la: " + BacLuong);
    }
}
 class DanhSachNhanVienBienChe
{
    CBienChe[] A;
    private int n;
    //Xu ly thong tin
    public void Nhap()
    {
        Console.Write("Nhap so luong nhan vien bien che: ");n = int.Parse(Console.ReadLine());
        for(int i=0;i<n;i++)
        {
           A[i] = new CBienChe();
            A[i].Nhap();
        }
    }
    public void Xuat()
    {
        
    }
}
 class Program
{
    static void Main(string[] args)
    {
        //CBienChe m = new CBienChe();
        //m.Nhap();
        //m.Xuat();
        //CHopDong n = new CHopDong();
        //n.Nhap();
        //n.Xuat();
        DanhSachNhanVienBienChe m = new DanhSachNhanVienBienChe();
        m.Nhap();
    }
}

Lỗi:

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