c# - NullReferenceException Controls.Add c# 错误

Question

开始调试程序后出现以下错误。你知道如何解决这个问题吗？

System.NullReferenceException was unhandled by user code
Message="Object reference not set to an instance of an object."

它指的是行:

pnlDropDownList.Controls.Add(ddl);

在 CreateDropDownLists 方法中。显然 ddl 必须是一个空对象，即使我之前用同样的方法初始化了 ddl。你明白我为什么会收到这个错误吗？请参阅下面的代码...

using System;
using System.Collections;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Diagnostics;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using ADONET_namespace;
using MatrixApp;

namespace AddFileToSQL
{
public partial class DataMatch : _Default
{

protected System.Web.UI.WebControls.PlaceHolder phTextBoxes;
protected System.Web.UI.WebControls.PlaceHolder phDropDownLists;
protected System.Web.UI.WebControls.Button btnAnotherRequest;
protected System.Web.UI.WebControls.Panel pnlCreateData;
protected System.Web.UI.WebControls.Literal lTextData;
protected System.Web.UI.WebControls.Panel pnlDisplayData;
Panel pnlDropDownList;
protected static string inputfile2;
static string[] headers = null;
static string[] data = null;
static string[] data2 = null;
static DataTable myInputFile = new DataTable("MyInputFile");
static string[] myUserSelections;

// a Property that manages a counter stored in ViewState
protected int NumberOfControls
{
get { return (int)ViewState["NumControls"]; }
set { ViewState["NumControls"] = value; }
}

public void EditRecord(object recordID)
{
SelectedRecordID = recordID;
// Load record from database and show in control
}

protected object SelectedRecordID
{
get
{
return ViewState["SelectedRecordID"];
}

set
{
ViewState["SelectedRecordID"] = value;
}
}

protected void OnPreLoad(object sender, EventArgs e)
{

//Create a Dynamic Panel
pnlDropDownList = new Panel();
pnlDropDownList.ID = "pnlDropDownList";
pnlDropDownList.BorderWidth = 1;
pnlDropDownList.Width = 300;
this.form1.Controls.Add(pnlDropDownList);
}

// Page Load 
private void Page_Load(object sender, System.EventArgs e)
{
if (!Page.IsPostBack)
{
this.NumberOfControls = 0;
}
}

// Add DropDownList Control to Placeholder
private void CreateDropDownLists()
{
for (int counter = 0; counter < NumberOfControls; counter++)
{
DropDownList ddl = new DropDownList();
SqlDataReader dr = ADONET_methods.DisplayTableColumns(targettable);
ddl.ID = "DropDownListID" + (counter + 1).ToString();
ddl.DataTextField = "COLUMN_NAME";
ddl.DataValueField = "COLUMN_NAME";
ddl.DataSource = dr;
ddl.DataBind();
ddl.AutoPostBack = true;
ddl.EnableViewState = true; //Preserves View State info on Postbacks
ddl.Style["position"] = "absolute";
ddl.Style["top"] = 100 * counter + 80 + "px";
ddl.Style["left"] = 250 + "px";
ddl.SelectedIndexChanged += new EventHandler(this.OnSelectedIndexChanged);
pnlDropDownList.Controls.Add(ddl);
pnlDropDownList.Controls.Add(new LiteralControl("<br><br><br>"));
dr.Close();
}
}

private void CreateLabels()
{
for (int counter = 0; counter < NumberOfControls; counter++)
{
Label lbl = new Label();
lbl.ID = "Label" + counter.ToString();
lbl.Text = headers[counter];
lbl.Style["position"] = "absolute";
lbl.Style["top"] = 100 * counter + 50 + "px";
lbl.Style["left"] = 250 + "px";
pnlDropDownList.Controls.Add(lbl);
pnlDropDownList.Controls.Add(new LiteralControl("<br><br><br>"));
}
}

// Add TextBoxes Control to Placeholder
private void RecreateDropDownLists()
{
for (int counter = 0; counter < NumberOfControls; counter++)
{
DropDownList ddl = new DropDownList();
SqlDataReader dr = ADONET_methods.DisplayTableColumns(targettable);
ddl.ID = "DropDownListID" + (counter + 1).ToString();
ddl.DataTextField = "COLUMN_NAME";
ddl.DataValueField = "COLUMN_NAME";
ddl.DataSource = dr;
ddl.DataBind();
myUserSelections[counter] = "";
dr.Close();
ddl.AutoPostBack = true;
ddl.EnableViewState = false; //Preserves View State info on Postbacks
ddl.Style["position"] = "absolute";
ddl.Style["top"] = 100 * counter + 80 + "px";
ddl.Style["left"] = 250 + "px";
pnlDropDownList.Controls.Add(ddl);
pnlDropDownList.Controls.Add(new LiteralControl("<br><br><br>"));
}
}

// Create TextBoxes and DropDownList data here on postback.
protected override void CreateChildControls()
{
// create the child controls if the server control does not contains child controls
this.EnsureChildControls();
// Creates a new ControlCollection. 
this.CreateControlCollection();
// Here we are recreating controls to persist the ViewState on every post back

if (Page.IsPostBack)
{
RecreateDropDownLists();
RecreateLabels();
}

// Create these conrols when asp.net page is created
else
{
PopulateFileInputTable();
CreateDropDownLists();
CreateLabels();
}

// Prevent child controls from being created again.
this.ChildControlsCreated = true;
}

private void AppendRecords()
{
switch (targettable)
{
case "ContactType":
for (int r = 0; r < myInputFile.Rows.Count; r++)
{ ADONET_methods.AppendDataCT(myInputFile.Rows[r]); }
break;
case "Contact":
for (int r = 0; r < myInputFile.Rows.Count; r++)
{ ADONET_methods.AppendDataC(myInputFile.Rows[r]); }
break;
case "AddressType":
for (int r = 0; r < myInputFile.Rows.Count; r++)
{ ADONET_methods.AppendDataAT(myInputFile.Rows[r]); }
break;

default: throw new ArgumentOutOfRangeException("targettable type", targettable);
}
}

// Read all the data from TextBoxes and DropDownLists 
protected void btnSubmit_Click(object sender, System.EventArgs e)
{
int cnt = FindOccurence("DropDownListID");
EditRecord("DropDownListID" + Convert.ToString(cnt + 1));
AppendRecords();
pnlDisplayData.Visible = false;
}

private int FindOccurence(string substr)
{
string reqstr = Request.Form.ToString();
return ((reqstr.Length - reqstr.Replace(substr, "").Length) / substr.Length);
}

#region Web Form Designer generated code

override protected void OnInit(EventArgs e)
{
InitializeComponent();
base.OnInit(e);
}

private void InitializeComponent()
{
(this.btnSubmit_Click);
}

#endregion

}
}

Answer 1

我认为它指的是“pnlDropDownList”是一个 Null 对象。该函数必须在“pnlDropDownList = new Panel();”之前被调用在“OnPreLoad”中。使用断点单步执行源代码并遵循代码路径。我相信您会发现情况就是如此。