System.NullReferenceException：未將對象引用設置爲對象iframe的實例src

Question

我在通過ASP.NET C＃傳遞iframe的src值時收到此錯誤。這裏isrc是一個字符串變量：

Line 26:   HtmlControl ifm = (HtmlControl)this.FindControl("ifm"); 
Line 27:   if (cover_id != null) 
Line 28:   ifm.Attributes["src"] = isrc; 
Line 29:   //Response.Redirect("Default2.aspx") 
Line 30:  } 

    \Default2.aspx.cs Line: 28 
    Source File: c:\Users\arsha\Documents\Visual Studio 2008\WebSites\hajcover\Default2.aspx.cs Line: 28 

堆棧跟蹤：

[NullReferenceException: Object reference not set to an instance of an object.] 
    Default2.btn1_Click(Object sender, EventArgs e) in c:\Users\arsha\Documents\Visual Studio 2008\WebSites\hajcover\Default2.aspx.cs:28 
    System.Web.UI.WebControls.Button.OnClick(EventArgs e) +111 
    System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +110 
    System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10 
    System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13 
    System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +36 
    System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1565 


Version Information: Microsoft .NET Framework Version:2.0.50727.4984; ASP.NET Version:2.0.50727.4971