Retrieve list item using CSOM on SharePoint online isn't working

Question

I've been stuck on this problem for couple days, I develop app model on SharePoint online using provider hosted model.

From clientwebpart, i want to access sharepoint list item. I can access list object but cant get the list item (always retrieve empty).

I already follow the sample code at "Apps for SharePoint sample pack - SharePoint 2013 Perform basic data access operations by using CSOM in apps", but still does not work.

Here are my code :

SharePointContextToken contextToken;
Uri sharepointUrl;
string accessToken;

TokenHelper.TrustAllCertificates();
string contextTokenString = TokenHelper.GetContextTokenFromRequest(Request);

if (contextTokenString != null)
{
    contextToken = TokenHelper.ReadAndValidateContextToken(contextTokenString, Request.Url.Authority);

    sharepointUrl = new Uri(Request.QueryString["SPHostUrl"]);
    accessToken = TokenHelper.GetAccessToken(contextToken, sharepointUrl.Authority).AccessToken;

    using (ClientContext clientContext = TokenHelper.GetClientContextWithAccessToken(sharepointUrl.ToString(), accessToken))
    {
        Web web = clientContext.Web;
        ListCollection lists = web.Lists;

        List selectedList = lists.GetByTitle("LeaveCategory");

        clientContext.Load<ListCollection>(lists); // this lists object is loaded successfully
        clientContext.Load<List>(selectedList);  // this list object is loaded successfully

        clientContext.ExecuteQuery();

        CamlQuery camlQuery = new CamlQuery();
        camlQuery.ViewXml = @"<View><Query><Where><IsNotNull><FieldRef Name='ID' /></IsNotNull></Where></Query><ViewFields><FieldRef Name='ID' /></ViewFields></View>";

        Microsoft.SharePoint.Client.ListItemCollection listItems = selectedList.GetItems(camlQuery);

        clientContext.Load<Microsoft.SharePoint.Client.ListItemCollection>(listItems); // problem here, this list items is return empty 

        clientContext.ExecuteQuery();
    }
}

Is there any trivia mistake that i miss ?

thank you in advance

Edit : I create fresh new project and follow instruction of How to: Create a basic provider-hosted app for SharePoint, and append code for retrieve list item, but still return 0 item.

Did anyone have ever succeed achieving this ?

Answer 1

Its working now,

its because we must add specific permission for the app at file AppManifest.xml

I add "List" at scope and give it permission "FullControl"

Answer 2

you can use this code to retrive the items from MSDN:

// Starting with ClientContext, the constructor requires a URL to the 
// server running SharePoint. 
ClientContext context = new ClientContext("http:SiteUrl"); 

// Assume the web has a list named "Announcements". 
List announcementsList = context.Web.Lists.GetByTitle("Announcements"); 

// This creates a CamlQuery that has a RowLimit of 100, and also specifies Scope="RecursiveAll" 
// so that it grabs all list items, regardless of the folder they are in. 
CamlQuery query = CamlQuery.CreateAllItemsQuery(100); 
ListItemCollection items = announcementsList.GetItems(query); 

// Retrieve all items in the ListItemCollection from List.GetItems(Query). 
context.Load(items); 
context.ExecuteQuery(); 
foreach (ListItem listItem in items) 
{ 
 // We have all the list item data. For example, Title. 
  label1.Text = label1.Text + ", " + listItem["Title"]; 
} 

Reference

Answer 3

I think issue is in your Caml Query, Just check removing Every condition from it i.e Where and also , if data is coming. After that you could cross-check your Query..