Office 365 Sandbox Solution EventReceiver throwing Remote Exception in ItemAdding

Question

Hi,
I created a sandbox webpart for O365 with EventReceivers with ItemAdding for Document Library and while i upload a document to library in O365  sharepoint site application throws below exception:-
System.Runtime.Remoting.RemotingException: Server encountered an internal error. For more information, turn off customErrors in the server's .config file.
Server stack trace: 
Exception rethrown at [0]: 
   at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)
   at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)
   at Microsoft.SharePoint.Administration.ISPUserCodeExecutionHostProxy.Execute(Type us
Same code works perfectly on my Development machine, Please help. Below is the Code
base.EventFiringEnabled = false;
bool isFile = (properties.AfterProperties["vti_filesize"] != null);
if (isFile == true)                   
SPWeb currentWeb = properties.OpenWeb();
// Get foldername from url like Document/EC10001/filename.txt                       
string folderName = properties.AfterUrl.Split(new char[] { '/' })[1];
SPList spList = currentWeb.Lists[properties.List.ID];                       
SPQuery spQuery = new SPQuery();                       
spQuery.Query = "<OrderBy><FieldRef Name='Modified' Ascending='FALSE'/></OrderBy>";
//Getting the folder object from the list                       
SPFolder folder = spList.RootFolder.SubFolders[folderName];
//Set the Folder property                       
spQuery.Folder = folder;
int fileSequenceId = 0;                      
SPListItemCollection items = spList.GetItems(spQuery);
if (items.Count > 0)                       
string documentID = items[0]["DocumentID"] != null ? items[0]["DocumentID"].ToString() : string.Empty;
if (!string.IsNullOrEmpty(documentID))                           
string splitNumber = documentID.Split(new char[] { '-' })[1];                               
fileSequenceId = Convert.ToInt32(splitNumber) + 1;                           
else                           
properties.ErrorMessage = "Unable to generate Document Id";                               
properties.Cancel = true;                           
else                       
fileSequenceId = 1;                       
// Set DocumentID like EC10001-001                       
properties.AfterProperties["DocumentID"] = folderName + "-" + fileSequenceId.ToString(ConstantsList(currentWeb, "DocumentID"));      
// Retrive "EEC000" string from Constant List               
properties.AfterProperties["vti_title"] = folderName + "-" + fileSequenceId.ToString(ConstantsList(currentWeb, "DocumentID"));   
// Retrive "EEC000" string from Constant List                
base.EventFiringEnabled = true;
Thanks,
Pranay Chandra Sapa

Answer 1

Hi,
According to your description, my understanding is that when you upload document in Office 365 site, the event receiver in sandbox solution throws error.
Per my knowledge, if you want to use event receiver in Office 365 environment, you need to use remote event receiver instead the normal event receiver in an app.
Here are some detailed articles for your reference:
Create a remote event receiver in apps for SharePoint
Handle events in apps for SharePoint
Thanks
Best Regards
TechNet Community Support
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