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

  string DURL = "http://myURL/";
                        using (SPSite site = new SPSite(DURL))
                        {
                            using (SPWeb web = site.OpenWeb())
                            {
                                web.AllowUnsafeUpdates = true;
                                SPList listLook = web .Lists ["Posts"];

                                SPList list = web.Lists["Comments"];
                                SPListItem Item1 = list.Items.Add();
                                Item1["Title"] = "My testing title";
                               
                                list.Fields["Post Id"].ReadOnlyField = false;
                                list.Update();
                                Item1["Post Id"] = 1;
                                Item1.Update();
                                
                                list.Fields["Post Title"].ReadOnlyField = true;
                                list.Fields["Post Id"].ReadOnlyField = true; 
                                list.Update();

                            }
                        }

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