Update the value of LookupMulti column in SharePoint using the Client Object Model

ClientContext ctx = new ClientContext(weburl);
ctx.Credentials = new SharePointOnlineCredentials(userName, passWord); 

List list = ctx.Web.Lists.GetByTitle("ListTitle");
ListItem currentItem = list.GetItemById(1);

FieldLookupValue[] lookupFieldValCollection = new FieldLookupValue[3];FieldLookupValue lookupFieldA = new FieldLookupValue();
lookupFieldA.LookupId = 2;
lookupFieldValCollection.SetValue(lookupFieldA, 0);
FieldLookupValue lookupFieldB = new FieldLookupValue();
lookupFieldB.LookupId = 4;
lookupFieldValCollection.SetValue(lookupFieldB, 1);
FieldLookupValue lookupFieldC = new FieldLookupValue();
lookupFieldC.LookupId = 6;
lookupFieldValCollection.SetValue(lookupFieldC, 2);

currentItem["MultiLookupValCol"] = lookupFieldValCollection;
currentItem.Update();

ctx.ExecuteQuery();  

Two important thing I would like to share here,

• Initially I tried to pass an array of an array of integer [containing the ids of the item] to the LookupMulti column which generated the error: Value does not fall between the specified range

• Another thing was then I created a List of type FieldLookupValue and then I passed the List to the column using its ToArray(). I guess we just had to pass the array of FieldLookupValue. However, this resulted in the error: Unknown Error. Don’t know what that means.

Finally, I can say that create the array of type FieldLookupValue and populate this array using its SetValue method only.