SharePoint CAML query gets (401) Unauthorized exception - sharepoint

Question

I have a query which looks like this:
var site = properties.Site;
var context = new ClientContext(site.Url);
List list = context.Web.Lists.GetByTitle(properties.ListTitle);
var query = new CamlQuery();
query.ViewXml = "<View><Query><Where><Eq><FieldRef Name='FileLeafRef'/>" +
"<Value Type='Text'>" + fieldName + "</Value></Eq></Where></Query></View>";
ListItemCollection itemCollection = list.GetItems(query);
context.Load(itemCollection);
context.ExecuteQuery();
In this scenario properties is an SPItemEventProperties.
Whenever it gets to ExecuteQuery(), it throws an exception: The remote server returned an error: (401) Unauthorized.
What could be the cause of this?

Answer 1

Looking at your code I guess this is RER associated with list item action. It happens that if you run it from host web (rather than app web) or you use CSOM it fails to authencitate back to SharePoint. Most likely when you will try to debug your code you will see that your RER has not received a valid context token (SPRemoteEventProperties.ContextToken is an empty string). Additionally the way you create your context using constructor
var context = new ClientContext(site.Url);
Does not deal with authenticateion. You have token methods to do that.
var context = TokenHelper.CreateRemoteEventReceiverClientContext(properties)
This will return a valid context as long as this event is on app web.
If this does not resolve your problem you will need to switch to use app permissions in your AppManifest and build a proper Realm from your SPHostUrl
var siteUrl = properties.Site.Url;
var siteUri = new Uri(siteUrl);
var realm = TokenHelper.GetRealmFromTargetUrl(siteUri);
var token = TokenHelper.GetAppOnlyAccessToken(TokenHelper.SharePointPrincipal, siteUri.Authority, realm).AccessToken;
using (var context = TokenHelper.GetClientContextWithAccessToken(siteUrl, token))
{
}
Let me know if that helps.