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

I have SharePoint blog and I created a custom web part to add the comments for a new post. But when i add comment item by using this web part then its giving an error "Invalid data has been used to update the list item. The field you are trying to update may be read only."

Please see the attached code here

Does any one have an Idea!

regards

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";
                               
                                Item1.Fields["Post Id"].ReadOnlyField = false;
                                Item1["Post Id"] = 1;
                                Item1.Update();
                                Item1.Fields["Post Title"].ReadOnlyField = true;
                                Item1.Fields["Post Id"].ReadOnlyField = true; 
                            }
                        }

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