问在C#中更改下拉菜单项时如何在文本框中插入内容

EN

  using System;
  using System.Collections.Generic;
  using System.Linq;
  using System.Web;
  using System.Web.UI;
  using System.Web.UI.WebControls;

  namespace OrderManagementSystem
  {
  public partial class MakePayment : System.Web.UI.Page
  {
    Dictionary<int, int> InvoiceAmount = new Dictionary<int, int>();



    protected void Page_Load(object sender, EventArgs e)
    {
        ChequeNumber.Enabled = false;

    }

    protected void InvoiceNo_SelectedIndexChanged(object sender, EventArgs e)
    {
        InvoiceAmount.Add(1111, 100000);
        InvoiceAmount.Add(2222, 200000);
        InvoiceAmount.Add(3333, 300000);
        InvoiceAmount.Add(4444, 400000);


        int InvoiceValue = Convert.ToInt32(InvoiceNo.SelectedValue);
        foreach (KeyValuePair<int, int> item in InvoiceAmount)
        {
            if (InvoiceValue == item.Key)
            {
                TotalAmount.Text = Convert.ToString(item.Value);
            }
        }
    }

    protected void PaymentMode_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (PaymentMode.SelectedValue == "Cheque")
        {

            ChequeNumber.Enabled = true;

            if (ChequeNumber.Text=="")
            {

            }
        }

    }

    protected void TotalAmount_TextChanged(object sender, EventArgs e)
    {

    }
  }
 }