Issue in adding new items to a O365 SharePoint Online List having lookup columns (Client Object Model)

Question

I have two Lists i.e. Publisher and Products in my SharePoint Online site. They are having the following structures:
1. Publisher:
Publisher (Single line of txt)
     A1
     A2
     A3
     A4
     A5
     A6
2. Products:
Publisher (lookup to the above column)       
ProductName (Single line of txt)
     A1                                                                   Apple
     A2                                                                   Samsung
     A3                                                                   Nokia
And I have an excel file named Products.xlsx in my local machine which has the following data:
Publisher        ProductName
     A1                   Apple
     A2                   Samsung
     A3                   Nokia
     A4                   Motorola
     A5                   LG
     A6                   HTC
Now I have written the below client-side (CSOM) code to fetch the data from the Excel and update the corresponding fields in the Products table:
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Web;
using System.IO;
using System.Data;
using System.Data.OleDb;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Xml.Linq;
using Microsoft.SharePoint;
using Microsoft.SharePoint.Client;
using File =
Microsoft.SharePoint.Client.File;
using System.Security;
namespace ExcelToSP
    public class
ExcelToSP
        //Main function to get the command line values and invoke the getSPList function to pull SP List data
        public
static void Main(string[] args)
            //Instantiate Class object
            ExcelToSP p
= new ExcelToSP();
            try
                p.LoadExcelData();
//Exit with Sucess code
Environment.Exit(0);
            catch
(Exception ex)
//Invoke writeErrorLog function to log the exception details
//p.WriteErrorLog(ex);
        public
void LoadExcelData()
            try
            string fileName
= @"E:\Products.xlsx";
            string fileExtension
= Path.GetExtension(fileName).ToUpper();
            string connectionString
= "";
             if
(fileExtension == ".XLS")
                connectionString
= "Provider=Microsoft.Jet.OLEDB.4.0;Data Source='"
+ fileName + "'; Extended Properties='Excel 8.0;HDR=YES;'";
            else
if (fileExtension ==
".XLSX")
                connectionString
= "Provider=Microsoft.ACE.OLEDB.12.0;Data Source='"
+ fileName + "';Extended Properties='Excel 12.0 Xml;HDR=YES;'";
            if
(!(string.IsNullOrEmpty(connectionString)))
string[] sheetNames =
GetExcelSheetNames(connectionString);
if ((sheetNames !=
null) &&
(sheetNames.Length
> 0))
DataTable dt = null;
OleDbConnection con =
new OleDbConnection(connectionString);
OleDbDataAdapter da =
new OleDbDataAdapter("SELECT * FROM ["
+ sheetNames[0]
+ "]", con);
                    dt
= new DataTable();
                    da.Fill(dt);
InsertIntoList(dt,"Products");
            catch
(Exception ex)
throw ex;
        private
string[] GetExcelSheetNames(string strConnection)
            var connectionString
= strConnection;
            String[] excelSheets;
            using
(var connection =
new OleDbConnection(connectionString))
                connection.Open();
var dt = connection.GetOleDbSchemaTable(OleDbSchemaGuid.Tables,
null);
if (dt ==
null)
return null;
                excelSheets
= new String[dt.Rows.Count];
int i = 0;
// Add the sheet name to the string array.
foreach (DataRow row
in dt.Rows)
                    excelSheets[i]
= row["TABLE_NAME"].ToString();
                    i++;
            return excelSheets;
        private
void InsertIntoList(DataTable listTable,
string ListName)
            try
string username = "[email protected]";
string pwd = "contoso@1234";//this.Dts.Variables["password"].Value.ToString();
ClientContext clientContext =
new ClientContext("https://contoso.sharepoint.com/teams/myPOC/");
SecureString password =
new SecureString();
char[] decryptpwd = pwd.ToCharArray();
foreach (char c
in decryptpwd)
                    password.AppendChar(c);
                clientContext.Credentials
= new SharePointOnlineCredentials(username, password);
                clientContext.ExecuteQuery();
//Setting SiteURL Client context
Web web = clientContext.Web;//
                clientContext.Load(web);
                clientContext.ExecuteQuery();
List lstProductFamily = web.Lists.GetByTitle("Products");
for (int iRow
= 0; iRow < listTable.Rows.Count; iRow++)
ListItemCreationInformation itemCreateInfo =
new ListItemCreationInformation();
ListItem newItem = lstProductFamily.AddItem(itemCreateInfo);
//FieldLookupValue flv = newItem["Publisher"] as FieldLookupValue ;
//string lkup = flv.LookupValue;
//int valueid = 0;
//valueid = flv.LookupId;
                    newItem["Publisher"]
= Convert.ToString(listTable.Rows[iRow][0]);
                    newItem["ProductName"]
= Convert.ToString(listTable.Rows[iRow][1]);
                    newItem.Update();
                    clientContext.ExecuteQuery();
            catch
(Exception ex)
throw ex;
But I'm getting the following error on running the above code "Invalid data has been used to update the list item. The field you are trying to update may be read only." 
Can anybody please help me out?

Answer 1

Hi,
According to your description, my understanding is that you want to update data to look up field.
I have a test about updating look up field using Client Object Model in my environment. lookup field will accept an array to set the field value.
Here is the code snippet:
Microsoft.SharePoint.Client.ClientContext ctx = new ClientContext("http://sp2013sps/sites/test/");
if (ctx != null)
List list = ctx.Web.Lists.GetByTitle("List3");
ListItem itemToUpdate = list.GetItemById(1);
ctx.Load(itemToUpdate);
ctx.ExecuteQuery();
FieldLookupValue newLookUpField = new FieldLookupValue();
newLookUpField.LookupId = 3;
FieldLookupValue newLookUpField1 = new FieldLookupValue();
newLookUpField1.LookupId = 4;
FieldLookupValue[] newarr = { newLookUpField, newLookUpField1 };
itemToUpdate["lookup"] = newarr;
itemToUpdate.Update();
ctx.Load(itemToUpdate);
ctx.ExecuteQuery();
Best regards
Patrick Liang
TechNet Community Support