access Sharepoint document library items from provider-hosted app

Question

So I'm trying to access files from a Sharepoint document library in C#. My app is a provider-hosted Sharepoint app. I seem to be able to access the library, but not the library's items.

Here is how I get my context in the controller:

var spContext = SharePointContextProvider.Current.GetSharePointContext(System.Web.HttpContext.Current);
using (var clientContext = spContext?.CreateUserClientContextForSPHost())
{
    if (clientContext != null)
    {
        template.SetMergefields(clientContext);
    }
}

And how I try to access the files:

Web web = clientContext.Web;

List templateList = web.Lists.GetByTitle(libraryName);
clientContext.Load(templateList);
clientContext.ExecuteQuery();

var templateFiles = templateList.RootFolder.Files;
clientContext.Load(templateFiles);
clientContext.ExecuteQuery();

var templateListItems = templateList.GetItems(CamlQuery.CreateAllItemsQuery());
clientContext.Load(templateListItems);
clientContext.ExecuteQuery();

At this point, the templateList has the property ItemCount = 8 which does match the number of the files in the library. Yet, both templateFiles and templateListItems, have Count = 0 so I don't seem to be able to access these 8 items.

I have also tried to access a single item by it's id which I looked up on Sharepoint:

var itemWithId1 = templateList.GetItemById(1);
clientContext.Load(itemWithId1);
clientContext.ExecuteQuery();

Yet this leads to the error:

Microsoft.SharePoint.Client.ServerException: 'Item does not exist. It may have been deleted by another user.'

Another approach I tried was using the GetFileByServerRelativeUrl to get a specific file:

File file = clientContext.Web.GetFileByServerRelativeUrl(serverRelativeUrl);
clientContext.Load(file);
clientContext.ExecuteQuery();

This gives me the following error:

Microsoft.SharePoint.Client.ServerUnauthorizedAccessException: 'Access denied. You do not have permission to perform this action or access this resource.'

And yes I've checked - I do have full permissions for this library and it's on default settings so item-level permissions do not vary from library's permissions.

Does anyone have an idea what I'm doing wrong or how to do it properly? The actual goal is to access a specific file from the library by the file's name, a file list would also work.

Answer 1

I finally got it to work! Turns out, that the permissions on SharePoint where wrong.

On SharePoint, in the "Apps in Testing" Tab, you can click the "..." next to your app, go to "MANAGE PERMISSIONS" and you will be forwarded to a page that asks you "Do you trust XXX?". I was so focused on the question and the "Trust It" and "Cancel" Buttons that I didn't see the dropdown on the left saying "Let it have full control of the list:" with "App Packages" pre-selected. When I changed it to the list I was trying to access, one of the approaches in my code suddenly worked:

List templateList = web.Lists.GetByTitle(libraryName);
clientContext.Load(templateList);
clientContext.ExecuteQuery();

ListItemCollection templateListItems = templateList.GetItems(CamlQuery.CreateAllItemsQuery());
clientContext.Load(templateListItems);
clientContext.ExecuteQuery();

Now templateListItems actually contains objects from my library!

One can of course "optimize" the code and delete the first ExecuteQuery(), or define the CamlQuery.CreateAllItemsQuery() as a separate variable and pass it to GetItems(), or use vars instead of explicit types - all of those work, it's up to your preferences.

Appreciate all the help, I hope I've got it from here on :)