简单的点击按钮后有关问题

简单的点击按钮后问题
前台 <asp:Button ID="btnAddBusiness" Width="120px" runat="server" OnClick="btnAddBusiness_Click" Text="Submit" />

  public string fileName;
  protected void Page_Load(object sender, System.EventArgs e)
  {
  if (!Page.IsPostBack)
  {
  fileName = "1";
   
  }
 
  }


  protected void btnAddBusiness_Click(object sender, EventArgs e)
  {
  Response.Write(fileName); // 怎么点击按钮后 fileName 变成null 了？
   
  }

  怎么点击按钮后 fileName 变成null 了？

[解决办法]
fileName赋一个初始值不就行了么

C# code

private string fileName="XX"; protected void Page_Load(object sender, System.EventArgs e)  {  if (!Page.IsPostBack)  {  ViewState["FileName"]= "1";      }    }  protected void btnAddBusiness_Click(object sender, EventArgs e)  {     Response.Write(this.FileName);   }