Invalid data has been used to update the list item. The field you are trying to update may be read only (Lookup Field).

Question

Hi.
I am getting below error while adding value to look-up field.
Invalid data has been used to update the list item. The field you are trying to update may be read only.
I have tried many forums ans post but didn't come to know what's the root cause of issue. I am also posting Code for creating and adding lookup field.
CAML to create lookup field (It works Fine)
string lkproductNumber = "<Field Type='Lookup' DisplayName='Product Number' StaticName='ProductNumber' ReadOnly='FALSE' List='" + pNewMaster.Id + "' ShowField='Product_x0020_Number' />";
Code to insert value to lookup field
ClientContext client = new ClientContext(SiteUrl);
client.Load(client.Web);
client.Credentials = new NetworkCredential(this.UserName, this.Password, this.Domain);
// Lookup Lists
List pmList = client.Web.Lists.GetByTitle("Product_Master");
//List Conatining Lookup Columns
List piList = client.Web.Lists.GetByTitle("Product_Inventory");
client.Load(piList);
query.ViewXml = "<View/>";
ListItemCollection collection = pmList.GetItems(query);
client.Load(collection);
client.ExecuteQuery();
int prodid=0;
foreach (ListItem item in collection)
if (Convert.ToString(item["Product_x0020_Number"]) == ProductNumber)
{ prodid = Convert.ToInt32(item["ID"]); }
ListItem piItem = piList.AddItem(new ListItemCreationInformation());
piItem["Product_x0020_Number"] = new FieldLookupValue() { LookupId = prodid };
piItem.Update();
client.ExecuteQuery();
Exception Detail
Microsoft.SharePoint.Client.ServerException was caught
Message=Invalid data has been used to update the list item. The field you are trying to update may be read only.
Source=Microsoft.SharePoint.Client.Runtime
ServerErrorCode=-2147352571
ServerErrorTypeName=Microsoft.SharePoint.SPException
ServerStackTrace=""
StackTrace:
at Microsoft.SharePoint.Client.ClientRequest.ProcessResponseStream(Stream responseStream)
at Microsoft.SharePoint.Client.ClientRequest.ProcessResponse()
at WebServiceProviders.ClientServices.NewProductInventory() in Z:\.............ClientServices.cs:line 889
InnerException:
Quick response is highly appreciated.
Thanks
Mehar

Answer 1

Try some thing like below,
your data value that needs to be update should be in this format "ID of the lookup";#"Title of the Lookup" 
For example,
listItem["Product_x0020_Number"]
= "1;#iPhone";
listItem["Product_x0020_Number"]
= "2;#Mobile";
Hope this helped you....