asp.net對象引用未設置錯誤

Question

我想從aspx頁面找到標籤控件。

Label labelmessageupdate; 
      labelmessageupdate = (System.Web.UI.WebControls.Label)FindControl("updateMessage1"); 

如果我設置labelmessageupdate.Text ="something"

它返回對象引用異常。

和標籤控制在更新面板內可能是問題所在。

Answer 1

只需檢查null。始終檢查空條件，以便它不會最終顯示對象引用錯誤。

if (labelmessageupdate != null) 
{ 
    labelmessageupdate.Text ="something" 
} 

Answer 2

我認爲它不能夠找到標籤控件指定

if(FindControl("updateMessage1") is Label) 
{ 
    labelmessageupdate = FindControl("updateMessage1") as Label; 
    labelmessageupdate.Text="This shoould work if available"; 
} 

Answer 3

試試這個，和你的控制試圖找到可能是在其他用戶的控制。

要使用

Label updateMessage = FindChildControl<Label>(base.Page, "updateMessage1"); 
if (updateMessage!=null) 
{ 
    updateMessage.Text = "new text"; 
} 

/// <summary>  
/// Similar to Control.FindControl, but recurses through child controls. 
/// Assumes that startingControl is NOT the control you are searching for. 
/// </summary> 
public static T FindChildControl<T>(Control startingControl, string id) where T : Control 
{ 
    T found = null; 

    foreach (Control activeControl in startingControl.Controls) 
    { 
     found = activeControl as T; 

     if (found == null || (string.Compare(id, found.ID, true) != 0)) 
     { 
      found = FindChildControl<T>(activeControl, id); 
     } 

     if (found != null) 
     { 
      break; 
     } 
    } 

    return found; 
}