C#中的嵌套类

我正在尝试研究c#中的嵌套类.在阅读了许多文档和goggling之后,我还不清楚何时使用嵌套类.但据我所知,我做了一个小样本程序.我在下面粘贴我的代码.这个嵌套类程序是否以正确的逻辑实现？ .实际上嵌套类用于什么？而且我对这个程序有疑问,我在程序中指出了这个疑问.请帮我 …

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        Bank bankObj = new Bank();
        bankObj.CreateAccount();
        bankObj.ShowMyAccountNumber();
    }
}

class Bank
{
    static int accountNumber; //  here if I just declare this as int accountNumber without static it showing an error in the CreatePersonalAccount(int accNo) method's first line ie accountNumber = accNo; as "Cannot access a non-static member of outer type." What actually this error mean ?

    public class BankAccountSection
    {
        public bool CreatePersonalAccount(int accNo)
        {
            accountNumber = accNo;
            return true;
        }
    }

    public void CreateAccount()
    {
        bool result = new BankAccountSection().CreatePersonalAccount(10001);
    }

    public void ShowMyAccountNumber()
    {
        MessageBox.Show(accountNumber.ToString());
    }
}