使用FindControl时，对象引用未设置为对象的实例

Question

I am using the following code.

.ascx file:

<div class="DemoArea">
    <asp:Button ID="btnCaseComplete" runat="server" Text="Case Complete" CssClass="btn_contentlist"
        onclick="btnCaseComplete_Click" OnClientClick="scroll(0,0);$.loading({mask: true, effect: 'ellipsis update'});"/>
       <ComponentArt:Dialog ID="caDropDownDialog" runat="server" Modal="true" Alignment="MiddleCentre" AllowDrag="true" AllowResize="false" AnimationDuration="1000" 
                            CloseTransition="Fade"  RenderOverWindowedObjects="true" ShowTransition="Fade" AnimationType="Outline" CssClass="ModalMask">
                            <Header><p class="header">Case Complete</p></Header>
                            <Content>
                                <asp:Panel ID="panSelectArea" runat="server" CssClass="modalMaskContent">

                                   <p><span class="red">Please Note:</span>Once you click
                                        <span class="bold">Ok</span>, your 
                                            case will be Submitted to ACR and you will not be able to edit the Case again. 

                                            <span class="style2">To continue editing the case, click </span>
                                            <span class="bold">Cancel</span>. You will be taken back 
                                            to the Case Wizard and your case will not be submitted to ACR.</p>
                                  </asp:Panel>
                            </Content>
                            <Footer>
                              <center class="modalMaskFooter">
                                    <asp:Button ID="btnOK" runat="server" CausesValidation="false" 
                                        CssClass="btn_contentlist" OnClientClick="caDropDownDialog.IsShowing=false" Text="OK" />
                                    <asp:Button ID="btnCancel" runat="server" CausesValidation="false" 
                                        CssClass="btn_contentlist" OnClientClick="caDropDownDialog.Close();" Text="Cancel" />
                                </center>
                            </Footer>
                        </ComponentArt:Dialog>
    </div>

Code behind:

CaseContentList obj = new CaseContentList(); // creating the object of case content list control to this page

        LinkButton lbtn = (LinkButton)((DataList)obj.FindControl("dlstContentList")).FindControl("lbtnDisplay");
        if (lbtn.Text == "Final Page")
        {
            caDropDownDialog.IsShowing = true;
        }
        else
            Page.ClientScript.RegisterStartupScript(this.GetType(), "Window", "alert('Add Final Page First.');", true);

But it's giving an error 'object reference not set to an instance of an object'.

Answer 1

It is caused by result of one of "FindControl()" method. The result is NULL and not instance of control.

You must check for null:

bool found = false;
var dlstContentList = obj.FindControl("dlstContentList");
if ( null != dlstConentList ) {
   var lbtnDisplay = dlstContentList.FindControl("lbtnDisplay");
   found = (null != lbtnDisplay);
}

if ( found ) {
   // ... do something
}
else {
  // ... do something else
}

Answer 2

CaseContentList cl = (CaseContentList)this.Parent.TemplateControl.FindControl("ContentList");
        if(cl.IsFinalPage)
            caDropDownDialog.IsShowing = true;
        else
          Page.ClientScript.RegisterStartupScript(this.GetType(), "Window", "alert('Please add Final Page to complete your case');", true);

Answer 3

If it is a DataList and you are looking for a LinkButton, then I would expect you would need to loop through each ListItem within the DataList for the one you want, then find the control in that ListItem

To loop through your ListItems within the DataList you could do this

foreach (DataListItem dataListItem in obj.Items) {

if (dataListItem .ItemType == ListItemType.AlternatingItem | dl.ItemType == ListItemType.Item) {
        //
        // find the control here.
}
}