無法將源類型「System.Web.UI.Control」爲目標類型「System.Web.UI.WebControls.TextBox」

Question

我有下面的代碼行

objCustomField = FindControl(oList.ID.Replace("ddl", "txt")); 

我繼承此頁面一個用visual basic .net編寫的遺留應用程序，我將它轉換爲c＃。

這種特殊的行拋出這個錯誤：

Cannot convert source type 'System.Web.UI.Control' to target type 'System.Web.UI.WebControls.TextBox'

這是整個程序，它被包含在：

private void PopulateDecodeDropDown(ref DropDownList oList, string vListItems, int vValueIndex, int vTextIndex, string vSelectedValue, string vCustomText , ref TextBox oCustomField, bool vIncludeBlank = true, string vBlankText = "", string vBlankValue = "") 
    { 


     TextBox objCustomField = default(TextBox); 
     ListItem objItem = default(ListItem); 
     string[] arItems = vListItems.Split(Convert.ToChar("|")); //added syntax to convert from string to Char 10/21/15 Max // 


     oList.Items.Clear(); 


     if (vIncludeBlank == true) 
     { 
      objItem = new ListItem(); 
      var _with7 = objItem; 
      objItem.Value = vBlankValue; 
      objItem.Text = vBlankText; 
      oList.Items.Add(objItem); 
     } 

     if (!string.IsNullOrEmpty(vCustomText)) 
     { 
      objItem = new ListItem(); 
      var _with8 = objItem; 
      objItem.Value = "-1"; 
      objItem.Text = vCustomText; 
      oList.Items.Add(objItem); 
     } 

     for (n = 0; n <= arItems.Count() - 1; n++) 
     { 
      objItem = new ListItem(); 
      var _with9 = objItem; 
      objItem.Value = mobjFormat.StripObjectToString(arItems[n]); //added square brackets to arItems to facilitate array 10/21/15 Max // 
      objItem.Text = mobjFormat.StripObjectToString(arItems[n]); //added square brackets to arItems to facilitate array 10/21/15 Max // 
      oList.Items.Add(objItem); 
     } 

     try 
     { 
      //set the value 
      oList.SelectedValue = vSelectedValue; 

      //if for some reason the value selected is different then 
      //we need to show custom something when wrong 
      if (oList.SelectedValue != vSelectedValue) 
      { 

       if (!string.IsNullOrEmpty(vCustomText)) 
       { 
        objCustomField = FindControl(oList.ID.Replace("ddl", "txt")); 
        //DropDownList objCustomField = (DropDownList) FindControl(oList.ID.Replace("ddl", "txt")); 
        oList.SelectedValue = "-1"; //added double quotes to facilitate conversion to string 10/21/15 Max // 
        oCustomField.Text = vSelectedValue; 
        oCustomField.Style.Add("display", ""); //changed to c# syntax 10/21/15 Max // 
       } 
       else 
       { 
        objItem = new ListItem(); 
        var _with10 = objItem; 
        objItem.Value = Strings.Trim(arItems(vSelectedValue)); 
        objItem.Text = Strings.Trim(arItems(vSelectedValue)); 
        oList.Items.Add(objItem); 
       } 


      } 

     } 
     catch (Exception ex) 
     { 

      if (!string.IsNullOrEmpty(vCustomText)) 
      { 
       objCustomField = FindControl(oList.ID.Replace("ddl", "txt")); 
       oList.SelectedValue = -1; 
       oCustomField.Text = vSelectedValue; 
       oCustomField.Style.Item("display") = ""; 
      } 
      else 
      { 
       objItem = new ListItem(); 
       var _with11 = objItem; 
       objItem.Value = Strings.Trim(arItems(vSelectedValue)); 
       objItem.Text = Strings.Trim(arItems(vSelectedValue)); 
       oList.Items.Add(objItem); 
      } 

     } 

    } 

我知道還有其他一些語法錯誤在這很好，但此錯誤對我來說是新的。有沒有人碰到這個錯誤或許可以解釋給我

Answer 1

FindControl()返回Control類型的引用。 Control是TextBox的基類，因此您可以輕鬆使用類型轉換：

objCustomField = (TextBox) FindControl(oList.ID.Replace("ddl", "txt")); 

Answer 2

你需要投這樣

objCustomField = (TextBox)FindControl(oList.ID.Replace("ddl", "txt"));